Category Archives: Quantum Physics

The year is 1641. We open in France, where confusingly, everyone is speaking English. A Scottish man has been caught selling weapons to enemies of Louis XIII, and as punishment is forced to wear a red-hot iron mask forever. Cut to the not too distant future, where the mans descendant, Christopher Eccleston, is presenting a lecture about newly weaponised flying metal bugs to some Nato employees. Originally developed to isolate and kill cancer cells, at MARS industries we discovered how to program nanomites to do almost anything. For example eat metal. It turns out nanomites can also be injected into rocket warheads, and thus the back story and premise of GI Joe: The Rise of Cobra is explained in less than a minute.

The opening sets the tone for the film that follows speedy, irony-free B-movie action nonsense, delivered to you with the efficiency of a Big Mac on a Friday night And if it requires Christopher Eccleston to do a PowerPoint presentation so we can get on with watching helicopters blow up in slow motion, then dammit Christopher Eccleston will do a PowerPoint. On top of which, this particular Big Mac is filled with Channing Tatum.

Despite his previous acting highlights including the Step Up dance movies and grinding topless in the background of the video for Ricky Martins She Bangs, when asked about GI Joe in an interview in 2012, Channing Tatum said, I fucking hate that movie. Luckily for us, in 2009 Channing Tatum did a three-movie deal with Paramount and was forced to accept the GI Joe role to avoid being sued.

Despite his dislike of the film, Channing Tatum is still Channing Tatum and both he and his massive arms give it their all and he has gone to the Michael Bay School of Turning Around in Slow Motion While Holding a Machine Gun. After turning around slowly, he and his partner Marlon Wayans load some nanomite warheads into a jeep, refer to a group of muscular male soldiers as ladies and tell them to mount up. Strap in, everyone.

What follows is a plot of such madness and a cast of characters so enormous (IMDb lists 144 in total) its understandable that it required a PowerPoint to set it up. The truck is ambushed by Channing Tatums ex-girlfriend, Sienna Miller, and after a lengthy fight in which several members of elite army unit GI Joe parachute in to save the day, Tatum and Wayans are transported to an underground base in the Egyptian desert to participate in a training montage soundtracked by the UK band Bus Stops dance rap cover of T-Rexs Get It On. (Fun fact: Bus Stop were fronted by rapper and professional football manager Darren Daz Sampson, who went on to represent Britain in 2006s Eurovision Song Contest.) Channing Tatum wins a gladiatorial pugil stick fight with GI Joes resident masked ninja, Snake Eyes, and to celebrate the boys all take their tops off.

A semi-naked Marlon Wayans attempts to charm one of the Joes (they are collectively referred to as Joes) confusingly named Scarlett OHara, as she jogs on a treadmill while reading a book about quantum physics. (It is not clear why she needs to read a book about quantum physics when her job is beating people up dont worry about it.) Tatum puts on something called a Delta 6 accelerator suit and travels to Paris to stop Sienna Miller blowing up the Eiffel Tower, before charging around the Champs-lyses running after tanks, jumping through bus windows and flipping over Renault Mganes. Joseph Gordon Levitt appears to explain cobras to everyone using a CGI snake in a glass box (They are vicious). Chaos reigns.

Writer/director Stephen Sommers was also in charge of both The Mummy and the 90s B-movie classic Deep Rising, and although in comparison GI Joe contains a more noughties post-Transformers fixation on guns and machinery than those two films, there is a similar air of fun, unapologetic action campness throughout. If youre happy to suspend your disbelief to its very limits and relax into 1 hour and 58 minutes of revolving door cast, plot delivered via flashbacks and laughably hammy dialogue, plus Channing Tatum blowing things up in slow motion this is the film for you. And give me that kind of Big Mac silliness over po-faced serious blockbuster action, any day of the week.

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Hear me out: why GI Joe: The Rise of Cobra isnt a bad movie - The Guardian

July 26, 2021• Physics 14, 105

Nanophotonic devices based on tin-vacancy qubits in diamond show promise as building blocks of quantum repeaters, an important step toward the realization of long-range quantum networks.

Building long-range quantum networks is one of the most important and ambitious goals of quantum science and engineering. To connect these networks into a quantum internet will require intermediate stations where quantum informationcarried by photonscan be manipulated and refreshed. These stations, known as quantum repeaters, contain long-lived qubits with an optical interface that allows the photons to be entangled with the spins encoding these qubits. Color centers in solids are prime candidates for quantum repeaters, as they can have long coherence times, spin-selective optical transitions, and compatibility with photonic devices, such as cavities, that facilitate photon emission and routing. The tin-vacancy (SnV) center in diamond, a relatively new and promising color center [1], features several of these key elements. Until recently, however, these SnV centers had not been integrated into cavities in which their emission would be enhanced. Now, a team led by Jelena Vukovi at Stanford University has succeeded in integrating an SnV center into a nanophotonic device and has achieved 90% photon emission into the desired cavity mode [2]. This work is an important step toward the realization of long-range quantum networks.

A color center forms in a crystalline solid when one or more lattice atoms are missing or substituted by another species. Such complexes can absorb and emit light and, if they have a ground state with nonzero spin, can be used as a qubit. Intense research is dedicated to exploring color centers in diamond as building blocks of quantum networks [3]. The most studied diamond color center for quantum technology applications is the nitrogen vacancy (NV). With its remarkably long coherence time and spin-selective transition, the NV center has played a central role in the development of quantum networks, including the recent seminal demonstration of a three-node network with entanglement swapping capabilities [4]. Despite its successes, though, the NV suffers from a few shortcomings that limit its suitability for quantum networks. One issue is vibrational noise, which causes the majority of photons to be emitted incoherently, thereby reducing the success probability of spin-photon entanglement schemes. The other issue is the permanent electric dipole of this color center, leading to sensitivity to nearby charges. Such charge noise, which can destroy information stored in a qubit, is exacerbated when the NV is near surfaces, as in nanophotonic devices.

These limitations have led researchers to consider alternative qubits. A prominent family of diamond color centers currently under intense investigation consists of complexes made up of two carbon vacancies between which is a group-IV atom, an atom with four valence electrons. The inversion symmetry of these systems leads to a vanishing permanent electric dipole moment, making them suitable for integration in nanophotonic structures. Moreover, most of the emitted photons have frequencies in the desirable zero-phonon line, where there is no vibrational noise. One of the most studied group-IV centers in diamond is the silicon-vacancy (SiV) center, which has been integrated into nanophotonic devices and used in the first demonstration of memory-enhanced quantum communication [5]. However, the operation of an SiV center requires complex and expensive cryogenic technology based on dilution refrigeration to reach temperatures at which the coherence times are sufficiently long for applications [6].

The SnV center [7] has emerged as a potential solution. It has a number of desirable attributes: It obeys inversion symmetry, making it insensitive to charge noise; it emits photons primarily through the zero-phonon line; it exhibits longer coherence times than SiV; and it can be operated at a few kelvin, a temperature that can be reached with simpler technology than dilution refrigerators. These features make SnV particularly promising for use as a quantum network node. While optical initialization of SnV spins has been demonstrated [7], this color center had notuntil nowbeen integrated into nanophotonic structures, as required for applications.

For their demonstration, the Stanford team fabricated high-quality nanophotonic cavities from a diamond plate [2]. They first implanted tin atoms within the diamond. Typically, this integration of heavy impurity atoms into nanophotonic devices comes at the price of damaging the diamond surface and degrading the emitter quality. The authors overcame this problem using their novel color-center generation method that ensures precise placement of Sn impurity atoms below a high-quality diamond substrate [8]. The team then constructed photonic crystal cavities into the diamond plate. Each long, thin cavity had an array of holes etched along it. The cavities were also partly suspended in the air to make the light confinement stronger.

The researchers succeeded in efficiently coupling light in and out of the cavities via inverse-designed couplers that they developed in earlier work [9]. They could tune the cavity wavelength by depositing condensed argon on the device. When the wavelength of the cavity matched that of the optical transition of the color centers, the team observed a 40-fold enhancement of emission intensity compared with the off-resonant case. Furthermore, while the photons retained their sharp linewidths, the spontaneous emission rate was considerably enhanced. This narrow emission meant that photons have nearly 100% probability of being emitted into a single cavity mode. These elements are necessary for establishing high enough entanglement rates in quantum repeaters.

These new results, together with the recent demonstration of coherent optical control of the SnV spin state [10], are key steps toward the creation of SnV-based networks. The remaining critical step toward this goal is the creation of a high-fidelity quantum memory register composed of carbon-13 atomsthe only isotope of carbon with nonzero spinin the diamond lattice that can store quantum information transferred to and from the SnV center. Upon such a demonstration, the SnV center would be in a position to outshine its diamond-based predecessors.

Evangelia Takou is a Ph.D. candidate in the Department of Physics at Virginia Tech. Prior to joining Virginia Tech, she obtained a masters degree in condensed-matter physics and atomic physics in 2019 and an undergraduate degree in physics in 2018 at the University of Crete in Greece. In the second year of her Ph.D. studies, she was awarded the Ray F. Tipsword graduate scholarship. She was also selected to receive a competitive graduate school doctoral assistantship from the College of Science at Virginia Tech. Currently, she is working on theoretical protocols for the control of qubits based on color centers.

Sophia Economou is a professor of physics at Virginia Tech. She obtained her Ph.D. in 2006 from the University of California, San Diego, where she worked on theoretical aspects of optically controlled spin qubits. After that, she spent several years at the Naval Research Laboratory in Washington, D.C., first as an NRC postdoc and later as a staff researcher. Her current research interests include quantum computing with various types of qubits, including spin-based, superconducting, and photons. She is also interested in photonic entanglement in quantum networks as well as quantum simulation algorithms for solving many-body problems on quantum devices.

Alison E. Rugar, Shahriar Aghaeimeibodi, Daniel Riedel, Constantin Dory, Haiyu Lu, Patrick J. McQuade, Zhi-Xun Shen, Nicholas A. Melosh, and Jelena Vukovi

Phys. Rev. X 11, 031021 (2021)

Published July 26, 2021

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Tin Qubits Give Diamond a New Shine - Physics

- Organized by IIIT Hyderabad, Quantum Ecosystems Technology Council of India, IEEE Quantum Initiative, in association with PSA, Govt of India - Launchpad for Quantum Ecosystems and Technology Council of India ( QETCI) HYDERABAD, India, July 26, 2021 /PRNewswire/ -- The National Quantum Science and Technology Symposium (NQSTS), organized by IIIT Hyderabad, Quantum Ecosystems Technology Council of India, IEEE Quantum Initiative, in association with PSA, Govt of India is being held online from 26 July - 3 August 2021.

Through talks delivered by some of India's best quantum experts from government, academia and industry, the symposium will cover diverse aspects of the field and provide an overview of the scope and impact of quantum computing in India.

NQSTS launched the Quantum Ecosystems and Technology Council of India (QETCI), headed by Reena Dayal,which will work closely with various members of quantum ecosystems across government, academia, industry, startups and investors to accelerate the quantum ecosystem in India.

The symposium features several eminent keynote speakers - Prof Vijay Raghavan, PSA to Govt of India; Prof K Sivan, Chairman ISRO; Ajay Prakash Sawhney, Secretary MEITY; Dr KR MuraliMohan, Mission Director NM-ICPS, DST; Jayesh Ranjan, Principal Secretary, IC&T Govt of Telangana and Prof P J Narayanan, Director IIIT Hyderabad. It also includes several keynote speakers from Microsoft, Amazon, IBM, QNU Labs, TCS and IQM.

Speaking at the inauguration, Prof. P J Narayanan, Director, IIITH, said, 'Quantum computing is a truly futuristic area with huge potential that we must invest in today to reap benefits in the near future. IIIT Hyderabad had recognized the importance of this area and started research in related areas about 10 years. Our group today has 6 faculty members with mix of expertise in Physics, Mathematics, Computer Science, etc. We have a productive group consisting of Masters, PhD, as well as B.Tech students conducting research on different aspects of quantum computing, producing papers in journals and conferences, etc. To give greater push to this area, we have formed a Centre for Quantum Science & Technology (CQST) and hope to develop it into a major national and international player in the area of Quantum Information Sciences, from creating quantum computers to developing quantum computing models to quantum algorithms and their applications to various areas like Healthcare, Sciences, Machine Learning, etc. It is only natural that IIIT Hyderabad stay ahead in these areas for years to come, just as we have done in other areas of computing and communications.' Prof K Sivan, Chairman ISRO, said, 'India will be quantum-enabled in this decade. We will infuse the encryption of the satellite data with the power of quantum mechanics.' Commending the symposium, Ajay Prakash Sawhney, Secretary MEITY, said, 'Congratulations to the Quantum Ecosystems and Technology Council of India and IIIT Hyderabad for working towards bringing together the quantum ecosystem through this symposium. This is the right time for international corporates to establish their quantum presence in India, and for Indian companies to have dedicated teams on quantum technologies to take up the challenges that abound in this field.' The symposium organising committee was led by Prof Indranil Chakravarty. More details on the symposium at https://nqsts.com About IIIT-Hyderabad The International Institute of Information Technology, Hyderabad (IIITH) is an autonomous research university founded in 1998 that focuses on the core areas of Information Technology, such as Computer Science, Electronics and Communications, and their applications in other domains through inter-disciplinary research that has a greater social impact. Some of its research domains include Visual Information Technologies, Human Language Technologies, Data Engineering, VLSI and Embedded Systems, Computer Architecture, Wireless Communications, Algorithms and Information Security, Robotics, Building Science, Cognitive Science, Earthquake Engineering, Computational Natural Sciences and Bioinformatics, Education Technologies, Power Systems, IT in Agriculture and e-Governance.

Website: https://www.iiit.ac.in/ Logo: https://mma.prnewswire.com/media/600789/IIIT_Hyderabad_Logo.jpg PWR PWR

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First-of-Its-Kind National Quantum Science And Technology Symposium - Yahoo India News

Can you guess the size of the thinnest magnet? It is just one atom thick.

Scientists from the University of California Berkeley have created the first 2D magnet, just an atom thick. This ultra-thin magnet is also chemically stable and retains magnetism at room temperatures. The research is published inNature Communications.

The new magnet can revolutionize the research of ferromagnetism and the development of new types of memory devices. It could be a game-changer for the field of quantum physics.

Previous ultra-thin 2D magnets had to be kept at ultracold conditions to retain the chemical properties and magnetism, making it impossible to use them in practical application.

According to material scientist Jie Yao from the University of California, State-of-the-art 2D magnets need very low temperatures to function. But for practical reasons, a data center needs to run at room temperature. Our 2D magnet is not only the first that operates at room temperature or higher, but it is also the first magnet to reach the true 2D limit: Its as thin as a single atom!

Scientists made this state-of-the-art magnet using cobalt-doped van der Waals zinc oxide. A carefully measured ratio of Graphene oxide is mixed in acetate dihydrates of zinc and cobalt. When this mixture is baked in a vacuum, the mixture cools into a single layer of zinc oxide interspersed with cobalt atoms sandwiched between layers of graphene. The graphene layer is burned off by burning in the air, leaving a single layer of cobalt-doped zinc oxide.

The amount of cobalt scattered among the zinc oxide determines the strength of magnetism. A sweet spot of 12 percentage of cobalt makes the layer strongly magnetic. The material also was found to be stable even at temperatures around 212 degrees Fahrenheit.

Electrons are small magnets with North and South poles. They have their own tiny magnetic field, and their magnetic orientations cancel each other out in most materials. However, in ferromagnetic materials, electrons with the same magnetic orientation group themselves in domains. All the domains are oriented in the same direction in a magnetic material.

According to the researchers, the free electrons in the zinc oxide could be acting as intermediaries to keep the film magnetic even at high temperatures.

This material opens up new possibilities in various technological fields include memory devices and quantum computing. Further analysis is required to understand the limitations of this material.

Source: ScienceAlert

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Physicists have created the Worlds thinnest magnet Just one atom thick - News Landed

"I cannot define the real problem, therefore I suspect there's no real problem, but I'm not sure there's no real problem."

The American physicist Richard Feynman said this about the notorious puzzles and paradoxes of quantum mechanics, the theory physicists use to describe the tiniest objects in the Universe. But he might as well have been talking about the equally knotty problem of consciousness.

Some scientists think we already understand what consciousness is, or that it is a mere illusion. But many others feel we have not grasped where consciousness comes from at all.

The perennial puzzle of consciousness has even led some researchers to invoke quantum physics to explain it. That notion has always been met with skepticism, which is not surprising: it does not sound wise to explain one mystery with another. But such ideas are not obviously absurd, and neither are they arbitrary.

For one thing, the mind seemed, to the great discomfort of physicists, to force its way into early quantum theory. What's more, quantum computers are predicted to be capable of accomplishing things ordinary computers cannot, which reminds us of how our brains can achieve things that are still beyond artificial intelligence. "Quantum consciousness" is widely derided as mystical woo, but it just will not go away.

Quantum mechanics is the best theory we have for describing the world at the nuts-and-bolts level of atoms and subatomic particles. Perhaps the most renowned of its mysteries is the fact that the outcome of a quantum experiment can change depending on whether or not we choose to measure some property of the particles involved.

When this "observer effect" was first noticed by the early pioneers of quantum theory, they were deeply troubled. It seemed to undermine the basic assumption behind all science: that there is an objective world out there, irrespective of us. If the way the world behaves depends on how or if we look at it, what can "reality" really mean?

The most famous intrusion of the mind into quantum mechanics comes in the "double-slit experiment"

Some of those researchers felt forced to conclude that objectivity was an illusion, and that consciousness has to be allowed an active role in quantum theory. To others, that did not make sense. Surely, Albert Einstein once complained, the Moon does not exist only when we look at it!

Today some physicists suspect that, whether or not consciousness influences quantum mechanics, it might in fact arise because of it. They think that quantum theory might be needed to fully understand how the brain works.

Might it be that, just as quantum objects can apparently be in two places at once, so a quantum brain can hold onto two mutually-exclusive ideas at the same time?

These ideas are speculative, and it may turn out that quantum physics has no fundamental role either for or in the workings of the mind. But if nothing else, these possibilities show just how strangely quantum theory forces us to think.

The most famous intrusion of the mind into quantum mechanics comes in the "double-slit experiment". Imagine shining a beam of light at a screen that contains two closely-spaced parallel slits. Some of the light passes through the slits, whereupon it strikes another screen.

Light can be thought of as a kind of wave, and when waves emerge from two slits like this they can interfere with each other. If their peaks coincide, they reinforce each other, whereas if a peak and a trough coincide, they cancel out. This wave interference is called diffraction, and it produces a series of alternating bright and dark stripes on the back screen, where the light waves are either reinforced or cancelled out.

The implication seems to be that each particle passes simultaneously through both slits

This experiment was understood to be a characteristic of wave behaviour over 200 years ago, well before quantum theory existed.

The double slit experiment can also be performed with quantum particles like electrons; tiny charged particles that are components of atoms. In a counter-intuitive twist, these particles can behave like waves. That means they can undergo diffraction when a stream of them passes through the two slits, producing an interference pattern.

Now suppose that the quantum particles are sent through the slits one by one, and their arrival at the screen is likewise seen one by one. Now there is apparently nothing for each particle to interfere with along its route yet nevertheless the pattern of particle impacts that builds up over time reveals interference bands.

The implication seems to be that each particle passes simultaneously through both slits and interferes with itself. This combination of "both paths at once" is known as a superposition state.

But here is the really odd thing.

If we place a detector inside or just behind one slit, we can find out whether any given particle goes through it or not. In that case, however, the interference vanishes. Simply by observing a particle's path even if that observation should not disturb the particle's motion we change the outcome.

The physicist Pascual Jordan, who worked with quantum guru Niels Bohr in Copenhagen in the 1920s, put it like this: "observations not only disturb what has to be measured, they produce it We compel [a quantum particle] to assume a definite position." In other words, Jordan said, "we ourselves produce the results of measurements."

If that is so, objective reality seems to go out of the window.

And it gets even stranger.

If nature seems to be changing its behaviour depending on whether we "look" or not, we could try to trick it into showing its hand. To do so, we could measure which path a particle took through the double slits, but only after it has passed through them. By then, it ought to have "decided" whether to take one path or both.

The sheer act of noticing, rather than any physical disturbance caused by measuring, can cause the collapse

An experiment for doing this was proposed in the 1970s by the American physicist John Wheeler, and this "delayed choice" experiment was performed in the following decade. It uses clever techniques to make measurements on the paths of quantum particles (generally, particles of light, called photons) after they should have chosen whether to take one path or a superposition of two.

It turns out that, just as Bohr confidently predicted, it makes no difference whether we delay the measurement or not. As long as we measure the photon's path before its arrival at a detector is finally registered, we lose all interference.

It is as if nature "knows" not just if we are looking, but if we are planning to look.

Whenever, in these experiments, we discover the path of a quantum particle, its cloud of possible routes "collapses" into a single well-defined state. What's more, the delayed-choice experiment implies that the sheer act of noticing, rather than any physical disturbance caused by measuring, can cause the collapse. But does this mean that true collapse has only happened when the result of a measurement impinges on our consciousness?

It is hard to avoid the implication that consciousness and quantum mechanics are somehow linked

That possibility was admitted in the 1930s by the Hungarian physicist Eugene Wigner. "It follows that the quantum description of objects is influenced by impressions entering my consciousness," he wrote. "Solipsism may be logically consistent with present quantum mechanics."

Wheeler even entertained the thought that the presence of living beings, which are capable of "noticing", has transformed what was previously a multitude of possible quantum pasts into one concrete history. In this sense, Wheeler said, we become participants in the evolution of the Universe since its very beginning. In his words, we live in a "participatory universe."

To this day, physicists do not agree on the best way to interpret these quantum experiments, and to some extent what you make of them is (at the moment) up to you. But one way or another, it is hard to avoid the implication that consciousness and quantum mechanics are somehow linked.

Beginning in the 1980s, the British physicist Roger Penrose suggested that the link might work in the other direction. Whether or not consciousness can affect quantum mechanics, he said, perhaps quantum mechanics is involved in consciousness.

What if, Penrose asked, there are molecular structures in our brains that are able to alter their state in response to a single quantum event. Could not these structures then adopt a superposition state, just like the particles in the double slit experiment? And might those quantum superpositions then show up in the ways neurons are triggered to communicate via electrical signals?

Maybe, says Penrose, our ability to sustain seemingly incompatible mental states is no quirk of perception, but a real quantum effect.

Perhaps quantum mechanics is involved in consciousness

After all, the human brain seems able to handle cognitive processes that still far exceed the capabilities of digital computers. Perhaps we can even carry out computational tasks that are impossible on ordinary computers, which use classical digital logic.

Penrose first proposed that quantum effects feature in human cognition in his 1989 book The Emperor's New Mind. The idea is called Orch-OR, which is short for "orchestrated objective reduction". The phrase "objective reduction" means that, as Penrose believes, the collapse of quantum interference and superposition is a real, physical process, like the bursting of a bubble.

Orch-OR draws on Penrose's suggestion that gravity is responsible for the fact that everyday objects, such as chairs and planets, do not display quantum effects. Penrose believes that quantum superpositions become impossible for objects much larger than atoms, because their gravitational effects would then force two incompatible versions of space-time to coexist.

Penrose developed this idea further with American physician Stuart Hameroff. In his 1994 book Shadows of the Mind, he suggested that the structures involved in this quantum cognition might be protein strands called microtubules. These are found in most of our cells, including the neurons in our brains. Penrose and Hameroff argue that vibrations of microtubules can adopt a quantum superposition.

But there is no evidence that such a thing is remotely feasible.

It has been suggested that the idea of quantum superpositions in microtubules is supported by experiments described in 2013, but in fact those studies made no mention of quantum effects.

Besides, most researchers think that the Orch-OR idea was ruled out by a study published in 2000. Physicist Max Tegmark calculated that quantum superpositions of the molecules involved in neural signaling could not survive for even a fraction of the time needed for such a signal to get anywhere.

Other researchers have found evidence for quantum effects in living beings

Quantum effects such as superposition are easily destroyed, because of a process called decoherence. This is caused by the interactions of a quantum object with its surrounding environment, through which the "quantumness" leaks away.

Decoherence is expected to be extremely rapid in warm and wet environments like living cells.

Nerve signals are electrical pulses, caused by the passage of electrically-charged atoms across the walls of nerve cells. If one of these atoms was in a superposition and then collided with a neuron, Tegmark showed that the superposition should decay in less than one billion billionth of a second. It takes at least ten thousand trillion times as long for a neuron to discharge a signal.

As a result, ideas about quantum effects in the brain are viewed with great skepticism.

However, Penrose is unmoved by those arguments and stands by the Orch-OR hypothesis. And despite Tegmark's prediction of ultra-fast decoherence in cells, other researchers have found evidence for quantum effects in living beings. Some argue that quantum mechanics is harnessed by migratory birds that use magnetic navigation, and by green plants when they use sunlight to make sugars in photosynthesis.

Besides, the idea that the brain might employ quantum tricks shows no sign of going away. For there is now another, quite different argument for it.

In a study published in 2015, physicist Matthew Fisher of the University of California at Santa Barbara argued that the brain might contain molecules capable of sustaining more robust quantum superpositions. Specifically, he thinks that the nuclei of phosphorus atoms may have this ability.

Phosphorus atoms are everywhere in living cells. They often take the form of phosphate ions, in which one phosphorus atom joins up with four oxygen atoms.

Such ions are the basic unit of energy within cells. Much of the cell's energy is stored in molecules called ATP, which contain a string of three phosphate groups joined to an organic molecule. When one of the phosphates is cut free, energy is released for the cell to use.

Cells have molecular machinery for assembling phosphate ions into groups and cleaving them off again. Fisher suggested a scheme in which two phosphate ions might be placed in a special kind of superposition called an "entangled state".

Phosphorus spins could resist decoherence for a day or so, even in living cells

The phosphorus nuclei have a quantum property called spin, which makes them rather like little magnets with poles pointing in particular directions. In an entangled state, the spin of one phosphorus nucleus depends on that of the other.

Put another way, entangled states are really superposition states involving more than one quantum particle.

Fisher says that the quantum-mechanical behaviour of these nuclear spins could plausibly resist decoherence on human timescales. He agrees with Tegmark that quantum vibrations, like those postulated by Penrose and Hameroff, will be strongly affected by their surroundings "and will decohere almost immediately". But nuclear spins do not interact very strongly with their surroundings.

All the same, quantum behaviour in the phosphorus nuclear spins would have to be "protected" from decoherence.

This might happen, Fisher says, if the phosphorus atoms are incorporated into larger objects called "Posner molecules". These are clusters of six phosphate ions, combined with nine calcium ions. There is some evidence that they can exist in living cells, though this is currently far from conclusive.

I decided... to explore how on earth the lithium ion could have such a dramatic effect in treating mental conditions

In Posner molecules, Fisher argues, phosphorus spins could resist decoherence for a day or so, even in living cells. That means they could influence how the brain works.

The idea is that Posner molecules can be swallowed up by neurons. Once inside, the Posner molecules could trigger the firing of a signal to another neuron, by falling apart and releasing their calcium ions.

Because of entanglement in Posner molecules, two such signals might thus in turn become entangled: a kind of quantum superposition of a "thought", you might say. "If quantum processing with nuclear spins is in fact present in the brain, it would be an extremely common occurrence, happening pretty much all the time," Fisher says.

He first got this idea when he started thinking about mental illness.

"My entry into the biochemistry of the brain started when I decided three or four years ago to explore how on earth the lithium ion could have such a dramatic effect in treating mental conditions," Fisher says.

At this point, Fisher's proposal is no more than an intriguing idea

Lithium drugs are widely used for treating bipolar disorder. They work, but nobody really knows how.

"I wasn't looking for a quantum explanation," Fisher says. But then he came across a paper reporting that lithium drugs had different effects on the behaviour of rats, depending on what form or "isotope" of lithium was used.

On the face of it, that was extremely puzzling. In chemical terms, different isotopes behave almost identically, so if the lithium worked like a conventional drug the isotopes should all have had the same effect.

But Fisher realised that the nuclei of the atoms of different lithium isotopes can have different spins. This quantum property might affect the way lithium drugs act. For example, if lithium substitutes for calcium in Posner molecules, the lithium spins might "feel" and influence those of phosphorus atoms, and so interfere with their entanglement.

We do not even know what consciousness is

If this is true, it would help to explain why lithium can treat bipolar disorder.

At this point, Fisher's proposal is no more than an intriguing idea. But there are several ways in which its plausibility can be tested, starting with the idea that phosphorus spins in Posner molecules can keep their quantum coherence for long periods. That is what Fisher aims to do next.

All the same, he is wary of being associated with the earlier ideas about "quantum consciousness", which he sees as highly speculative at best.

Physicists are not terribly comfortable with finding themselves inside their theories. Most hope that consciousness and the brain can be kept out of quantum theory, and perhaps vice versa. After all, we do not even know what consciousness is, let alone have a theory to describe it.

We all know what red is like, but we have no way to communicate the sensation

It does not help that there is now a New Age cottage industry devoted to notions of "quantum consciousness", claiming that quantum mechanics offers plausible rationales for such things as telepathy and telekinesis.

As a result, physicists are often embarrassed to even mention the words "quantum" and "consciousness" in the same sentence.

But setting that aside, the idea has a long history. Ever since the "observer effect" and the mind first insinuated themselves into quantum theory in the early days, it has been devilishly hard to kick them out. A few researchers think we might never manage to do so.

In 2016, Adrian Kent of the University of Cambridge in the UK, one of the most respected "quantum philosophers", speculated that consciousness might alter the behaviour of quantum systems in subtle but detectable ways.

Kent is very cautious about this idea. "There is no compelling reason of principle to believe that quantum theory is the right theory in which to try to formulate a theory of consciousness, or that the problems of quantum theory must have anything to do with the problem of consciousness," he admits.

Every line of thought on the relationship of consciousness to physics runs into deep trouble

But he says that it is hard to see how a description of consciousness based purely on pre-quantum physics can account for all the features it seems to have.

One particularly puzzling question is how our conscious minds can experience unique sensations, such as the colour red or the smell of frying bacon. With the exception of people with visual impairments, we all know what red is like, but we have no way to communicate the sensation and there is nothing in physics that tells us what it should be like.

Sensations like this are called "qualia". We perceive them as unified properties of the outside world, but in fact they are products of our consciousness and that is hard to explain. Indeed, in 1995 philosopher David Chalmers dubbed it "the hard problem" of consciousness.

"Every line of thought on the relationship of consciousness to physics runs into deep trouble," says Kent.

This has prompted him to suggest that "we could make some progress on understanding the problem of the evolution of consciousness if we supposed that consciousnesses alters (albeit perhaps very slightly and subtly) quantum probabilities."

"Quantum consciousness" is widely derided as mystical woo, but it just will not go away

In other words, the mind could genuinely affect the outcomes of measurements.

It does not, in this view, exactly determine "what is real". But it might affect the chance that each of the possible actualities permitted by quantum mechanics is the one we do in fact observe, in a way that quantum theory itself cannot predict. Kent says that we might look for such effects experimentally.

He even bravely estimates the chances of finding them. "I would give credence of perhaps 15% that something specifically to do with consciousness causes deviations from quantum theory, with perhaps 3% credence that this will be experimentally detectable within the next 50 years," he says.

If that happens, it would transform our ideas about both physics and the mind. That seems a chance worth exploring.

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The strange link between the human mind and quantum physics

The history of quantum mechanics is a fundamental part of the history of modern physics. Quantum mechanics' history, as it interlaces with the history of quantum chemistry, began essentially with a number of different scientific discoveries: the 1838 discovery of cathode rays by Michael Faraday; the 185960 winter statement of the black-body radiation problem by Gustav Kirchhoff; the 1877 suggestion by Ludwig Boltzmann that the energy states of a physical system could be discrete; the discovery of the photoelectric effect by Heinrich Hertz in 1887; and the 1900 quantum hypothesis by Max Planck that any energy-radiating atomic system can theoretically be divided into a number of discrete "energy elements" (Greek letter epsilon) such that each of these energy elements is proportional to the frequency with which each of them individually radiate energy, as defined by the following formula:

where h is a numerical value called Planck's constant.

Then, Albert Einstein in 1905, in order to explain the photoelectric effect previously reported by Heinrich Hertz in 1887, postulated consistently with Max Planck's quantum hypothesis that light itself is made of individual quantum particles, which in 1926 came to be called photons by Gilbert N. Lewis. The photoelectric effect was observed upon shining light of particular wavelengths on certain materials, such as metals, which caused electrons to be ejected from those materials only if the light quantum energy was greater than the work function of the metal's surface.

The phrase "quantum mechanics" was coined (in German, Quantenmechanik) by the group of physicists including Max Born, Werner Heisenberg, and Wolfgang Pauli, at the University of Gttingen in the early 1920s, and was first used in Born's 1924 paper "Zur Quantenmechanik".[1] In the years to follow, this theoretical basis slowly began to be applied to chemical structure, reactivity, and bonding.

Ludwig Boltzmann suggested in 1877 that the energy levels of a physical system, such as a molecule, could be discrete (as opposed to continuous). He was a founder of the Austrian Mathematical Society, together with the mathematicians Gustav von Escherich and Emil Mller. Boltzmann's rationale for the presence of discrete energy levels in molecules such as those of iodine gas had its origins in his statistical thermodynamics and statistical mechanics theories and was backed up by mathematical arguments, as would also be the case twenty years later with the first quantum theory put forward by Max Planck.

In 1900, the German physicist Max Planck reluctantly introduced the idea that energy is quantized in order to derive a formula for the observed frequency dependence of the energy emitted by a black body, called Planck's law, that included a Boltzmann distribution (applicable in the classical limit). Planck's law[2] can be stated as follows: I ( , T ) = 2 h 3 c 2 1 e h k T 1 , {displaystyle I(nu ,T)={frac {2hnu ^{3}}{c^{2}}}{frac {1}{e^{frac {hnu }{kT}}-1}},} where:

The earlier Wien approximation may be derived from Planck's law by assuming h k T {displaystyle hnu gg kT} .

Moreover, the application of Planck's quantum theory to the electron allowed tefan Procopiu in 19111913, and subsequently Niels Bohr in 1913, to calculate the magnetic moment of the electron, which was later called the "magneton;" similar quantum computations, but with numerically quite different values, were subsequently made possible for both the magnetic moments of the proton and the neutron that are three orders of magnitude smaller than that of the electron.

In 1905, Albert Einstein explained the photoelectric effect by postulating that light, or more generally all electromagnetic radiation, can be divided into a finite number of "energy quanta" that are localized points in space. From the introduction section of his March 1905 quantum paper, "On a heuristic viewpoint concerning the emission and transformation of light", Einstein states:

"According to the assumption to be contemplated here, when a light ray is spreading from a point, the energy is not distributed continuously over ever-increasing spaces, but consists of a finite number of 'energy quanta' that are localized in points in space, move without dividing, and can be absorbed or generated only as a whole."

This statement has been called the most revolutionary sentence written by a physicist of the twentieth century.[3] These energy quanta later came to be called "photons", a term introduced by Gilbert N. Lewis in 1926. The idea that each photon had to consist of energy in terms of quanta was a remarkable achievement; it effectively solved the problem of black-body radiation attaining infinite energy, which occurred in theory if light were to be explained only in terms of waves. In 1913, Bohr explained the spectral lines of the hydrogen atom, again by using quantization, in his paper of July 1913 On the Constitution of Atoms and Molecules.

These theories, though successful, were strictly phenomenological: during this time, there was no rigorous justification for quantization, aside, perhaps, from Henri Poincar's discussion of Planck's theory in his 1912 paper Sur la thorie des quanta.[4][5] They are collectively known as the old quantum theory.

The phrase "quantum physics" was first used in Johnston's Planck's Universe in Light of Modern Physics (1931).

In 1923, the French physicist Louis de Broglie put forward his theory of matter waves by stating that particles can exhibit wave characteristics and vice versa. This theory was for a single particle and derived from special relativity theory. Building on de Broglie's approach, modern quantum mechanics was born in 1925, when the German physicists Werner Heisenberg, Max Born, and Pascual Jordan[6][7] developed matrix mechanics and the Austrian physicist Erwin Schrdinger invented wave mechanics and the non-relativistic Schrdinger equation as an approximation of the generalised case of de Broglie's theory.[8] Schrdinger subsequently showed that the two approaches were equivalent.

Heisenberg formulated his uncertainty principle in 1927, and the Copenhagen interpretation started to take shape at about the same time. Starting around 1927, Paul Dirac began the process of unifying quantum mechanics with special relativity by proposing the Dirac equation for the electron. The Dirac equation achieves the relativistic description of the wavefunction of an electron that Schrdinger failed to obtain. It predicts electron spin and led Dirac to predict the existence of the positron. He also pioneered the use of operator theory, including the influential braket notation, as described in his famous 1930 textbook. During the same period, Hungarian polymath John von Neumann formulated the rigorous mathematical basis for quantum mechanics as the theory of linear operators on Hilbert spaces, as described in his likewise famous 1932 textbook. These, like many other works from the founding period, still stand, and remain widely used.

The field of quantum chemistry was pioneered by physicists Walter Heitler and Fritz London, who published a study of the covalent bond of the hydrogen molecule in 1927. Quantum chemistry was subsequently developed by a large number of workers, including the American theoretical chemist Linus Pauling at Caltech, and John C. Slater into various theories such as Molecular Orbital Theory or Valence Theory.

Beginning in 1927, researchers attempted to apply quantum mechanics to fields instead of single particles, resulting in quantum field theories. Early workers in this area include P.A.M. Dirac, W. Pauli, V. Weisskopf, and P. Jordan. This area of research culminated in the formulation of quantum electrodynamics by R.P. Feynman, F. Dyson, J. Schwinger, and S. Tomonaga during the 1940s. Quantum electrodynamics describes a quantum theory of electrons, positrons, and the electromagnetic field, and served as a model for subsequent quantum field theories.[6][7][9]

The theory of quantum chromodynamics was formulated beginning in the early 1960s. The theory as we know it today was formulated by Politzer, Gross and Wilczek in 1975.

Building on pioneering work by Schwinger, Higgs and Goldstone, the physicists Glashow, Weinberg and Salam independently showed how the weak nuclear force and quantum electrodynamics could be merged into a single electroweak force, for which they received the 1979 Nobel Prize in Physics.

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History of quantum mechanics - Wikipedia

Quantum theory is the theoretical basis of modern physics that explains the nature and behavior of matter and energy on the atomic and subatomic level.The nature and behavior of matter and energy at that level is sometimes referred to as quantum physics and quantum mechanics. Organizations in several countries have devoted significant resources to the development of quantum computing, which uses quantum theory to drastically improve computing capabilities beyond what is possible using today's classical computers.

In 1900, physicist Max Planck presented his quantum theory to the German Physical Society. Planck had sought to discover the reason that radiation from a glowing body changes in color from red, to orange, and, finally, to blue as its temperature rises. He found that by making the assumption that energy existed in individual units in the same way that matter does, rather than just as a constant electromagnetic wave - as had been formerly assumed - and was therefore quantifiable, he could find the answer to his question. The existence of these units became the first assumption of quantum theory.

Planck wrote a mathematical equation involving a figure to represent these individual units of energy, which he called quanta. The equation explained the phenomenon very well; Planck found that at certain discrete temperature levels (exact multiples of a basic minimum value), energy from a glowing body will occupy different areas of the color spectrum. Planck assumed there was a theory yet to emerge from the discovery of quanta, but, in fact, their very existence implied a completely new and fundamental understanding of the laws of nature. Planck won the Nobel Prize in Physics for his theory in 1918, but developments by various scientists over a thirty-year period all contributed to the modern understanding of quantum theory.

The two major interpretations of quantum theory's implications for the nature of reality are the Copenhagen interpretation and the many-worlds theory. Niels Bohr proposed the Copenhagen interpretation of quantum theory, which asserts that a particle is whatever it is measured to be (for example, a wave or a particle), but that it cannot be assumed to have specific properties, or even to exist, until it is measured. In short, Bohr was saying that objective reality does not exist. This translates to a principle called superposition that claims that while we do not know what the state of any object is, it is actually in all possible states simultaneously, as long as we don't look to check.

To illustrate this theory, we can use the famous and somewhat cruel analogy of Schrodinger's Cat. First, we have a living cat and place it in a thick lead box. At this stage, there is no question that the cat is alive. We then throw in a vial of cyanide and seal the box. We do not know if the cat is alive or if the cyanide capsule has broken and the cat has died. Since we do not know, the cat is both dead and alive, according to quantum law - in a superposition of states. It is only when we break open the box and see what condition the cat is that the superposition is lost, and the cat must be either alive or dead.

The second interpretation of quantum theory is the many-worlds (or multiverse theory. It holds that as soon as a potential exists for any object to be in any state, the universe of that object transmutes into a series of parallel universes equal to the number of possible states in which that the object can exist, with each universe containing a unique single possible state of that object. Furthermore, there is a mechanism for interaction between these universes that somehow permits all states to be accessible in some way and for all possible states to be affected in some manner. Stephen Hawking and the late Richard Feynman are among the scientists who have expressed a preference for the many-worlds theory.

Although scientists throughout the past century have balked at the implications of quantum theory - Planck and Einstein among them - the theory's principles have repeatedly been supported by experimentation, even when the scientists were trying to disprove them. Quantum theory and Einstein's theory of relativity form the basis for modern physics. The principles of quantum physics are being applied in an increasing number of areas, including quantum optics, quantum chemistry, quantum computing, and quantum cryptography.

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What is quantum theory? - Definition from WhatIs.com

One of the most important open questions in science is how our consciousness is established. In the 1990s, long before winning the 2020 Nobel Prize in Physics for his prediction of black holes, physicist Roger Penrose teamed up with anaesthesiologist Stuart Hameroff to propose an ambitious answer.

They claimed that the brains neuronal system forms an intricate network and that the consciousness this produces should obey the rules of quantum mechanics the theory that determines how tiny particles like electrons move around. This, they argue, could explain the mysterious complexity of human consciousness.

Penrose and Hameroff were met with incredulity. Quantum mechanical laws are usually only found to apply at very low temperatures. Quantum computers, for example, currently operate at around -272C. At higher temperatures, classical mechanics takes over. Since our body works at room temperature, you would expect it to be governed by the classical laws of physics. For this reason, the quantum consciousness theory has been dismissed outright by many scientists though others are persuaded supporters.

Instead of entering into this debate, I decided to join forces with colleagues from China, led by Professor Xian-Min Jin at Shanghai Jiaotong University, to test some of the principles underpinning the quantum theory of consciousness.

In our new paper, weve investigated how quantum particles could move in a complex structure like the brain but in a lab setting. If our findings can one day be compared with activity measured in the brain, we may come one step closer to validating or dismissing Penrose and Hameroffs controversial theory.

Our brains are composed of cells called neurons, and their combined activity is believed to generate consciousness. Each neuron contains microtubules, which transport substances to different parts of the cell. The Penrose-Hameroff theory of quantum consciousness argues that microtubules are structured in a fractal pattern which would enable quantum processes to occur.

Fractals are structures that are neither two-dimensional nor three-dimensional, but are instead some fractional value in between. In mathematics, fractals emerge as beautiful patterns that repeat themselves infinitely, generating what is seemingly impossible: a structure that has a finite area, but an infinite perimeter.

Read more: Explainer: what are fractals?

This might sound impossible to visualise, but fractals actually occur frequently in nature. If you look closely at the florets of a cauliflower or the branches of a fern, youll see that theyre both made up of the same basic shape repeating itself over and over again, but at smaller and smaller scales. Thats a key characteristic of fractals.

The same happens if you look inside your own body: the structure of your lungs, for instance, is fractal, as are the blood vessels in your circulatory system. Fractals also feature in the enchanting repeating artworks of MC Escher and Jackson Pollock, and theyve been used for decades in technology, such as in the design of antennas. These are all examples of classical fractals fractals that abide by the laws of classical physics rather than quantum physics.

Its easy to see why fractals have been used to explain the complexity of human consciousness. Because theyre infinitely intricate, allowing complexity to emerge from simple repeated patterns, they could be the structures that support the mysterious depths of our minds.

But if this is the case, it could only be happening on the quantum level, with tiny particles moving in fractal patterns within the brains neurons. Thats why Penrose and Hameroffs proposal is called a theory of quantum consciousness.

Were not yet able to measure the behaviour of quantum fractals in the brain if they exist at all. But advanced technology means we can now measure quantum fractals in the lab. In recent research involving a scanning tunnelling microscope (STM), my colleagues at Utrecht and I carefully arranged electrons in a fractal pattern, creating a quantum fractal.

When we then measured the wave function of the electrons, which describes their quantum state, we found that they too lived at the fractal dimension dictated by the physical pattern wed made. In this case, the pattern we used on the quantum scale was the Sierpiski triangle, which is a shape thats somewhere between one-dimensional and two-dimensional.

This was an exciting finding, but STM techniques cannot probe how quantum particles move which would tell us more about how quantum processes might occur in the brain. So in our latest research, my colleagues at Shanghai Jiaotong University and I went one step further. Using state-of-the-art photonics experiments, we were able to reveal the quantum motion that takes place within fractals in unprecedented detail.

We achieved this by injecting photons (particles of light) into an artificial chip that was painstakingly engineered into a tiny Sierpiski triangle. We injected photons at the tip of the triangle and watched how they spread throughout its fractal structure in a process called quantum transport. We then repeated this experiment on two different fractal structures, both shaped as squares rather than triangles. And in each of these structures we conducted hundreds of experiments.

Our observations from these experiments reveal that quantum fractals actually behave in a different way to classical ones. Specifically, we found that the spread of light across a fractal is governed by different laws in the quantum case compared to the classical case.

This new knowledge of quantum fractals could provide the foundations for scientists to experimentally test the theory of quantum consciousness. If quantum measurements are one day taken from the human brain, they could be compared against our results to definitely decide whether consciousness is a classical or a quantum phenomenon.

Our work could also have profound implications across scientific fields. By investigating quantum transport in our artificially designed fractal structures, we may have taken the first tiny steps towards the unification of physics, mathematics and biology, which could greatly enrich our understanding of the world around us as well as the world that exists in our heads.

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Can consciousness be explained by quantum physics? My research takes us a step closer to finding out - The Conversation UK

In therecent Nature study, Basov and his colleagues recreated Fizeaus experiments on a speck of graphene made up of a single layer of carbon atoms. Hooking up the graphene to a battery, they created an electrical current reminiscent of Fizeaus water streaming through a pipe. But instead of shining light on the moving water and measuring its speed in both directions, as Fizeau did, they generated an electromagnetic wave with a compressed wavelengtha polaritonby focusing infrared light on a gold nub in the graphene. The activated stream of polaritons look like light but are physically more compact due to their short wavelengths.

The researchers clocked the polaritons speed in both directions. When they traveled with the flow of the electrical current, they maintained their original speed. But when launched against the current, they slowed by a few percentage points.

We were surprised when we saw it, saidstudy co-author Denis Bandurin, a physics researcher at MIT. First, the device was still alive, despite the heavy current we passed through itit hadnt blown up. Then we noticed the one-way effect, which was different from Fizeaus original experiments.

The researchers repeated the experiments over and over, led by the studys first-author, Yinan Dong, a Columbia graduate student. Finally, it dawned on them. Graphene is a material that turns electrons into relativistic particles, Dong said. We needed to account for their spectrum.

A group at Berkeley Lab founda similar result, published in the same issue of Nature. Beyond reproducing the Fizeau effect in graphene, both studies have practical applications. Most natural systems are symmetric, but here, researchers found an intriguing exception. Basov said he hopes to slow down, and ultimately, cut off the flow of polaritons in one direction. Its not an easy task, but it could hold big rewards.

Engineering a system with a one-way flow of light is very difficult to achieve, saidMilan Delor, a physical chemist working on light-matter interactions at Columbia who was not involved in the research. As soon as you can control the speed and direction of polaritons, you can transmit information in nanoscale circuits on ultrafast timescales. Its one of the ingredients currently missing in photon-based circuits.

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Physicists Show That a Quantum Particle Made of Light and Matter Can Be Dragged by a Current of Electrons - Columbia University

We take for granted that an event in one part of the world cannot instantly affect what happens far away. This principle, which physicists call locality, was long regarded as a bedrock assumption about the laws of physics. So when Albert Einstein and two colleagues showed in 1935 that quantum mechanics permits spooky action at a distance, as Einstein put it, this feature of the theory seemed highly suspect. Physicists wondered whether quantum mechanics was missing something.

Then in 1964, with the stroke of a pen, the Northern Irish physicist John Stewart Bell demoted locality from a cherished principle to a testable hypothesis. Bell proved that quantum mechanics predicted stronger statistical correlations in the outcomes of certain far-apart measurements than any local theory possibly could. In the years since, experiments have vindicated quantum mechanics again and again.

Bells theorem upended one of our most deeply held intuitions about physics, and prompted physicists to explore how quantum mechanics might enable tasks unimaginable in a classical world. The quantum revolution thats happening now, and all these quantum technologies thats 100% thanks to Bells theorem, says Krister Shalm, a quantum physicist at the National Institute of Standards and Technology.

Heres how Bells theorem showed that spooky action at a distance is real.

The spooky action that bothered Einstein involves a quantum phenomenon known as entanglement, in which two particles that we would normally think of as distinct entities lose their independence. Famously, in quantum mechanics a particles location, polarization and other properties can be indefinite until the moment they are measured. Yet measuring the properties of entangled particles yields results that are strongly correlated, even when the particles are far apart and measured nearly simultaneously. The unpredictable outcome of one measurement appears to instantly affect the outcome of the other, regardless of the distance between them a gross violation of locality.

To understand entanglement more precisely, consider a property of electrons and most other quantum particles called spin. Particles with spin behave somewhat like tiny magnets. When, for instance, an electron passes through a magnetic field created by a pair of north and south magnetic poles, it gets deflected by a fixed amount toward one pole or the other. This shows that the electrons spin is a quantity that can have only one of two values: up for an electron deflected toward the north pole, and down for an electron deflected toward the south pole.

Imagine an electron passing through a region with the north pole directly above it and the south pole directly below. Measuring its deflection will reveal whether the electrons spin is up or down along the vertical axis. Now rotate the axis between the magnet poles away from vertical, and measure deflection along this new axis. Again, the electron will always deflect by the same amount toward one of the poles. Youll always measure a binary spin value either up or down along any axis.

It turns out its not possible to build any detector that can measure a particles spin along multiple axes at the same time. Quantum theory asserts that this property of spin detectors is actually a property of spin itself: If an electron has a definite spin along one axis, its spin along any other axis is undefined.

Armed with this understanding of spin, we can devise a thought experiment that we can use to prove Bells theorem. Consider a specific example of an entangled state: a pair of electrons whose total spin is zero, meaning measurements of their spins along any given axis will always yield opposite results. Whats remarkable about this entangled state is that, although the total spin has this definite value along all axes, each electrons individual spin is indefinite.

Suppose these entangled electrons are separated and transported to distant laboratories, and that teams of scientists in these labs can rotate the magnets of their respective detectors any way they like when performing spin measurements.

When both teams measure along the same axis, they obtain opposite results 100% of the time. But is this evidence of nonlocality? Not necessarily.

Alternatively, Einstein proposed, each pair of electrons could come with an associated set of hidden variables specifying the particles spins along all axes simultaneously. These hidden variables are absent from the quantum description of the entangled state, but quantum mechanics may not be telling the whole story.

Hidden variable theories can explain why same-axis measurements always yield opposite results without any violation of locality: A measurement of one electron doesnt affect the other but merely reveals the preexisting value of a hidden variable.

Bell proved that you could rule out local hidden variable theories, and indeed rule out locality altogether, by measuring entangled particles spins along different axes.

Suppose, for starters, that one team of scientists happens to rotate its detector relative to the other labs by 180 degrees. This is equivalent to swapping its north and south poles, so an up result for one electron would never be accompanied by a down result for the other. The scientists could also choose to rotate it an in-between amount 60 degrees, say. Depending on the relative orientation of the magnets in the two labs, the probability of opposite results can range anywhere between 0% and 100%.

Without specifying any particular orientations, suppose that the two teams agree on a set of three possible measurement axes, which we can label A, B and C. For every electron pair, each lab measures the spin of one of the electrons along one of these three axes chosen at random.

Lets now assume the world is described by a local hidden variable theory, rather than quantum mechanics. In that case, each electron has its own spin value in each of the three directions. That leads to eight possible sets of values for the hidden variables, which we can label in the following way:

The set of spin values labeled 5, for instance, dictates that the result of a measurement along axis A in the first lab will be up, while measurements along axes B and C will be down; the second electrons spin values will be opposite.

For any electron pair possessing spin values labeled 1 or 8, measurements in the two labs will always yield opposite results, regardless of which axes the scientists choose to measure along. The other six sets of spin values all yield opposite results in 33% of different-axis measurements. (For instance, for the spin values labeled 5, the labs will obtain opposite results when one measures along axis B while the other measures along C; this represents one-third of the possible choices.)

Thus the labs will obtain opposite results when measuring along different axes at least 33% of the time; equivalently, they will obtain the same result at most 67% of the time. This result an upper bound on the correlations allowed by local hidden variable theories is the inequality at the heart of Bells theorem.

Now, what about quantum mechanics?Were interested in the probability of both labs obtaining the same result when measuring the electrons spins along different axes. The equations of quantum theory provide a formula for this probability as a function of the angles between the measurement axes.

According to the formula, when the three axes are all as far apart as possible that is, all 120 degrees apart, as in the Mercedes logo both labs will obtain the same result 75% of the time. This exceeds Bells upper bound of 67%.

Thats the essence of Bells theorem: If locality holds and a measurement of one particle cannot instantly affect the outcome of another measurement far away, then the results in a certain experimental setup can be no more than 67% correlated. If, on the other hand, the fates of entangled particles are inextricably linked even across vast distances, as in quantum mechanics, the results of certain measurements will exhibit stronger correlations.

Since the 1970s, physicists have made increasingly precise experimental tests of Bells theorem. Each one has confirmed the strong correlations of quantum mechanics. In the past five years, various loopholes have been closed. Locality that long-held assumption about physical law is not a feature of our world.

Editors note: The author is currently a postdoctoral researcher at JILA in Boulder, Colorado.

Read more from the original source:

How Bell's Theorem Proved 'Spooky Action at a Distance' Is Real - Quanta Magazine