Monthly Archives: July 2021

Strange ideas can come from ordinary places. This one came from Texas. In 1981, John A. Wheeler, the father of the black hole and a theoretical physicist at the University of Texas in Austin, threw a party. The guests were all young physicists with a common interest in the foundations of computing, a topic that Wheeler believed--correctly--would become increasingly important in the years to come.

It was at this party that a conversation with Charles Bennett, an IBM physicist, sparked an idea in the mind of Oxford University researcher David Deutsch. It struck him that computer theory was based on Newton's laws, not the more fundamental description of the universe provided by quantum theory.

At the time, the computer industry was beginning to fret over the future of microchips. How many calculations per second would be ultimately possible, how much heat would this produce, and could silicon survive the constant baking? To help them, computer scientists turned to the theory developed in the 1930s by the pioneer of their field, Alan Turing. But at Wheeler's party, said Deutsch, "I could see immediately that using the laws [of quantum mechanics] would give a different answer."

Deutsch began work on a paper that is now generally regarded as a classic in the field. Published in 1985, it describes how a computer might run using the strange rules of quantum mechanics and why such a computer differs fundamentally from ordinary computers.

Fifteen years later, the revolution that Deutsch started has reached global proportions. Quantum computers are no longer seen as weird curiosities but as the powerful future of the computer industry, and the debate is shifting from whether they will ever become a reality to when they will do so. The excitement is not due to their power, although they undoubtedly will be more powerful than today's models. Their big selling point, the killer app if you like, is that they can solve problems and carry out simulations that are basically impossible on conventional computers.

Such is the potential of these devices that the list of companies funding research programs sounds like a roll call of the world's biggest telecommunications and computer businesses. They include IBM, Hewlett-Packard, Lucent Technologies, AT&T, and Microsoft. There is even a New York Citybased start-up called MagiQ Technologies that hopes to make money by developing intellectual property in this field.

One of the strongest forces driving the development of quantum computers is the fear they will crack with ease secret codes that are impervious to other computers. The alarm bells started ringing in 1994, when Peter Shor of AT&T's Bell Laboratories in New Jersey showed that quantum computers were far faster than their ordinary brethren at factoring numbers.

Finding the factors of large numbers is so difficult for conventional computers that code-makers rely on this weakness of theirs to protect sensitive data. With the development of quantum computers, these codes will be obsolete. As soon as the first modest-sized quantum computer is switched on, governments and their militaries will be forced to concede that many of their codes are unsafe. Understandably, they are keen to find out just what quantum computers can do, and various national laboratories have begun substantial programs, in particular the U.S. National Institute of Standards and Technology in Boulder, Colo.; Los Alamos National Laboratory in New Mexico; and the United Kingdom equivalent, the Defence Evaluation and Research Agency in Malvern.

Aside from its promise for espionage there is the new physics unveiled almost daily by scientists trying to understand quantum information and how to control it. Quantum computers are becoming tiny laboratories in which scientists can test the theories of quantum mechanics with greater precision than ever before. Arguably the strongest team in the world making such discoveries is at the University of Oxford. Smaller groups exist at places such as MIT, Caltech, and a group of Australian universities, with influential individuals scattered throughout the United States, Europe, and Israel. After a late start, Japan has begun a concerted effort to catch up.

ILLUSTRATION: STEVE STANKIEWICZ

How Spin States Can Make Qubits: The spin of a particle in a dc magnetic field is analogous to a spinning top that is precessing around the axis of the field. In such a field, the particle assumes one of two states, spin up or spin down, which can represent 0 and 1 in digital logic. A particle in one spin state can be pushed toward another by a radio frequency pulse perpendicular to the magnetic field. A pulse of the right frequency and duration will flip the spin completely [top]. A shorter RF pulse will tip the spin into a superposition of the up and down state [bottom], allowing simultaneous calculations on both states.

Quantum information

Digital information appears mundane stuff. The 0s and 1s of binary code can be easily measured, copied, and moved around. But assign a piece of information to a quantum particle, and it takes on the bizarre characteristics of the quantum world. This fundamental unit of quantum information is called a quantum bit, or qubit (pronounced cue bit), and it is quite different from its classical counterpart.

For a start, a qubit can be both a 0 and 1 at the same time. Take the spin of an electron--a property that can be imagined as the spin of a top with its axis pointing either up or down [see figure, above}. The up or down spin can correspond to a 0 or 1. But the electron can also be placed in a ghostly dual existence, known as a superposition of states, in which it is both up and down, a 0 and a 1, at the same time. Carry out a calculation using the electron, and you perform it simultaneously on both the 0 and the 1, two calculations for the price of one.

At first glance, this may not seem impressive, but add more qubits and the numbers become much more persuasive. While 1 qubit can be in a superposition of two states, 0 and 1, two qubits can be in a superposition of four states--00, 01, 10, and 11--representing four numbers at once. The increase is exponential: with m qubits, it is possible to carry out a single calculation on 2m numbers in parallel. With only a few hundred qubits, it is possible to represent simultaneously more numbers than there are atoms in the universe.

Algorithms, entanglement, and error correction

Of course, once the calculation has finished, the answer must be obtained. A simple measurement destroys the superposition, leaving the system in one state or another. Unfortunately, it is rarely possible to determine in advance which state this will be, and that is a problem. The goal is to ensure that the measurement produces the answer of interest, and it can be reached by exploiting the phenomenon of quantum interference. Each of the superposed states has a probability associated with it that has a wavelike behavior--it can interfere with the probabilities of other states destructively or constructively. Getting the desired answer to a calculation means processing the information in such a way that undesired solutions interfere destructively, leaving only the wanted state, or a few more or less wanted states, at the end. The process is known as a quantum algorithm, and its design challenges physicists, mathematicians, and computer scientists. A final measurement then gives the desired answer, or in the case of a few final states, a series of measurements gives their probability distribution from which the desired answer can be calculated.

Quantum algorithms have the potential to be dramatically faster than their conventional counterparts. A good example is an algorithm for searching through lists that was developed by Lov Grover at Lucent Technologies' Bell Laboratories, in Murray Hill, N.J. The problem is to find a person's name in a telephone directory, given his or her phone number. If the directory contains N entries, then on average, you would have to search through N/2 entries before you find it. Grover's quantum algorithm does better. It finds the name after searching through only (check)N entries, on average. So for a directory of 10 000 names, the task would require (check)(10 000) = 100 steps, rather than 5000. The algorithm works by first creating a superposition of all 10 000 entries in which each entry has the same likelihood of appearing in response to a measurement made on the system. Then, to increase the probability of a measurement producing the required entry, the superposition is subjected to a series of quantum operations that recognize the required entry and increase its chances of appearing. (Remember that the recognition is possible because you have the phone number but not the name.)

ILLUSTRATION: STEVE STANKIEWICZ

Entangled Particles: If two particles, both in states of superposition, are entangled, measuring one forces both to assume complementary states.

Coherence/Decoherence: the ability of a quantum system to maintain a superposition of states. Decoherence is the process by which interactions with the environment destroy superposition, forcing a system into one state or another.

Entanglement: the state in which two quantum systems in indeterminate states are linked so that measuring or manipulating one system instantaneously manipulates the second.

Qubit: a unit of information used in quantum computing. It is distinct from an ordinary bit in that it can encode a superposition of values.

Spin: a quantum mechanical property of particles that in certain cases can take only two mutually exclusive values. It is used widely in nuclear magnetic resonance.

Superposition: if a physical system such as a particle can be found in more than one state and its state is unknown, it exists in a superposition of those states. That is, if there are two possible states, the system can be said to exist in both at once until its state is actually measured. Such a measurement collapses the system onto one state or another.

Teleportation: communication between two parties using entangled particles. Through the entanglement the state of one particle can be transferred to another distant particle with which it is entangled.

Vibrational State: the quantized state of the collective motion of ions in a linear ion trap. The vibrational state can encode a qubit and is used to link the ions during calculations.

As if superposed values and probability waves were not counterintuitive enough, another strange phenomenon is prominent in the new science of quantum information. In the '30s, scientists fiercely debated whether what quantum mechanics predicted had a real existence or whether its strangeness was due to some deficiency in the theory. In particular, Albert Einstein could not believe that the universe was built as quantum mechanics claimed. So, together with his colleagues Boris Podolsky and Nathan Rosen, he devised a thought experiment to find holes in the new theory.

The thought experiment centers on the behavior of pairs of particles that, according to quantum theory, are joined together--entangled--in a profound way that has no analog in the classical world. Prod one, and it seems the other instantly feels the influence, no matter how far away it might be [see figure, above]. The three scientists pointed out that this process would have to involve a faster-than-light signal passing between the particles--an impossibility. Their conclusion became known as the EPR (Einstein-Podolsky-Rosen) paradox and the entangled particles as EPR pairs.

The debate was resolved by John Bell, a theorist at CERN, the European laboratory for particle physics near Geneva, and the French physicist Alain Aspect. They proved that the Siamese twins of the quantum world, EPR pairs, indeed behave in the way predicted by quantum mechanics. However, the experiment also showed that there is no faster-than-light signal and that entanglement cannot be used for superluminal communication. Rather than communicating, EPR pairs share the same existence, the same destiny, if you like. Entanglement is now one of the key phenomena exploited in quantum information processing. Today the EPR experiment is performed almost daily around the world.

If creating entanglement and superposition has become a commonplace event compared with 10 years ago, quantum information remains fragile stuff. Ordinary interactions with the environment destroy qubits and the information they contain, a process known as decoherence. (Its opposite, coherence, is the ability of a qubit to maintain such quantum characteristics as superposition.) If quantum information is to pass into the world of computer science, a process of error correction is needed to protect against decoherence [see Defining Terms, left].

Initially, physicists believed that such a technique was impossible, because detecting and correcting errors would mean measuring the state of a quantum system and so destroying the information it contained. Still, by the early '90s Deutsch had shown this need not be the case. And in 1994 Andrew Steane at the University of Oxford and Peter Shor at AT&T's Bell Laboratories in New Jersey independently discovered practical quantum error-correction algorithms.

The problem is similar to reproducing in one place a message that has been constructed in another. If the message is sent over a channel or stored in a place noisy enough to distort some of the bits in the sequence, how can the receiver recognize the message? By adding redundancy to the message so that the sender can correct bits that have been distorted.

Shor and Steane came up with the quantum equivalent of sending the same bit three times. The extra qubits are known as ancillas. Measuring these qubits tells the receiver what errors have occurred and how to correct the qubits that are part of the message.

NMR leads the charge

The first big breakthrough for scientists building actual quantum computers came in the mid-'90s, when they discovered how to carry out calculations using the techniques of nuclear magnetic resonance (NMR). The key idea was that a single molecule can act like a tiny computer. Information is stored in the orientation of nuclear spins in the molecule, each nucleus holding one qubit. And the interaction between the nuclear spins, known as spin-spin coupling, serves to mediate logic operations. In a strong magnetic field, these nuclei precess around the direction of the magnetic field at frequencies that depend on their chemical environment.

For instance, in a 9.3-tesla field, a carbon-13 nucleus in a chloroform molecule precesses at about 100 MHz. By zapping the molecule with radio waves tuned to these resonant frequencies, it is possible to manipulate each nucleus individually to carry out logic operations. The manipulation might involve flipping a nucleus from a 1 to a 0, a so-called one-qubit operation or single-bit rotation; or it might involve two linked nuclei in a two-qubit operation, in which the value of one nucleus is flipped in a way that depends on the value of the other.

Chloroform made with the carbon-13 isotope is a good example of a molecule that can act as a two-qubit quantum computer, because its hydrogen and carbon-13 nuclei can be addressed individually by the radio waves. A quantum calculation is then carried out by encoding a program--a sequence of one- and two-qubit operations--as a series of RF pulses. The results are then read out by listening for the magnetic induction signal generated by the precessing nuclei at the end of the calculation. That signal indicates the orientation of the nuclear spin.

Nuclear magnetic resonance sounds like the dream solution to a thorny problem. Nuclei are naturally isolated from the noise of the outside world and so can maintain coherence for many seconds, enough time to perform hundreds of logic operations. In addition, NMR is a mature technology, having been used over many years for imaging and chemical analysis.

But the technique has some severe limitations. Single molecules do not produce a signal strong enough to be observed. Instead, NMR experiments must involve huge numbers of molecules (of the order of 1023) so that their combined magnetic induction signal is large enough to be picked up. (These molecules are usually distributed in a solvent, so the first quantum computers actually have liquid hearts.)

To begin a calculation, the initial state of the computer must be known. But in a material at room temperature, the spin up and spin down states are distributed almost equally and at random. In other words, the state of each of the many computers in solution cannot be known, rendering any subsequent calculation meaningless.

ILLUSTRATION: STEVE STANKIEWICZ

Quantum Logic: One of the most important logic elements in quantum computing is the controlled-NOT gate, similar to a controllable inverter circuit. In such an element, the state of one qubit, the control qubit, determines whether the final state of a second qubit, the input qubit, will be inverted by a series of RF pulses.

But never say die. In 1997, two groups independently came to quantum computing's rescue. Isaac Chuang, now at IBM's Almaden Laboratory near San Jose, Calif., and Neil Gershenfeld at the Massachusetts Institute of Technology (MIT), in Cambridge, found that they could turn a small natural bias--say, toward spinning up rather than down with respect to the magnetic field--in the nuclei of some molecules to advantage. They could use it to establish a kind of artificial ground state (00 for a two-qubit stystem) from which to start a calculation. At the same time, David Cory, also at MIT, and Amr Fahmy and Timothy Havel, both from Harvard University, in Cambridge, Mass., discovered that by bombarding the sample with radio pulses they could effectively "jam" the signal from all but the ground state.

To carry out useful calculations, the computer must be able to perform any logical operation. For quantum computers, there are two logic operations from which all other operations can be derived, rather like the AND and NOT gates in classical computing. One involves rotating a single qubit. The other, carried out on two qubits and called a controlled-NOT gate, flips or fails to flip one qubit depending on the state of another to which it is coupled [see figure, above]. Both these operations are straightforward: simply bombard the liquid sample with the appropriate sequence of radio pulses. Since 1997, these two groups and others, notably at Los Alamos and Oxford University, have built liquid NMR quantum computers with up to seven qubits to perform simple algorithms, one of which even belongs to the mathematical family of Shor's code-cracking formula [see "Quantum Code Cracking Creeps Closer," Spectrum, October 2000].

Unfortunately, quantum computers based on liquid NMR will never be much more powerful than this. The readout signals they produce plummet exponentially with the number of qubits involved in the calculation, because the proportion of molecules found in the appropriate starting state decreases. So scientists do not expect to be able to handle any more than a dozen qubits or so before the signal becomes indistinguishable from the background. Attempts to build machines that can handle more than 10 qubits continue, but if nontrivial quantum computing is ever to become possible, some other approach is needed.

Refrigerated ions

A technology that is less in the public eye than NMR has attracted others. In 1995 Ignacio Cirac and Peter Zoller of the University of Innsbruck, in Austria, suggested using ion traps to build quantum logic gates. The technology behind ion traps is already used for spectroscopy and to improve time and frequency standards, but huge advances are needed for quantum computation. The idea is that a number of ultra-cold ions can be trapped using a device known as a linear radio-frequency Paul trap. This device sets up a high-frequency RF field that holds the ions tightly in two dimensions but only weakly in the third dimension. Because the ions have the same charge, they repel each other and tend to arrange themselves in a straight line, equally spaced, like beads on an elastic string. The arrangement allows them to vibrate as a group in ways important for quantum computing.

The qubits are initially stored in the internal spin states of the ions relative to a background magnetic field. They are written to the ions using a pulsed, oscillating magnetic field, which flips the bits or places them in a superposition of up and down states, depending on its duration. An advantage of ion traps is that this superposition is extremely robust, lasting for at least as long as the qubits in NMR, ample time to carry out the desired logic operations.

ILLUSTRATION: STEVE STANKIEWICZ

Computing in an Ion Trap: Ions are lined up in a trap by RF energy from four electrodes, then chilled using lasers [top]. The electrostatic repulsion between the ions couples their individual motion as if they were connected by springs [middle]. The coupled motion, or vibrational state, can be used to transfer quantum information from one qubit to another. Basically, a pulse of energy equal to the difference between the quantum state of the ion and the vibrational state of the two ions (0 or 1) leads the ion to swap its internal state for the vibrational state. A similar pulse to the other ion performs another swap, transferring the original state of the first qubit to the second.

To share the qubits between the ions, scientists turn to the ion vibrations. The aim is to chill the ions until as a group they are absolutely still. This is the ground state of the system. Inject a little energy, and the ions begin to vibrate. But being quantum particles, the ions can exist in a superposition of the ground state and the vibratory state, so the vibration can be used to store a qubit. Because the ions all take part in the vibration, this qubit is shared among them. It's as if this collective motion is a kind of databus, allowing all the ions to temporarily share the information and become entangled. This sharing allows the IF and THEN type operations that are the building blocks of computer logic gates. For example, an instruction might be: IF the vibrational state is 1, THEN flip the qubit in the first ion's internal spin state. Researchers at the National Institute of Science and Technology (NIST) have already demonstrated that a string of four ions can be entangled and have said that more should be possible.

At least five groups around the world are working on ion trap quantum computers, but David Wineland's team at NIST is widely regarded as the leader. His group has built a 2-qubit logic gate using a single beryllium ion cooled to its vibrating ground state. Using a laser focused on the ion, the group superimposes on the background magnetic field a second magnetic field with a magnitude that varies with the position of the ions. The ion's vibration causes it to experience an oscillating magnetic field, and when the frequency of the oscillation matches the energy difference between the ion's two spin states, energy is transferred from the spin to the vibrational state, mapping the quantum information to the vibrational from the spin state [see figure, above]. This is the basis of a controlled-NOT gate and was realized in 1995 only a few months after Cirac and Zoller's announcement. Reading the data involves scattering light off the ion, since a spin up ion can be made to scatter strongly, while a spin down ion will scatter hardly at all.

Ion traps, too, have their limitations. One is the short decoherence time of the qubits after transfer to the vibrational "databus." Because the ions are charged, the vibrations are strongly influenced by stray electric fields, causing decoherence. Nonetheless, the group is confident that this tendency can be overcome by isolating the trap better from the environment. Ion traps also suffer from problems of scalability. The more ions there are in the trap, the greater the risk of tapping into uncontrollable vibrational states and so destroying the calculation. The next step will be to build adjacent traps, each holding only a few ions, and sending quantum information from one trap to another, either by physically moving the ions or by a phenomenon peculiar to quantum information called teleportation.

The alternatives

While liquid NMR is doomed because of the problems of working at room temperature, several groups are looking into carrying out NMR-type manipulations on single atoms in the solid state. A proposal from Bruce Kane at the University of Maryland in particular has attracted attention. His idea is to bury an array of phosphorus atoms in silicon and overlay it with an insulating layer, on top of which sits a like array of electrodes, each of which can apply a voltage to the atom beneath it. The ingenious aspect of this setup is how Kane proposes to control the spin of each nucleus.

Just as in NMR, the spin of the nuclei can be flipped by being zapped with radio waves of just the right energy--but, of course, these radio waves would flip every nucleus. Now phosphorus atoms have a single electron in their outer shell that interacts with the nuclear spin in a complex way. Applying a voltage to the atom changes the energy required to address both the nuclear and the electronic spin, and therefore it changes the frequency of the radio waves needed to flip the nucleus. So by applying a voltage to a specific electrode and zapping the array with the new frequency, it is possible to address a single nucleus.

But to perform a controlled-NOT logic operation, two qubits have to become entangled. Kane also has a way of doing this. Voltages applied between adjacent phosphorus atoms in the array can turn on and off the interactions between the outer electrons in each atom, allowing two-qubit operations.

Of course, the theory is all very well. The difficulty is actually building such a device, and Kane's collaborators are already working on it. At the Centre for Quantum Computer Technology at the University of New South Wales, in Australia, Robert Clark heads a team that is hoping to overcome many of the obstacles Kane's device faces. First up is the difficulty of creating the atomic array and preventing the phosphorus atoms from migrating within the silicon.

Kane is setting up a lab to study another challenging aspect of his device: the readout. Once the one- or two-qubit operation has been completed, the result has to be read out from the nuclear spins. Once again, Kane relies on the link between nuclear and electronic spins to get an answer. By very carefully measuring the spin of the electron, he said, it is possible to infer the spin of the nucleus. Measuring the spin of a single electron has never been done, but Kane said this should be possible shortly.

Kane's idea has attracted so much attention because many of these logic gates can be linked together to form a large quantum computer, though doing so may take some time. New South Wales's Clark believes that a handful of qubits might be possible in the medium term.

The quantum phenomena of superconductivity may also prove useful for building quantum computers. In 1999, at the Delft University of Technology in the Netherlands, a team designed a superconducting circuit in which superposed counter-rotating currents could prove useful for storing and manipulating qubits. The circuit consists of a loop with three or four Josephson junctions for measuring the circuit's state. The fact that it is made by conventional electron-beam lithographic techniques makes it particularly conducive to large-scale integration. However, superconducting circuits have short decoherence times, and today's techniques for measuring the states of the circuits are too invasive for useful manipulation of qubits.

A more advanced solid-state technology is the quantum dot, essentially a semiconductor trap holding a discrete number of electrons. These have been studied since the early 1990s because the trapped electrons act like artificial atoms, with their own periodic table and chemistry. Then in 1998, David DiVincenzo of IBM and Daniel Loss of the University of Basel, in Switzerland, proposed using quantum dots as the building block of a quantum computer, and a variety of ideas have since been put forward for exploiting the dots' quantum properties for computation. One idea is a two-qubit system consisting of two electrons shared by four quantum dots in a square. The electrons, seeking to minimize their energy, occupy opposite corners of the square, and since this arrangement has two configurations, they exist as a superposition that is manipulable through electrodes at the corners of the square. A number of other techniques involve reading and writing data to the dots with laser pulses and placing a single nucleus at the center of each dot that can be addressed with NMR techniques, rather as in Kane's proposal.

A quantum Internet

The problems in scaling up many of these ideas have persuaded many scientists that if quantum computing is to become useful any time soon, it will have to involve networking small quantum computers together. But sending quantum information from one place to another is tricky. One option is to physically move the qubits, but then they would be liable to decoherence. In 1993, however, Charles Bennett, from IBM's Thomas J. Watson Laboratory in Yorktown Heights, N.Y., and a few colleagues came up with a different option: teleportation.

Teleportation utilizes the deep link that entanglement sets up between one point in the universe and another. Bennett theorized that entanglement could act as a kind of phone line down which to send quantum information--in other words, create an entangled pair of particles and send one of them to the receiver while keeping the other [see "Quantum Teleportation"]. This process links these two points in a way that allows the exchange of quantum information from one qubit to another.

Bennett and his colleagues had to wait four years to see their predictions verified. In 1997, in a small room at the University of Innsbruck, in Austria, a group of physicists led by Anton Zeilinger performed the first teleportation experiment. Zeilinger's travelers were photons and he was sending them only a meter or so, from one side of the lab to the other. Today, more than three years later, Zeilinger is working on the next step, which is to teleport photons over distances of a kilometer.

Soon after Zeilinger's breakthrough, Cirac and Zoller proposed that teleportation could become the basis of a kind of quantum Internet. And in March of 2000, Seth Lloyd and Selim Shahriar at MIT and Philip Hemmer at the U.S. Air Force Research Laboratory, in Lincoln, Mass., suggested sending entangled photons over optical fibers to nodes containing cold atoms that would absorb the photons and so store the entanglement. This entanglement could then be used for error correction, teleportation, and various other valuable applications. A number of groups are working on this idea, including Jeff Kimble at the California Institute of Technology and Eli Yablonovitch at the University of California at Los Angeles. They hope to have a three-node network running within 10 years.

Some scientists hope for even greater things from entanglement, believing it will be so useful that it will one day be traded as a currency over the quantum Internet. Considerable progress will be required before anything remotely like that becomes possible. Even so, the pace of innovation in quantum computing has already exceeded most scientists' wildest dreams. Only five years ago, many were confident that quantum computers would not be built for 20 years, yet NMR proved them wrong within a year. Only the bravest forecaster would dare to predict how the field will stand five years from now.

With only a few hundred qubits it is possible to represent simultaneously more numbers than there are atoms in the universe

Spectrum Editor: Samuel K. Moore

JUSTIN MULLINS [p. 42], a contributing editor, is a freelance science writer based in Oxford, England. He is a consulting technology editor for New Scientist.

For an overview of quantum computing techniques and the peculiarities of quantum information, try Introduction to Quantum Computing and Information, edited by Hoi-Kwong Lo, Sandu Popescu, and Tim Spiller, and published in 1998 by World Scientific (Singapore).

Some important papers in quantum computing include: "Bulk Spin-Resonance Quantum Computation," by N. Gershenfeld and I. L. Chuang, Science, Vol. 275, p. 350 (1997); "Quantum Logic Gates in Optical Lattices," by G. Brennen, C. Caves, I. Deutsch, and P. Jessen, Physics Review Letters, Vol. 82, p. 1060 (1999); "A Silicon-Based Nuclear Spin Quantum Computer," by B. E. Kane, Nature, Vol. 393, p. 133 (1998); and Teleportation and the Quantum Internet, by S. Lloyd, M. Shahriar, and P. Hemmer, available from the Los Alamos Archive (http://xxx.lanl.gov).

An informative Web site explaining quantum teleportation is http://info.uibk.ac.at/c/c7/c704/qo/ photon/_teleport/index.html.

Nuclear magnetic resonance (NMR) is explained in an on-line book at http://www.cis.rit.edu/htbooks/nmr/. The author is Joseph P. Hornak.

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The Topsy Turvy World of Quantum Computing - IEEE Spectrum

In quantum physics, one breakthrough can quickly lead to several more.

This could happen in the wake of a brand new particle recently discovered by a group of scientists with the Large Hadron Collider (LHCb), called Tcc+and dubbed a tetraquark, according to a recent presentation at the European Physical Society Conference on High Energy Physics (EPS-HEP). The new particle is an exotic hadron comprised of two quarks and two antiquarks.

Crucially, this exotic matter particle lives longer than any other ever discovered, in addition to containing two heavy quarks and two light antiquarks, in another first.

All matter is comprised of fundamental building blocks, called quarks, which can fuse to form hadrons, including baryons, like the neutron and proton of conventional atomic theory. These contain three quarks, in addition to mesons, which come into being as quark-antiquark pairs. In the last several years, numerous "exotic" hadrons, particles dubbed as such because they possess four or five quarks (instead of two or three, which is more normal), were discovered. But the recent study has revealed the existence of an especially distinguished exotic hadron, or super-exotic hadron, if you can believe it.

The exceptionally unique hadron contains two charm quarks, in addition to both an up and a down antiquark. In recent years, multiple tetraquarks were discovered, one of which had two charm quarks, and two charm antiquarks. But the newly-discovered one has two charm quarks, without the extra two charm antiquarks that previously discovered hadrons had. Called "open charm", or "double open charm", these particles are different from other quarks that have an equal balance of quarks and antiquarks that cancel one another out (like a zero-sum game). But in the case of the new "super" exotic hadron (super quote not official), the charm number adds up to two, according to a Phys.org report.

But there's more to thisTcc+ super exotic hadron than charm. It's also the first particle discovered that's a member of a category of tetraquarks with a pair of both light and heavy antiquarks. This class of particles decays via a transformation into a pair of mesons, each of which comes into being via one of the heavy and one of the light antiquarks. Some theoretical predictions predicate the mass of tetraquarks of this kind to be near the sum of masses of the two mesons. In other words, their masses are very close, which creates "difficulty" for decay processes. What this does is extend the lifetime of the particle, compared to other ones, which is whyTcc+is the longest-lived exotic hadron ever discovered in the history of quantum physics.

Everyone knows quantum theory is famously difficult to parse, but this discovery will open the door to the discovery of even more novel particles of this class. Ones that are heavier, with one or two charm quarks that are replaced with bottom quarks. The theorized particle with two bottom quarks should have a mass smaller than the sum of any two B mesons, which, in simpler terms, means decay will be extremely difficult: Lacking the ability to decay via strong interaction, heavier particles than the newly-discovered one would have a lifetime that's several orders of magnitude longer than any exotic hadron observed before. Finally, this novelTcc+particle exhibit an exceptional level of precision on its mass, and enable further studies of quantum numbers of the particle. With these, physicists will finally be able to observe effects on quantum levelsthat no one has successfully studied before.

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Physicists at CERN Just Discovered a Brand New Particle - Interesting Engineering

The Ohio State University has joined the Chicago Quantum Exchange, a growing intellectual hub for the research and development of quantum technology.

The exchange, based at the University of Chicagos Pritzker School of Molecular Engineering, announced the addition today of Ohio State and the Weizmann Institute of Science as partners, referring to both as world-leading research institutions at the forefront of quantum information science and engineering.

Quantum information technology presents unique opportunities for students and researchers to engage in curiosity-driven and cutting-edge work that solves the problems people face in their everyday lives, said Ohio State President Kristina M. Johnson. As a result of this partnership with CQE, Ohio State faculty and students will have the opportunity to learn alongside brilliant collaborators and make a real-world and far-reaching impact.

Ohio State is the Chicago Quantum Exchanges first regional partner, strengthening the organizations connections throughout the Midwest and the nation. The lead member institution in the multi-institutional quantum education initiative QuSTEAM, the university is dedicated to preparing a quantum-ready workforce that can meet the existing and growing demand across the communications, optics, computing and materials industries.

The exchange is composed of a community of researchers aiming to accelerate discovery and innovation in quantum technology and develop new ways of understanding the laws of quantum mechanics, the theory that governs nature at its smallest scales. Anchored by the University of Chicago, Argonne National Laboratory, Fermi National Accelerator Laboratory and the University of Illinois at Urbana-Champaign, CQE also includes the University of Wisconsin-Madison and Northwestern University as well as a range of industry partners.

Having partners across the world, and across the Midwest, broadens our perspectives and as we continue to grow our community from the heart of U.S. quantum research in Chicago, said David Awschalom, the Liew Family Professor in Molecular Engineering and Physics at the University of Chicago and director of the Chicago Quantum Exchange. We look forward to collaborating with Ohio State and the Weizmann Institute to advance quantum science and technology and develop a strong, diverse quantum workforce.

In addition to advancing research in multiple quantum and physics areas as well as such disciplines as nanomechanics and physical chemistry, the exchange seeks to attract talent, funding and industry to the Chicago area to become the source for tomorrows leading quantum engineers.

Working with leaders at Ohio State University and the Weizmann Institute has reinforced for us the deep value of global collaboration on quantum science and technology, said Juan de Pablo, vice president for national laboratories, science strategy, innovation and global initiatives at the University of Chicago. Quantum information science is poised to make a profound impact on research, technology and business growth around the globe, and we are excited to continue advancing that work with some of the worlds great research organizations.

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Ohio State joins national initiative to accelerate innovation in quantum technology - The Ohio State University News

Illustration: Emily Cooper

Since the invention of paper money, counterfeiters have churned out fake bills. Some of their handiwork, created with high-tech inks, papers, and printing presses, is so good that its very difficult to distinguish from the real thing. National banks combat the counterfeiters with difficult-to-copy watermarks, holograms, and other sophisticated measures. But to give money the ultimate protection, some quantum physicists are turning to the weird quirks that govern natures fundamental particles.

At the moment, the idea of quantum money is very much on the drawing board. That hasnt stopped researchers from pondering what encryption schemes they might apply for it, or from wondering how the technologies used to create quantum states could be shrunk down to the point of fitting it in your wallet, says Scott Aaronson, an MIT computer scientist who works on quantum money. This is science fiction, but its science fiction that doesnt violate any of the known laws of physics.

The laws that govern subatomic particles differ dramatically from those governing everyday experience. The relevant quantum law here is the no-cloning theorem, which says it is impossible to copy a quantum particles state exactly. Thats because reproducing a particles state involves making measurementsand the measurements change the particles overall properties. In certain cases, where you already know something about the state in question, quantum mechanics does allow you to measure one attribute of a particle. But in doing so youve made it impossible to measure the particles other attributes.

This rule implies that if you use money that is somehow linked to a quantum particle, you could, in principle, make it impossible to copy: It would be counterfeit-proof.

The visionary physicist Stephen Wiesner came up with the idea of quantum money in 1969. He suggested that banks somehow insert a hundred or so photons, the quantum particles of light, into each banknote. He didnt have any clear idea of how to do that, nor do physicists today, but never mind. Its still an intriguing notion, because the issuing bank could then create a kind of minuscule secret watermark by polarizing the photons in a special way.

To validate the note later, the bank would check just one attribute of each photon (for example, its vertical or horizontal polarization), leaving all other attributes unmeasured. The bank could then verify the notes authenticity by checking its records for how the photons were set originally for this particular bill, which the bank could look up using the bills printed serial number.

Thanks to the no-cloning theorem, a counterfeiter couldnt measure all the attributes of each photon to produce a copy. Nor could he just measure the one attribute that mattered for each photon, because only the bank would know which attributes those were.

But beyond the daunting engineering challenge of storing photons, or any other quantum particles, theres another basic problem with this scheme: Its a private encryption. Only the issuing bank could validate the notes. The ideal is quantum money that anyone can verify, Aaronson saysjust the way every store clerk in the United States can hold a $20 bill up to the light to look for the embedded plastic strip.

That would require some form of public encryption, and every such scheme researchers have created so far is potentially crackable. But its still worth exploring how that might work. Verification between two people would involve some kind of black boxa machine that checks the status of a piece of quantum money and spits out only the answer valid or invalid. Most of the proposed public-verification schemes are built on some sort of mathematical relationship between a bank notes quantum states and its serial number, so the verification machine would use an algorithm to check the math. This verifier, and the algorithm it follows, must be designed so that even if they were to fall into the hands of a counterfeiter, he couldnt use them to create fakes.

As fast as quantum money researchers have proposed encryption schemes, their colleagues have cracked them, but its clear that everyones having a great deal of fun. Most recently, Aaronson and his MIT collaborator Paul Christiano put forth a proposal [PDF] in which each banknotes serial number is linked to a large number of quantum particles, which are bound together using a quantum trick known as entanglement.

All of this is pie in the sky, of course, until engineers can create physical systems capable of retaining quantum states within moneyand that will perhaps be the biggest challenge of all. Running a quantum economy would require people to hold information encoded in the polarization of photons or the spin of electrons, say, for as long as they required cash to sit in their pockets. But quantum states are notoriously fragile: They decohere and lose their quantum properties after frustratingly short intervals of time. Youd have to prevent it from decohering in your wallet, Aaronson says.

For many researchers, that makes quantum money even more remote than useful quantum computers. At present, its hard to imagine having practical quantum money before having a large-scale quantum computer, says Michele Mosca of the Institute for Quantum Computing at the University of Waterloo, in Canada. And these superfast computers must also overcome the decoherence problem before they become feasible.

If engineers ever do succeed in building practical quantum computersones that can send information through fiber-optic networks in the form of encoded photonsquantum money might really have its day. On this quantum Internet, financial transactions would not only be secure, they would be so ephemeral that once the photons had been measured, there would be no trace of their existence. In todays age of digital cash, we have already relieved ourselves of the age-old burden of carrying around heavy metal coins or even wads of banknotes. With quantum money, our pockets and purses might finally be truly empty.

Michael Brooks, a British science journalist, holds a Ph.D. in quantum physics from the University of Sussex, which prepared him well to tackle the article Quantum Cash and the End of Counterfeiting. He says he found the topic of quantum money absolutely fascinating, and adds, I just hope I get to use some in my lifetime. He is the author, most recently, of Free Radicals: The Secret Anarchy of Science (Profile Books, 2011).

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Quantum Cash and the End of Counterfeiting - IEEE Spectrum

A quirky type of subatomic particle known as the charm meson has the seemingly magical ability to switch states between matter and antimatter (and back again), according to the team of over 1,000 physicists who were involved in documenting the phenomenon for the first time.

Oxford researchers, using data from the second run of the Large Hadron Collider (LHC)a particle accelerator at the Switzerland-based European Organization for Nuclear Research (known internationally as CERN)made the determination by taking extremely precise measurements of the masses of two particles: the charm meson in both its particle and antiparticle states.

Yes, this breakthrough in quantum physics is as heady as it sounds. A charm meson particle, after all, can exist in a state where it is both itself and its evil twin (the antiparticle version) at once. This state is known as "quantum superposition," and it's at the heart of the famous Schrdinger's Cat thought experiment.

As a result of this situation, the charm meson exists as two distinct particles with two distinct masses. But the difference between the two is infinitesimally small0.00000000000000000000000000000000000001 grams to be exact, according to the scientists' research, described in a new paper published last month on the arXiv preprint server (that means the work hasn't been peer-reviewed yet). They've recently submitted the work for publication in the journal Physical Review Letters.

While the findings are basically the definition of minuscule, the ramifications are anything but; the physicists say the charm meson particle's ability to exist as both itself and its alter-ego could shake up our assumptions about the very nature of reality.

To understand what's going on here, we first have to unpack the meson particle. These are extremely short-lived subatomic particles with a balanced number of quarks and antiquarks. In case you skipped that lecture in quantum physics, quarks are particles that combine together to form "hadrons," some of which are protons and neutronsthe basic components of atomic nuclei.

Via Symmetry Magazine: a joint Fermilab/SLAC publication. Artwork by Sandbox Studio, Chicago.

There are six "flavors" of quark: up, down, charm, strange, top, and bottom. Each also has an antiparticle, called an antiquark. Quarks and antiquarks vary because they have different propertieslike electrical charge of equal magnitude, but opposite sign.

Back to mesons: They're almost the size of neutrons or protons, but are extremely unstable. So, they're uncommon in nature itself, but physicists are interested in studying them in artificial environments (like in the LHC) because they want to better understand quarks. That's because, along with leptons, quarks make up all known matter.

Charm mesons can travel as a mixture of both its particle and antiparticle states (a phenomenon appropriately called "mixing"). Physicists have known that for over a decade, but the new research shows for the first time that charm mesons can actually oscillate back and forth between the two states.

Antiquarks are the opposite of quarks and are considered a type of antimatter. These particles can cancel out normal matterwhich is kind of a problem if you want the universe to, well, exist. The various kinds of antimatter are almost all named using the anti- prefix, like quark versus antiquark. More specifically, a charm meson typically has a charm quark and an up antiquark, and its anti- partner has a charm antiquark and an up quark.

It's important to note the charm meson is not the only particle that can oscillate between matter and antimatter states. Physicists have previously observed three other particles in the Standard Modelthe theory that explains particle physicsdoing so. That includes strange mesons, beauty mesons, and strange-beauty mesons.

Why was the charm meson a holdout for so long? The charm meson oscillates incredibly slowly, meaning physicists had to take measurements at an extremely fine degree of detail. In fact, most charm mesons will fully decay before a complete oscillation can even take place, like an aging person with a very slow-growing tumor.

CERN

The large-scale undertaking that produced the charm meson data is called the Large Hadron Collider beauty experiment. It seeks to examine why we live in a world full of matter, but seemingly no antimatter, according to CERN.

Using a vast amount of data from the charm mesons generated at the LHC, the scientists measured particles to a difference of 1 x 10^-38 grams. With that unbelievably fine-toothed comb, they were able to observe the superposition oscillation of the charm mesons.

How did scientists measure this incredibly tiny difference in mass? In short, the LHC regularly produces mesons of all kinds as part of its scientists' work.

"Charm mesons are produced at the LHC in proton-proton collisions, and normally they only travel a few millimeters before they decay into other particles," according to a University of Oxford press release. "By comparing the charm mesons that tend to travel further versus those that decay sooner, the team identified differences in mass as the main factor that drives whether a charm meson turns into an anti-charm meson or not."

ALFRED PASIEKA/SCIENCE PHOTO LIBRARYGetty Images

Now that scientists have finally observed charm meson oscillation, they're ready to open up a whole new can of worms in their experimentation, hoping to unearth the mysteries of the oscillation process itself.

That path of study could lead to a new understanding about how our world began in the first place. Per the Standard Model of particle physics, the Big Bang should have produced matter and antimatter in equal parts. Thankfully, that didn't happenbecause if it had, all of the antimatter particles would have collided with the matter particles, destroying everything.

Clearly, physicists say, there is an imbalance in matter and antimatter collisions in our world, and the answer to that mystery could lie in the incomprehensibly small oscillations of particles like the charm meson. Now, scientists want to understand if the rate of transition from particle to antiparticle is the same as the rate of transition from antiparticle to particle.

Depending on what they find, our very conceptions of how we existwhy we live in a world full of matter rather than antimatter, and how we got herecould change forever.

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Charm Meson Particle | What Is Antimatter? - Popular Mechanics

Jul 31st 2021

AS HE LIKED to tell it, there were three epiphanies in Steven Weinbergs life. The first came in a wooden box. It was a chemistry set, passed on by a cousin who was tired of it. As he played with the chemicals in it, and found that each reacted differently because of atoms, a vast thought struck him: if he learned about atoms, he would know how the whole world worked.

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The second epiphany came when, as a teenager, he paid a routine visit to his local library in New York. On the table was a book called Heat, open to a page of equations. Among them was the elegant, unknown swirl of an integral sign. It showed that with a mathematical formula, and a magic symbol, science could express something as rudimentary as the glow of a candle flame. His third awakening, when he was in his 20s and already a professor of physics, was the discovery that a mathematical theory could be applied to the whole dazzling array of stars and planets, dark space beyond them and, he concluded, everything.

All regularities in nature followed from a few simple laws. Not all were known yet; but they would be. In the end he was sure they would combine into a set of equations simple enough to put on a T-shirt, like Einsteins E=mc2. It was just a matter of continually querying and searching. In the strange circumstance of finding himself conscious and intelligent on a rare patch of ordinary matter that was able to sustain life, doggedly asking questions was the least he could do.

His signal achievement was to discover, in the 1960s, a new level of simplicity in the universe. There were then four known universal forcesgravity and electromagnetism, both of which operate at large scales, and the strong and weak nuclear forces, both of which are appreciable only at small scales. Electromagnetism was explained by a quantum field theory; similar theories for the nuclear forces were eagerly being sought.

In quantum field theories, forces are mediated by particles called bosons; the boson involved in electromagnetism is the photon, the basic particle of light. He and others showed that a theory of the weak force required three bosons: the W+ and the W-, which carried electric charges, and the Z0, which did not. The W particles were at play in the observable universe; they were responsible for some sorts of radioactive decay. The Z was notional until, in 1973, researchers at CERN, Europes great particle-physics lab, observed neutral currents between the particles they were knocking together. These had never been seen before, and could be explained only by the Z. In 1979 the Nobel prize duly followed.

In his understated way, he called his contribution very satisfactory. It was not just that the weak force and the electromagnetic force could be explained by similar tools. At high energies they were basically the same thing.

That triumph of unification increased his curiosity about the only point where such high energies were known to have existed: the Big Bang. In his book The First Three Minutes, in 1977, he described the immediate aftermath, to the point where the hyper-hot cosmic soup had cooled enough for atomic nuclei to form. He saw early on how deeply particle physics and cosmology were intertwined, and became fascinated by the idea of a universe dominated by unobservable dark energy and dark matter in which ordinary matter (the stars and the planets and us) was merely a small contamination. He longed for CERN s Large Hadron Collider to find evidence of dark matter. It caused him lasting frustration that Congress in 1993 had cancelled the Superconducting Super Collider, which was to have been even bigger.

Whatever was found, he was sure it would fit into the simple scheme of natures laws. Quantum mechanics, however, troubled him. He worried that its determinism implied that the world was endlessly splitting, generating myriad parallel histories and universes in which the constants in nature would have different values. Goodbye to a unified theory of everything, if that were so.

Such a unified law would have given him satisfaction but, he knew, no comfort. Natures laws were impersonal, cold and devoid of purpose. Certainly there was no God-directed plan. As he wrote at the end of The First Three Minutes, the more the universe seemed comprehensible, the more it seemed pointless. No saying of his became more famous, but the next paragraph softened it: humans gave the universe their own point and purpose by the way they lived, by loving each other and by creating art.

He set the example by marrying Louise, his college sweetheart, devouring opera and theatre, revelling in the quirky liberalism of Austin, where he taught at the University of Texas for almost four decades, and looking for theories in physics that would carry the same sense of inevitability he found so beautiful in chamber music, or in poetry. He still thought of human existence as accidental and tragic, fundamentally. But from his own little island of warmth and love, art and science, he managed a wry smile.

What angered him most was the persistence of religion. It had not only obstructed and undermined science in the age of Galileo and Copernicus; it had also survived Darwin, whose theory of evolution had shocked it more sharply than anything physics did. And it was still there, an alternative theory of the world that corroded free inquiry. For even if the laws of nature could be reduced to one, scientists would still ask: Why? Why this theory, not another? Why in this universe, and not another?

There was, he reflected, no end to the chain of whys. So he did not stop asking or wondering. He liked to review and grade his predecessors, from the ancient Greeks onwards, chastising them for failing to use the data they had, but also sympathising with their lack of machines advanced enough to prove their ideas. The human tragedy was never to understand why things were as they were. Yet, for all that, he could echo Ptolemy: I know that I am mortal and the creature of a day, but when I search out the massed wheeling circles of the stars, my feet no longer touch the EarthI take my fill of ambrosia, the food of the gods.

This article appeared in the Obituary section of the print edition under the headline "Natures laws"

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In search of nature's laws Steven Weinberg died on July 23rd - The Economist

Researchers from the University of Bristols School of Physics used some of Europes strongest continuous magnetic fields to uncover evidence of exotic charge carriers in the metallic state of copper-oxide high-temperature superconductors.

Their results have been published this week in Nature. In a related publication in SciPost Physics last week, the team postulated that it is these exotic charge carriers that form the superconducting pairs, in marked contrast with expectations from conventional theory.

Conventional superconductivity

Superconductivity is a fascinating phenomenon in which, below a so-called critical temperature, a material loses all its resistance to electrical currents. In certain materials, at low temperatures, all electrons are entangled in a single, macroscopic quantum state, meaning that they no longer behave as individual particles but as a collective resulting in superconductivity. The general theory for this collective electron behaviour has been known for a long time, but one family of materials, the cuprates, refuses to conform to the paradigm. They also possess the highest ambient-pressure superconducting transition temperatures known to exist. It was long thought that for these materials the mechanism that glues together the electrons must be special, but recently the attention has shifted and now physicists investigate the non-superconducting states of cuprates, hoping to find clues to the origin of high-temperature superconductivity and its distinction from normal superconductors.

High-temperature superconductivity

Most superconductors, when heated to exceed their critical temperature, change into ordinary metals. The quantum entanglement that causes the collective behaviour of the electrons fades away, and the electrons start to behave like an ordinary gas of charged particles.

Cuprates are special, however. Firstly, as mentioned above, because their critical temperature is considerably higher than that of other superconductors. Secondly, they have very special measurable properties even in their metallic phase. In 2009, physicist Prof Nigel Hussey and collaborators observed experimentally that the electrons in these materials form a new type of structure, different from that in ordinary metals, thereby establishing a new paradigm that scientists now call the strange metal. Specifically, the resistivity at low temperatures was found to be proportional to temperature, not at a singular point in the temperature versus doping phase diagram (as expected for a metal close to a magnetic quantum critical point) but over an extended range of doping. This extended criticality became a defining feature of the strange metal phase from which superconductivity emerges in the cuprates.

Magnetoresistance in a strange metal

In the first of these new reports, EPSRC Doctoral Prize Fellow Jakes Ayres and PhD student Maarten Berben (based at HFML-FELIX in Nijmegen, the Netherlands) studied the magnetoresistance the change in resistivity in a magnetic field and discovered something unexpected. In contrast to the response of usual metals, the magnetoresistance was found to follow a peculiar response in which magnetic field and temperature appear in quadrature. Such behaviour had only been observed previously at a singular quantum critical point, but here, as with the zero-field resistivity, the quadrature form of the magnetoresistance was observed over an extended range of doping. Moreover, the strength of the magnetoresistance was found to be two orders of magnitude larger than expected from conventional orbital motion and insensitive to the level of disorder in the material as well as to the direction of the magnetic field relative to the electrical current. These features in the data, coupled with the quadrature scaling, implied that the origin of this unusual magnetoresistance was not the coherent orbital motion of conventional metallic carriers, but rather a non-orbital, incoherent motion from a different type of carrier whose energy was being dissipated at the maximal rate allowed by quantum mechanics.

From maximal to minimal dissipation

Prof Hussey said: Taking into account earlier Hall effect measurements, we had compelling evidence for two distinct carrier types in cuprates - one conventional, the other strange. The key question then was which type was responsible for high-temperature superconductivity? Our team led by Matija ulo and Caitlin Duffy then compared the evolution of the density of conventional carriers in the normal state and the pair density in the superconducting state and came to a fascinating conclusion; that the superconducting state in cuprates is in fact composed of those exotic carriers that undergo such maximal dissipation in the metallic state. This is a far cry from the original theory of superconductivity and suggests that an entirely new paradigm is needed, one in which the strange metal takes centre stage.

Paper:

'Incoherent transport across the strange-metal regime of overdoped cuprates' in Nature by Nigel Hussey et al.

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July: Superconductivity in cuprates | News and features - University of Bristol

From the earliest batteries through vacuum tubes, solid state, and integrated circuits, electronics has staved off stagnation. Engineers and scientists have remade it repeatedly, vaulting it over one hurdle after another to keep alive a record of innovation unmatched in industrial history.

It is a spectacular and diverse account through which runs a common theme. When a galvanic pile twitches a frog's leg, when a triode amplifies a signal, or when a microprocessor stores a bit in a random access memory, the same agent is at work: the movement of electric charge. Engineers are far from exhausting the possibilities of this magnificent mechanism. But even if a dead end is not yet visible, the foreseeable hurdles are high enough to set some searching for the physics that will carry electronics on to its next stage. In so doing, it could help up the ante in the semiconductor stakes, ushering in such marvels as nonvolatile memories with enormous capacity, ultrafast logic devices that can change function on the fly, and maybe even processors powerful enough to begin to rival biological brains.v A growing band of experimenters think they have seen the future of electronics, and it is spin. This fundamental yet elusive property of electrons and other subatomic particles underlies permanent magnetism, and is often regarded as a strange form of nano-world angular momentum.

Microelectronics researchers have been investigating spin for at least 20 years. Indeed, their discoveries revolutionized hard-disk drives, which since 1998 have used a spin-based phenomenon to cram more bits than ever on to their disks. Within three years, Motorola Inc. and IBM Corp. are expected to take the next step, introducing the first commercial semiconductor chips to exploit spin--a new form of random access memory called M (for magnetic) RAM. Fast, rugged, and nonvolatile, MRAMs are expected to carve out a niche from the US $10.6-billion-a-year flash memory market. If engineers can bring the costs down enough, MRAMs may eventually start digging into the $35 billion RAM market as well.

The sultans of spin say memory will be just the beginning. They have set their sights on logic, emboldened by experimental results over the past two or three years that have shown the budding technologies of spin to be surprisingly compatible with the materials and methods of plain old charge-based semiconductor electronics. In February 2000, the Defense Advance Research Projects Agency announced a $15-million-a-year, five-year program to focus on new kinds of semiconductor materials and devices that exploit spin. It was the same Arlington, Va., agency's largesse of $60 million or so over the past five years that helped move MRAMs from the blackboard to the verge of commercial production.

Subatomic spookiness

Now proponents envision an entirely new form of electronics, called spintronics. It would be based on devices that used the spin of electrons to control the movement of charge. Farther down the road (maybe a lot farther), researchers might even succeed in making devices that used spin itself to store and process data, without any need to move charge at all. Spintronics would use much less power than conventional electronics, because the energy needed to change a spin is a minute fraction of what is needed to push charge around.

Other advantages of spintronics include nonvolatility: spins don't change when the power is turned off. And the peculiar nature of spin--and the quantum theory that describes it--points to other weird, wonderful possibilities, such as: logic gates whose function--AND, OR, NOR, and so on--could be changed a billion times a second; electronic devices that would work directly with beams of polarized light as well as voltages; and memory elements that could be in two different states at the same time. "It offers completely different types of functionality" from today's electronics, said David D. Awschalom, who leads the Center for Spintronics and Quantum Computation at the University of California at Santa Barbara. "The most exciting possibilities are the ones we're not thinking about."

Much of the research is still preliminary, Awschalom cautions. A lot of experiments are still performed at cryogenic temperatures. And no one has even managed to demonstrate a useful semiconductor transistor or transistor-like device based on spin, let alone a complex logic circuit. Nevertheless, researchers at dozens of organizations are racing to make spin-based transistors and logic, and encouraging results from groups led by Awschalom and others have given ground for a sense that major breakthroughs are imminent.

"A year and a half ago, when I was giving a talk [and] said something about magnetic logic, before I went on with the rest of my talk I'd preface my statement with, '...and now, let's return to the planet Earth,'" said Samuel D. Bader, a group leader in the materials science division at Argonne National Laboratory, in Illinois. "I can drop that line now," he added.

Quantum mechanical mystery

Spin remains an unplumbed mystery. "It has a reputation of not being really fathomable," said Jeff M. Byers, a leading spin theorist at the Naval Research Laboratory (NRL), in Washington, D.C. "And it's somewhat deserved."

Physicists know that spin is the root cause of magnetism, and that, like charge or mass, it is an intrinsic property of the two great classes of subatomic particles: fermions, such as electrons, protons, and neutrons; and bosons, including photons, pions, and more. What distinguishes them, by the way, is that a boson's spin is measurable as an integer number (0, 1, 2...) of units, whereas fermions have a spin of 1/2, 3/2, 5/2.... units.

Much of spin's elusiveness stems from the fact that it goes right to the heart of quantum theory, the foundation of modern physics. Devised in the early decades of the 20th century, quantum theory is an elaborate conceptual framework, based on the notion that the exchange of energy at the subatomic level is constrained to certain levels, or quantities--in a word, quantized.

Paul Dirac, an electrical engineering graduate of Bristol University, in England, turned Cambridge mathematician, postulated the existence of spin in the late 1920s. In work that won him a Nobel prize, he reconciled equations for energy and momentum from quantum theory with those of Einstein's special theory of relativity.

Spin is hard to grasp because it lacks an exact analog in the macroscopic world we inhabit. It is named after its closest real-world counterpart: the angular momentum of a spinning body. But whereas the ordinary angular momentum of a whirling planet, say, or curve ball vanishes the moment the object stops spinning and hence is extrinsic, spin is a kind of intrinsic angular momentum that a particle cannot gain or lose.

"Imagine an electronics technology founded on such a bizarre property of the universe," said Byers.

Of course, the analogy between angular momentum and spin only goes so far. Particle spin does not arise out of rotation as we know it, nor does the electron have physical dimensions, such as a radius. So the idea of the electron having angular momentum in the classical meaning of the term doesn't make sense. Confused? "Welcome to the club," Byers said, with a laugh.

The smallest magnets

Fortunately, a deep grasp of spin is not necessary to understand the promise of the recent advances. The usual imperfect analogies that somehow manage to render the quantum world meaningful for mortal minds turn out to be rather useful--as is spin's role in magnetism, a macroscopic manifestation of spin.

Start with the fact that spin is the characteristic that makes the electron a tiny magnet, complete with north and south poles. The orientation of the tiny magnet's north-south axis depends on the particle's axis of spin. In the atoms of an ordinary material, some of these spin axes point "up" (with respect to, say, an ambient magnetic field) and an equal number point "down." The particle's spin is associated with a magnetic moment, which may be thought of as the handle that lets a magnetic field torque the electron's axis of spin. Thus in an ordinary material, the up moments cancel the down ones, so no surplus moment piles up that could hold a picture to a refrigerator.

For that, you need a ferromagnetic material, such as iron, nickel, or cobalt. These have tiny regions called domains in which an excess of electrons have spins with axes pointing either up or down--at least, until heat destroys their magnetism, above the metal's Curie temperature. The many domains are ordinarily randomly scattered and evenly divided between majority-up and majority-down. But an externally applied magnetic field will move the walls between the domains and line up all the domains in the direction of the field, so that they point in the same direction. The result is a permanent magnet.

Ferromagnetic materials are central to many spintronics devices. Use a voltage to push a current of electrons through a ferromagnetic material, and it acts like a spin polarizer, aligning the spin axes of the transiting electrons so that they are up or down. One of the most basic and important spintronic devices, the magnetic tunnel junction, is just two layers of ferromagnetic material separated by an extremely thin, nonconductive barrier [see figure, "How a Magnetic Tunnel Junction Works" ]. The device was first demonstrated by the French physicist M. Jullire in the mid-1970s.

ILLUSTRATIONS: STEVE STANKIEWICZ

How a Magnetic Tunnel Junction Works: One of the most fundamental spintronic devices, the magnetic tunnel junction, is just two layers of ferromagnetic material [light blue] separated by a nonmagnetic barrier [darker blue]. In the top illustration, when the spin orientation [white arrows] of the electrons in the two ferromagnetic layers are the same, a voltage is quite likely to pressure the electrons to tunnel through the barrier, resulting in high current flow. But flipping the spins in one of the two layers [yellow arrows, bottom illustration], so that the two layers have oppositely aligned spins, restricts the flow of current.

It works like this: suppose the spins of the electrons in the ferromagnetic layers on either side of the barrier are oriented in the same direction. Then applying a voltage across the three-layer device is quite likely to cause electrons to tunnel through the thin barrier, resulting in high current flow. But flipping the spins in one of the two ferromagnetic layers, so that the two layers have opposite alignment, restricts the flow of current through the barrier [bottom]. Tunnel junctions are the basis of the MRAMs developed by IBM and Motorola, one per memory cell.

Any memory device can also be used to build logic circuits, in theory at least, and spin devices such as tunnel junctions are no exception. The idea has been explored by Mark Johnson, a leading spin researcher at the Naval Research Laboratory, and others. Lately, work in this area has shifted to a newly formed program at Honeywell Inc., Minneapolis, Minn. The challenges to the devices' use for programmable logic are formidable. To quote William Black, principal engineer at the Rocket Chips subsidiary of Xilinx, a leading maker of programmable logic in San Jose, Calif., "The basic device doesn't have gain and the switching threshold typically is not very well controlled." To call that "the biggest technical impediment," as he does, sounds like an understatement.

Relativistic transistors

Already on the drawing board are spin-based devices that would act something like conventional transistors--and that might even produce gain. There are several competing ideas. The most enduring one is known as the spin field-effect transistor (FET). A more recent proposal puts a new spin, so to speak, on an almost mythical device physicists have pursued for decades: the resonant tunneling transistor.

In an ordinary FET, a metal gate controls the flow of current from a source to a drain through the underlying semiconductor. A voltage applied to the gate sets up an electric field, and that field in turn varies the amount of current that can flow between source and drain. More voltage produces more current.

In 1990 Supriyo Datta and Biswajit A. Das, then both at Purdue University, in West Lafayette, Ind., proposed a spin FET in a seminal article published in the journal Applied Physics Letters. The two theorized about an FET in which the source and drain were both ferromagnetic metals, with the same alignment of electron spins. Electrons would be injected into the source, which would align the spins so that their axes were oriented the same way as those in the source and drain. These spin-polarized electrons would shoot through the source and travel at 1 percent or so of the speed of light toward the drain.

This speed is important, because electrons moving at so-called relativistic speeds are subject to certain significant effects. One is that an applied electric field acts as though it were a magnetic field. So a voltage applied to the gate would torque the spin-polarized electrons racing from source to drain and flip their direction of spin. Thus electron spins would become polarized in the opposite direction to the drain, and could not enter it so easily. The current going from the source to the drain would plummet.

Note that the application of the voltage would cut off current, rather than turn it on, as in a conventional FET. Otherwise, the basic operation would be rather similar--but with a couple of advantages. To turn the current on or off would require only the flipping of spins, which takes very little energy. Also, the polarization of the source and drain could be flipped independently, offering intriguing possibilities unlike anything that can be done with a conventional FET. For example, Johnson patented the idea of using an external circuit to flip the polarization of the drain, turning the single-transistor device into a nonvolatile memory cell.

A recent German breakthrough will "revolutionize" a majorspintronics subfield, one expert declared

Alas, 11 years after the paper by Datta and Das, no one has managed to make a working spin FET. Major efforts have been led by top researchers, such as Johnson at the NRL, Michael Flatt at the University of Iowa, Michael L. Roukes at the California Institute of Technology, Hideo Ohno of Tohoku University in Japan, Laurens W. Molenkamp, then at the University of Aachen in Germany, and Anthony Bland at the University of Cambridge in England. The main problem has been maintaining the polarization of the spins: the ferromagnetic source does in fact align the spins of electrons injected into it, but the polarization does not survive as the electrons shoot out of the source and into the semiconductor between the source and drain.

Recent work in Berlin, Germany, may change all that. In a result published last July in Physical Review Letters, Klaus H. Ploog and his colleagues at the Paul Drude Institute disclosed that they had used a film of iron, grown on gallium arsenide, to polarize spins of electrons injected into the GaAs. Not only was the experiment carried out at room temperature, but the efficiency of the injection, at 2 percent, was high in comparison with similar experiments. The work was "extremely important," said the Naval Research Laboratory's Johnson. "It will revolutionize this subfield. A year from now many spin-FET researchers will be working with iron."

The other kind of proposed spin transistor would exploit a quantum phenomenon called resonant tunneling. The device would be an extension of the resonant tunneling diode. At the heart of this device is an infinitesimal region, known as a quantum well, in which electrons can be confined. However, at a specific, resonant voltage that corresponds to the quantum energy of the well, the electrons tend to slip--the technical term is "tunnel"--freely through the barriers enclosing the well.

Generally, the spin state of the electron is irrelevant to the tunneling, because the up and down electrons have the same amount of energy. But by various means, researchers can design a device in which the spin-up and spin-down energy levels are different, so that there are two different tunneling pathways. The two tunnels would be accessed with different voltages; each voltage would correspond to one or the other of the two spin states. At one voltage, a certain level of spin-down current would flow. At some other voltage, a different level of spin-up current would go through the quantum well's barriers.

One way of splitting the energy levels is to make the two barriers of different materials, so that the potential energy that confines the electrons within the quantum well is different on either side of the well. That difference in the confining potentials translates, for a moving electron, into two regions within the quantum well, which have magnetic fields that are different from each other. Those asymmetric fields in turn give rise to the different resonant energy levels for the up and down spin states. A device based on these principles is the goal of a team led by Thomas McGill at the California Institute of Technology, with members at HRL Laboratories LLC, Jet Propulsion Laboratory, Los Alamos National Laboratory, and the University of Iowa.

Another method of splitting the energy levels is to simply put them in a magnetic field. This approach is being taken by a collaborative effort of nine institutions, led by Bruce D. McCombe at the University at Buffalo, New York.

Neither team has managed to build a working device, but the promise of such a device has kept interest high. A specific voltage would produce a certain current of, say, spin-up electrons. Using a tiny current to flip the spins would enable a larger current of spin-down electrons to flow at the same voltage. Thus a small current could, in theory anyway, be amplified.

Ray of hope

As these researchers refine the resonant and ballistic devices, they are looking over their shoulders at colleagues who are forging a whole new class of experimental device. This surging competition is based on devices that create or detect spin-polarized electrons in semiconductors, rather than in ferromagnetic metals. In these experiments, researchers use lasers to get around the difficulties of injecting polarized spin into semiconductors. By shining beams of polarized laser light onto ordinary semiconductors, such as gallium arsenide and zinc selenide, they create pools of SPIN-POLARIZED ELECTRONS.

Some observers lament the dependence on laser beams. They find it hard to imagine how the devices could ever be miniaturized to the extent necessary to compete with conventional electronics, let alone work smoothly with them on the same integrated circuit. Also, in some semiconductors, such as GaAs, the spin polarization persists only at cryogenic temperatures.

In an early experiment, Michael Oestreich, then at Philips University in Marburg, Germany, showed that electric fields could push pools of spin-polarized electrons through nonmagnetic semiconductors such as GaAs. The experiment was reported in the September 1998 Applied Physics Letters.

Then over the past three years, a breathtaking series of findings has turned the field into a thriving subdiscipline. Several key results were achieved in Awschalom's laboratory at Santa Barbara. He and his co-workers demonstrated that pools of spin-coherent electrons could retain their polarization for an unexpectedly long time--hundreds of nanoseconds. Working separately, Awschalom, Oestreich, and others also created pools of spin-polarized electrons and moved them across semiconductor boundaries without the electrons' losing their polarization.

If not for these capabilities, spin would have no future in electronics. Recall that a practical device will be operated by altering its orientation of spin. That means that the spin coherence has to last, at a minimum, longer than it takes to alter the orientation of that spin polarization. Also, spintronic devices, like conventional ones, will be built with multiple layers of semiconductors, so moving spin-polarized pools across junctions between layers without losing the coherence will be essential.

Awschalom and his co-workers used a pulsed, polarized laser to establish pools of spin-coherent electrons. The underlying physics revolves around the so-called selection rules. These are quantum-theoretical laws describing whether or not an electron can change energy levels by absorbing or emitting a photon of light. According to those selection rules, light that is circularly polarized will excite only electrons of one spin orientation or the other. Conversely, when spin-coherent electrons combine with holes, the result is photons of circularly polarized light.

Puzzling precession

In his most recent work, Awschalom and his graduate student, Irina Malajovich, collaborated with Nitin Samarth of Pennsylvania State University in University Park and his graduate student, Joseph Berry. As he has in the past, Awschalom performed the experiment on pools of electrons that were not only spin polarized but were also precessing. Precession occurs when a pool of spin-polarized electrons is put in a magnetic field: the field causes their spin axes to rotate in a wobbly way around that field. The frequency and direction of rotation depend on the strength of the magnetic field and on characteristics of the material in which the precession is taking place.

The Santa BarbaraPenn State team used circularly polarized light pulses to create a pool of spin-coherent electrons in GaAs. They applied a magnetic field to make the electrons precess, and then used a voltage to drag the precessing electrons across a junction into another semiconductor, ZnSe. The researchers found that if they used a low voltage to drag the electrons into the ZnSe, the electrons took on the precession characteristics of the ZnSe as soon as they got past the junction. However, if they used a higher voltage, the electrons kept on precessing, as though they were still in the GaAs [see illustration, "Precessional Mystery" ].

ILLUSTRATIONS: STEVE STANKIEWICZ

Precessional Mystery: Given the right circumstances, electrons will synchronously "precess," or whirl about an axis that is itself moving. The angle and rate of this wobbly spin depend in part on the material in which it occurs. Thus, if a voltage pushes an electron out of gallium arsenide [light blue] into zinc selenide [yellow], the electron's precession characteristics change [top]. However, if a higher voltage pushes the electron sharply enough into the ZnSe, the precession characteristics do not change but remain those of GaAs for a while [bottom]. Some researchers believe they will be able to exploit this variability in future devices.

"You can tune the whole behavior of the current, depending on the electric field," Awschalom said in an interview. "That's what was so surprising to us." The group reported its results in the 14 June issue of Nature, prompting theorists around the world to wear out their pencils trying to explain the findings.

Other results from the collaboration were even more intriguing. The Santa Barbara and Penn State researchers performed a similar experiment, except with p-type GaAs and n-type ZnSe. N-type materials rely on electrons to carry current; p-type, on holes. Because the materials were of two different charge-carrier types, an electric field formed around their junction. That field, the experimenters found, was strong enough to pull a pool of spin-coherent electrons from the GaAs immediately into the ZnSe, where the coherence persisted for hundreds of nanoseconds.

The result was encouraging for two reasons. As Awschalom put it, "It showed that you can build n-type and p-type materials and spin can get through the interfaces between them just fine." Equally important, it demonstrated that the spin can be moved from one kind of semiconductor into another without the need for external electric fields, which wouldn't be practical in a commercial device.

"The next big opportunity is to make a spin transistor," Awschalom added. "These results show, in principle, that there is no obvious reason why it won't work well."

Such a device is at least several years away. But even if researchers were on the verge of getting a spin transistor to work in the laboratory, more breakthroughs would be necessary before the device could be practical. For example, the fact that the device would need pulses of circularly polarized laser light would seem an inconvenience, although Awschalom sees a bright side. The gist is that the photons would be used for communications among chips, the magnetic elements for memory, and the spin-based devices for fast, low-power logic.

It's far-fetched now--but no more so than the idea of 1GB DRAMs would have seemed in the days when triodes ruled.

Hot off the presses is Semiconductor Spintronics and Quantum Computation, edited by David D. Awschalom, Nitin Samarth, and Daniel Loss. The 250-page book was released last October by Springer Verlag, Berlin/Heidelberg; ISBN: 3540421769.

The November/December issue of American Scientist, published by the scientific research society Sigma Xi, included an eight-page overview titled "Spintronics" by Sankar Das Sarma. See Vol. 89, pp. 516523.

Honeywell Inc.'s Romney R. Katti and Theodore Zhu described the company's magnetic RAM technology in "Attractive Magnetic Memories," IEEE Circuits & Devices, Vol. 17, March 2001, pp. 2634.

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The Quest for the Spin Transistor - IEEE Spectrum

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