Category Archives: Quantum Physics

IISER partners with international collaborators for breakthrough in the field of Quantum Communication  The Financial Express

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IISER partners with international collaborators for breakthrough in the field of Quantum Communication - The Financial Express

'QBism': quantum mechanics is not a description of objective reality it reveals a world of genuine free will  The Conversation

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'QBism': quantum mechanics is not a description of objective reality it reveals a world of genuine free will - The Conversation

Scientists think it will be particularly useful for problems that involve many variables, such as analyzing financial risk, encrypting data, and studying the properties of materials.

Researchers doubt that individuals will own personal quantum computers in near future. Instead, theyll be housed at academic institutions and private companies where they can be accessed through a cloud service.

Quantum computers use fundamental units of information similar to the bits used in classical computing. These are called qubits.

However, unlike conventional computer bitswhich convey information as a 0 or 1qubits convey information through a combination of quantum states, which are unique conditions found only on the subatomic scale.

For example, one quantum state that could be used to encode information is a property called spin, which is the intrinsic angular momentum of an electron. Spin can be thought of like a tiny compass needle that points either up or down. Researchers can manipulate that needle to encode information into the electrons themselves, much like they would with conventional bitsbut in this case, the information is encoded in a combination of possible states. Qubits are not either 0 or 1, but rather both and neither, in a quantum phenomenon called superposition.

This allows quantum computers to process information in a wholly different way than their conventional counterparts, and therefore they can solve certain types of problems that would take even the largest supercomputers decades to complete. These are problems like factoring large numbers or solving complex logistics calculations (see the traveling salesman problem). Quantum computers would be especially useful for cryptography as well as discovering new types of pharmaceutical drugs or new materials for solar cells, batteries, or other technologies.

But to unlock that potential, a quantum computer must be able to process a large number of qubitsmore than any single machine can manage at the moment. That is, unless several quantum computers could be joined through the quantum internet and their computational power pooled, creating a far more capable system.

There are several different types of qubits in development, and each comes with distinct advantages and disadvantages. The most common qubits being studied today are quantum dots, ion traps, superconducting circuits, and defect spin qubits.

Like many scientific advances, we wont understand everything the quantum internet can do until its been fully developed.

Few could imagine 60 years ago that a handful of interconnected computers would one day spawn the sprawling digital landscape we know today. The quantum internet presents a similar unknown, but a number of applications have been theorized and some have already been demonstrated.

Thanks to qubits unique quantum properties, scientists think the quantum internet will greatly improve information security, making it nearly impossible for quantum encrypted messages to be intercepted and deciphered. Quantum key distribution, or QKD, is a process by which two parties share a cryptographic key over a quantum network that cannot be intercepted. Several private companies already offer the process, and it has even been used to secure national elections.

At the same time, quantum computers pose a threat to traditional encrypted communication. RSA, the current standard for protecting sensitive digital information, is nearly impossible for modern computers to break; however, quantum computers with enough processing power could get past RSA encryption in a matter of minutes or seconds.

A fully-realized quantum network could significantly improve the precision of scientific instruments used to study certain phenomena. The impact of such a network would be wide-ranging, but early interest has centered on gravitational waves from black holes, microscopy, and electromagnetic imaging.

Creating a purely quantum internet would also relieve the need for quantum information to transition between classical and quantum systems, which is a considerable hurdle in current systems. Instead, it would allow a set of individual quantum computers to process information as one conglomerate machine, giving them far greater computational power than any single system could command on its own.

"The quantum internet represents a paradigm shift in how we think about secure global communication," said David Awschalom, the Liew Family Professor in Molecular Engineering and Physics at the University of Chicago, director of the Chicago Quantum Exchange, and director of Q-NEXT, a Department of Energy Quantum Information Science Center at Argonne. "Being able to create an entangled network of quantum computers would allow us to send unhackable encrypted messages, keep technology in perfect sync across long distances using quantum clocks, and solve complex problems that one quantum computer might struggle with alone--and those are just some of the applications we know about right now. The future is likely to hold surprising and impactful discoveries using quantum networks."

To date, no one has been able to successfully create a sustained quantum network on a large scale, but there have been major advances.

In 2017 researchers at the University of Science and Technology of China used lasers to successfully transmit entangled photons between a satellite in orbit and ground stations more than 700 miles below. The experiment showed the possibility of using satellites to form part of a quantum network, but the system was only able to recover one photon out of every 6 milliontoo few to be used for reliable communication.

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What is the quantum internet? | University of Chicago News

Yesterday, data was simply stored and managed. Today, data is an essential differentiator. At Quantum, we believe it's time to shift the focus from accumulating data to making it work much harder. Its a new data reality thats endlessly alive. Its massively growing, widely distributed, unstructured, and its gaining value at every turn. Your video and unstructured data not only needs to be fully protected, but it is also full of possibility. Quantum partners with you so you can shape it, use it, and transform it into the information you need to drive forward. With Quantum, you can enrich, orchestrate, protect, and archive your video and unstructured data, securely and at scalenow and for decades to come.

Its not only about managing data. Its about making sure you can extract value from it to gain a competitive edge. Between 80-90% of data collected today is unstructured. Locked inside these video and audio files, photos, security camera footage, sensor data, scientific data, and satellite imagery is a wealth of information that holds the key to informed decision-making.

We enable a world where data is alive. We make it right-time, right-place data so its available, discoverable, and safe. With Quantum, you have the insights you need to drive new opportunities, explore new paths, or accelerate the next groundbreaking discovery. Our bold, innovative, end-to-end data solutions allow forward-thinking organizationslike yoursto harness the enriched world of living data.

Solutions to Securely Scale Your Organization

Quantum allows you to focus on growing your business, not on managing your data. With the security of onsite data and the ease of the cloud, our software, subscriptions, and services help to power your data infrastructure. You no longer must choose how much of your valuable data to saveour edge-to-core-to-cloud solutions are designed with smart economics in mind. And, since we build in security at the foundation of our data solutions, you never have to sacrifice flexibility for data safety.

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This isnt inflexible, one-size-fits-all data management. Its innovative technology that supports your business, your needs, and your budget through the entire lifecyclefrom where data is captured to where its stored to where its used. From the worlds fastest file system for video to OPEX-friendly software subscriptions and as-a-Service options, Quantum solutions support your business every step of the way. Whether your business is helping to keep the world safe, making breakthrough discoveries, or creating entertainment, our end-to-end data solutions are built for living data.

The Tools You Need to Add Value to Your Data

Quantum builds in data enrichment at the foundation of our solutions, so getting valuable information from your data is not an afterthought. With complete, ecosystem-friendly solutions, you can store as much data as you neednow and in the futureand leverage rich information about your business. Quantum solutions allow you to avoid overprovisioning your data infrastructure through scalable on-prem solutions and subscription-based models. So, with Quantum, your data works for you.

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About Quantum | Quantum

Yoram AlhassidFrederick Phineas Rose Professor of PhysicsSPL 50yoram.alhassid@yale.edu203-432-6922Research Website Theorist

Current Projects:

The nuclear many-body problem;Femtoscience and nanoscience: nuclei quantum dots and nanoparticles;Cold atomic Fermi gases

Current Projects:

Quadratic Echo Line-Narrowing, Imaging Hard and Soft Solids, Advancing Spectral Reconstruction with Undersampled Data Sets, Custom NMR/MRI Probe Design and Construction

Current Projects:

Ultracold atomic physics in optical lattices

Current Projects:

Optomechanics: Radiation Pressure - Radiation pressure in the quantum engine, Optical control of microstructures, Mechanical control of nonclassical light and Persistent Current - Microcantilevers and probes of closed mesoscopic systems, In-situ electron thermometry, Persistent currents in normal-metal rings

Current Projects:

Haloscope At Yale Sensitive to Axion CDM (HAYSTAC), Electric dipole moment, Casimir effect

Current Projects:

Cryogenic Underground Observatory for Rare Events (CUORE), IceCube Neutrino Obervatory, CUORE Upgrade with Particle IDentification (CUPID), ATLAS, COSINE-100, DM-Ice, Haloscope At Yale Sensitive To Axion CDM (HAYSTAC)

Current Projects:

Quantum error correction when the noise is biased, Scalable fault-tolerant quantum error correction with bosonic qubits

Current Projects:

Exciton Transport & Diffusion; Time-Dependent Phenomena; Heterojunctions, Interfaces and Substrates; Defects

Current Projects:

The study of problems at the interface of optical and condensed matter physics

Current Projects:

Quantum transport phenomena in disordered media, mesoscopic electron physics, non-linear and chaotic dynamics, quantum and wave chaos, quantum measurement and quantum computing. Laser physics, non-linear optics, microcavity and random lasers.

Current Projects:

Quantum transductionfrom microwave to optical photons,Quantum networksand quantum communications,Superconducting quantum detectors

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Quantum Physics | Department of Physics - Yale University

Origin of Quantum Theory

In the 1900s, the field of Physics seemed, for the briefest time, to be at a halt. We were, however, at the brink of the discovery of one of the most revolutionary theories proposed to date: quantum physics. This theory was developed by a German physicist named Max Planck: he proposed that the energy of electromagnetic waves, unlike previously thought, was not a continuum, but rather, that there was a minimal, unbreakable, quantifiable unit of energy, that we call quanta. Plank's theory was shortly thereafter extended by Einstein, who found that the quantization of radiation provided an explanation for the photoelectric effect.

A lot of experiments, including the double-slit experiment by the famous mathematician Thomas Young, and Einstein's aforementioned photoelectric effect, provided extensive proof that waves and particles were not two different physical objects, but rather, that at a small enough scale, particles exhibit wave-like behavior, and electromagnetic waves (that is, light) behave as if they were tiny massless particles made of units of energy: the famous quanta. Wener Heisenberg would further build on this idea of particle-wave duality and postulate in 1927 one of the main axioms of quantum mechanics: the uncertainty principle. This theorem tells us that, for subatomic particles, we cannot exactly measure simultaneously their position and their velocity, giving a fundamental limit to measurements, a property that is based on the wave-like nature of particles.

Finally, it was Paul Dirac and Erwin Schrodinger who formulated the mathematical framework that tied together atomic theory with quantum mechanics. They developed the Schrodinger equation, which allows us to compute the wavefunction of a quantum system, and the more general, relativistic version, called the Dirac equation. Their work ultimately led to both of them winning a Nobel prize in Physics in 1933.

The fundamental postulates of quantum theory are:

In simpler terms, a wavefunction is a probabilistic description of a system. Many times it is said that a quantum system can be in a superposition of different states, and indeed, the wavefunction represents all of the possible states on which you can find a quantum particle (for example, all of the different possible positions) and their associated probability.

$$i hbar frac{partial}{partial t}Psi(t,x) = left( - frac{hbar ^2} {2m} frac{partial^2}{partial x^2 } + V(t,x) right) Psi(t,x) $$

This equation appears difficult at first sight, but it can be broken down into pieces in the following way: the left-hand side of the equation indicates the time evolution. {eq}hbar {/eq} is the plank constant, which gives us a sense of the energy scale of the system we are working with. On the right-hand side, we find two different terms, the first one involving the velocity of the particle, and thus corresponding to the kinetic energy, and the second one, {eq}V(t,x) {/eq}, corresponding to the potential energy.

When we measure a particle, the wavefunction is said to "collapse" - that is, the particle will not be in a superposition of states with associated probabilities anymore, but in a definite single state, corresponding to the measured quantity.

These principles have many consequences and have been used to derive many important theorems, but two of them are the most notable: Heisenberg's uncertainty principle and the existence of entanglement.

As mentioned before, Heisenberg's uncertainty principle states that one cannot know with perfect accuracy both the position and the velocity of a quantum particle. Plainly said, there is a fundamental limit on the information one can extract out of a quantum system, and this is because when we measure a particle and the wavefunction collapses, there is a loss of information happening during that collapse. Another way to understand this principle is to think about it in practice: Let's say that we have an electron and that we wish to measure its location. For that, we would need to look at the electron, either by shining a laser at it or by taking a photograph. Both processes are invasive and disrupt the state of the electron, causing a change in its internal energy, since we would be bombarding it with a ray of light. This change of energy, which is needed to determine the position, will affect the velocity of the electron. Therefore, we cannot know with exactitude the speed of the electron anymore. the same happens if we try to measure the velocity: in that case, it will be the position of the electron the one we would not be able to determine exactly.

Entanglement is a different phenomenon that occurs when two quantum particles interact. Sometimes, two interacting quantum particles can stay connected - in quantum terms this means that instead of there being two different wavefunctions, one representing each particle, there is a bigger, single wavefunction representing both of them at the same time. The result is that both of these particles stay connected, and can influence each other even if they are thousands of miles apart. The coolest (and most spooky!) thing is that, because we know that measurements affect the state of a quantum system, that means that if you separate two entangled particles and then you measure the state of one of them, the state of the other one will immediately be affected as well, no matter how far it is located.

Let's try to clarify all of these ideas with a famous example: Schrodinger's Cat.

This is a thought experiment that is aimed at illustrating the concept of superposition. Let's say we have a hypothetical cat, and that we put it inside of a box with poison, and then we close the box. We will also assume that the cat has a 50/50 chance of eating the poison. Therefore, until we open the box, we do not know with certainty if the cat will be dead or alive - there is a 50/50 chance of him being either. At that stage, if we imagine that the cat is a quantum particle, we can write the cat's wavefunction, for example, to be something like this:

$$Psi = 0.5 |text{dead}> + 0.5 |text{alive}> $$

This means that the cat is in a state which is a superposition of dead and alive: it is dead with a 50% chance, and alive with a 50% chance. Let's say that we now open the box, and find that the kitty is alive. The measurement, or the act of opening the box, has made the wavefunction collapse, and now we find the cat in a state of 100% aliveness.

Another spooky and plain amazing phenomenon that quantum particles can exhibit is that of quantum tunneling.

In the classical world, when you throw a ball against a wall, you know exactly what will happen. The wall acts as a barrier that is impenetrable, and the ball will certainly bounce back. However, this is no longer true for quantum particles. Given that quantum particles can be in a superposition of position states, they sometimes exhibit really spooky characteristics. When you throw a quantum particle against a barrier, there might be a small probability of finding the particle on the other side of the barrier. This means that, if you repeat the experiment again and again measuring the position of the particle right after hitting the barrier, while most of the times you will find that the quantum particle bounced back, a small portion of the times you will find the particle on the other side of the barrier: this is called quantum tunneling.

Today we learned that Quantum Theory is the branch of physics that studies atomic and subatomic particles, and their associated phenomena. It was developed in the early 1900s by Max Plank, and the theory was extended by many physicists including Einstein, Heisenberg, Dirac, and Schrodinger.

Quantum particles are described by a wavefunction, and when we observe them (that is when we measure them) we can alter their state. Quantum particles can be found in a superposition of states, but we do not know which one until we measure them: this is best exemplified by the hypothetical Schrodinger's cat, a thought experiment consisting of putting a cat on a box with poison, which results in the cat being in a superposition of dead and alive, with the observer not knowing until they open the box.

Some important principles of Quantum theory include the Heisenberg uncertainty principle, which indicates that we cannot know with perfect accuracy both the position and the velocity of a quantum particle, and the existence of entanglement, a long-range interaction effect that interacting quantum particles can have on one another.

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Quantum Physics | What is Quantum Theory? - Video & Lesson Transcript ...

The quantum world defies common sense at every turn. Shaped across hundreds of thousands of years by biological evolution, our modern human brain struggles to comprehend things outside our familiar naturalistic context. Understanding a predator chasing prey across a grassy plain is easy; understanding most anything occurring at subatomic scales may require years of intense scholarship and oodles of gnarly math. Its no surprise, then, that every year physicists deliver mind-boggling new ideas and discoveries harvested from realitys deep underpinnings, well beyond the frontiers of our perception. Here, Scientific American highlights some of our favorites from 2022.

This years Nobel Prize in Physics went to researchers who spent decades proving the universe is not locally reala feat that, to quote humorist Douglas Adams, has made a lot of people very angry and been widely regarded as a bad move. Local here means any objectan apple, for instancecan be influenced only by its immediate surroundings, not by happenings on the other side of the universe. Real means every object has definite properties regardless of how it is observedno amount of squinting will change an apple from red to green. Except careful, repeated experimentation with entangled particles has conclusively shown such seemingly sensible restrictions do not always apply to the quantum realm, the most fundamental level of reality we can measure. If youre uncertain as to what exactly the demise of local realism means for life, the universe and, well, everything, dont worry: youre not alonephysicists are befuddled, too.

Despite seeming like plot elements of a cult-classic science-fiction film, two unrelated papers published earlier this year describe not-at-all-fictitious ways of harnessing light at the quantum frontier. In one study, researchers reported the first-ever construction of laser-based time crystals, quantum systems that exhibit crystallike periodic structures not in space but in time. In the other, a team detailed how precise patterns of laser pulses coaxed strings of ions into behaving like a never-before-seen phase of matter occupying two time dimensions. The former study could lead to cheap, rugged microchips for making time crystals outside of laboratories. The latter suggests a method for enhancing the performance of quantum computers. For most of us, though, these studies may be most useful for sounding smart at cocktail parties.

The Mermin-Peres magic square (MPMS) game is the sort of competition one can win only by not playing. This dismal relative of Sudoku involves two participants taking turns adding the value of either +1 or 1 to cells in a three-by-three grid to collaboratively fulfill a win condition. Although the players must coordinate their actions to succeed, they are not allowed to communicate. And even if each correctly guesses the others move, the pair can still only win eight out of the games nine roundsunless, that is, they play a quantum version. If qubits (which can swap values between +1 and 1) are used to fill each cell, two players can, in theory, pull off a perfect run by avoiding conflicting moves for all nine rounds. In practice, however, the odds of guessing each move correctly are vanishingly slim. Yet by carefully leveraging entanglement between the qubits, during each turn, the players can surmise each others actions without actually communicatinga vexing technique known as quantum pseudotelepathy. In July researchers published a paper reporting their successful real-world demonstration of this strategy to achieve flawless performance. This isnt all fun and games, either: such work probes the fundamental limits of how information can be shared between entangled particles.

According to the tenets of quantum field theoryan uneasy union between Einsteins special theory of relativity and quantum mechanics used to model the behavior of subatomic particlesempty space isnt actually empty. Instead what we perceive as the void is filled with overlapping energetic fields. Fluctuations in these fields can produce photons, electrons and other particles essentially out of nothing. Among the various bizarre phenomena predicted to arise from such curious circumstances, the strangest might be the Unruh effect, a warm shroud of ghostly particles summoned by any object accelerating through a vacuum. Named for theorist Bill Unruh, who described it in 1976, this effect is so subtle that it has yet to be observed. That soon could change if a tabletop experiment proposed in April is successfully performed. The experiment involves accelerating a single electron through an intense and carefully configured electromagnetic field. This setup should lower the threshold of acceleration for the Unruh effect to visibly manifest, boosting the chances for catching a glimpse of its elusive quantum glow, the proposers say.

Not all counterintuitive quirks of quantum physics are linked to natural causes. Some are arguably more self-inflicted, arising from researchers questionable choices in how they name and describe certain phenomena. Consider the case of quantum spin, the label affixed to the angular momentum that is intrinsic to elementary particles. The term is confusing because such particles cannot physically spinif they were simply ever twirling subatomic gyroscopes, their rotation would be impossibly fast, well in excess of the speed of light. But quantum spin is crucial to accounting for the observed behavior of electrons and other particles: although they may not actually be physically spinning, the particles are clearly doing something. Exactly what that something is can be captured with utmost accuracy by mathematical equations, but its causal physical basis remains murky. One relatively new (and highly controversial) hypothesis appeals to quantum field theory for an explanation. In this proposal, particles (which arise from fluctuations in quantum fields) gain their spin (angular momentum) from their originating fields, somewhat like a turbine being spun by the wind. If this is where the angular momentum resides, Scientific Americans article on the idea noted, the problem of an electron spinning faster than the speed of light vanishes; the region of the field carrying an electrons spin is far larger than the purportedly pointlike electron itself.

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6 Times Quantum Physics Blew Our Minds in 2022

The Primacy of Doubt: From Quantum Physics to Climate Change, How the Science of Uncertainty Can Help Us Understand Our Chaotic World  Next Big Idea Club Magazine

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The Primacy of Doubt: From Quantum Physics to Climate Change, How the Science of Uncertainty Can Help Us Understand Our Chaotic World - Next Big Idea...

Set of statements that attempt to explain how quantum mechanics informs our understanding of nature

An interpretation of quantum mechanics is an attempt to explain how the mathematical theory of quantum mechanics might correspond to experienced reality. Although quantum mechanics has held up to rigorous and extremely precise tests in an extraordinarily broad range of experiments, there exist a number of contending schools of thought over their interpretation. These views on interpretation differ on such fundamental questions as whether quantum mechanics is deterministic or stochastic, which elements of quantum mechanics can be considered real, and what the nature of measurement is, among other matters.

Despite nearly a century of debate and experiment, no consensus has been reached among physicists and philosophers of physics concerning which interpretation best "represents" reality.[1][2]

The definition of quantum theorists' terms, such as wave function and matrix mechanics, progressed through many stages. For instance, Erwin Schrdinger originally viewed the electron's wave function as its charge density smeared across space, but Max Born reinterpreted the absolute square value of the wave function as the electron's probability density distributed across space.[3]:2433

The views of several early pioneers of quantum mechanics, such as Niels Bohr and Werner Heisenberg, are often grouped together as the "Copenhagen interpretation", though physicists and historians of physics have argued that this terminology obscures differences between the views so designated.[3][4] Copenhagen-type ideas were never universally embraced, and challenges to a perceived Copenhagen orthodoxy gained increasing attention in the 1950s with the pilot-wave interpretation of David Bohm and the many-worlds interpretation of Hugh Everett III.[3][5][6]

The physicist N. David Mermin once quipped, "New interpretations appear every year. None ever disappear."[7] As a rough guide to development of the mainstream view during the 1990s and 2000s, a "snapshot" of opinions was collected in a poll by Schlosshauer et al. at the "Quantum Physics and the Nature of Reality" conference of July 2011.[8]The authors reference a similarly informal poll carried out by Max Tegmark at the "Fundamental Problems in Quantum Theory" conference in August 1997. The main conclusion of the authors is that "the Copenhagen interpretation still reigns supreme", receiving the most votes in their poll (42%), besides the rise to mainstream notability of the many-worlds interpretations: "The Copenhagen interpretation still reigns supreme here, especially if we lump it together with intellectual offsprings such as information-based interpretations and the Quantum Bayesian interpretation. In Tegmark's poll, the Everett interpretation received 17% of the vote, which is similar to the number of votes (18%) in our poll."

Some concepts originating from studies of interpretations have found more practical application in quantum information science.[9][10]

More or less, all interpretations of quantum mechanics share two qualities:

Two qualities vary among interpretations:

In philosophy of science, the distinction of knowledge versus reality is termed epistemic versus ontic. A general law is a regularity of outcomes (epistemic), whereas a causal mechanism may regulate the outcomes (ontic). A phenomenon can receive interpretation either ontic or epistemic. For instance, indeterminism may be attributed to limitations of human observation and perception (epistemic), or may be explained as a real existing maybe encoded in the universe (ontic). Confusing the epistemic with the ontic, if for example one were to presume that a general law actually "governs" outcomesand that the statement of a regularity has the role of a causal mechanismis a category mistake.

In a broad sense, scientific theory can be viewed as offering scientific realismapproximately true description or explanation of the natural worldor might be perceived with antirealism. A realist stance seeks the epistemic and the ontic, whereas an antirealist stance seeks epistemic but not the ontic. In the 20th century's first half, antirealism was mainly logical positivism, which sought to exclude unobservable aspects of reality from scientific theory.

Since the 1950s, antirealism is more modest, usually instrumentalism, permitting talk of unobservable aspects, but ultimately discarding the very question of realism and posing scientific theory as a tool to help humans make predictions, not to attain metaphysical understanding of the world. The instrumentalist view is carried by the famous quote of David Mermin, "Shut up and calculate", often misattributed to Richard Feynman.[11]

Other approaches to resolve conceptual problems introduce new mathematical formalism, and so propose alternative theories with their interpretations. An example is Bohmian mechanics, whose empirical equivalence with the three standard formalismsSchrdinger's wave mechanics, Heisenberg's matrix mechanics, and Feynman's path integral formalismhas been demonstrated.

The Copenhagen interpretation is a collection of views about the meaning of quantum mechanics principally attributed to Niels Bohr and Werner Heisenberg. It is one of the oldest attitudes towards quantum mechanics, as features of it date to the development of quantum mechanics during 19251927, and it remains one of the most commonly taught.[14][15] There is no definitive historical statement of what is the Copenhagen interpretation, and there were in particular fundamental disagreements between the views of Bohr and Heisenberg.[16][17] For example, Heisenberg emphasized a sharp "cut" between the observer (or the instrument) and the system being observed,[18]:133 while Bohr offered an interpretation that is independent of a subjective observer or measurement or collapse, which relies on an "irreversible" or effectively irreversible process which imparts the classical behavior of "observation" or "measurement".[19][20][21][22]

Features common to Copenhagen-type interpretations include the idea that quantum mechanics is intrinsically indeterministic, with probabilities calculated using the Born rule, and the principle of complementarity, which states that objects have certain pairs of complementary properties which cannot all be observed or measured simultaneously. Moreover, the act of "observing" or "measuring" an object is irreversible, no truth can be attributed to an object except according to the results of its measurement. Copenhagen-type interpretations hold that quantum descriptions are objective, in that they are independent of physicists' mental arbitrariness.[23]:8590 The statistical interpretation of wavefunctions due to Max Born differs sharply from Schrdinger's original intent, which was to have a theory with continuous time evolution and in which wavefunctions directly described physical reality.[3]:2433[24]

The many-worlds interpretation is an interpretation of quantum mechanics in which a universal wavefunction obeys the same deterministic, reversible laws at all times; in particular there is no (indeterministic and irreversible) wavefunction collapse associated with measurement. The phenomena associated with measurement are claimed to be explained by decoherence, which occurs when states interact with the environment. More precisely, the parts of the wavefunction describing observers become increasingly entangled with the parts of the wavefunction describing their experiments. Although all possible outcomes of experiments continue to lie in the wavefunction's support, the times at which they become correlated with observers effectively "split" the universe into mutually unobservable alternate histories.

Quantum informational approaches[25][26] have attracted growing support.[27][8] They subdivide into two kinds.[28]

The state is not an objective property of an individual system but is that information, obtained from a knowledge of how a system was prepared, which can be used for making predictions about future measurements....A quantum mechanical state being a summary of the observer's information about an individual physical system changes both by dynamical laws, and whenever the observer acquires new information about the system through the process of measurement. The existence of two laws for the evolution of the state vector...becomes problematical only if it is believed that the state vector is an objective property of the system...The "reduction of the wavepacket" does take place in the consciousness of the observer, not because of any unique physical process which takes place there, but only because the state is a construct of the observer and not an objective property of the physical system.[31]

The essential idea behind relational quantum mechanics, following the precedent of special relativity, is that different observers may give different accounts of the same series of events: for example, to one observer at a given point in time, a system may be in a single, "collapsed" eigenstate, while to another observer at the same time, it may be in a superposition of two or more states. Consequently, if quantum mechanics is to be a complete theory, relational quantum mechanics argues that the notion of "state" describes not the observed system itself, but the relationship, or correlation, between the system and its observer(s). The state vector of conventional quantum mechanics becomes a description of the correlation of some degrees of freedom in the observer, with respect to the observed system. However, it is held by relational quantum mechanics that this applies to all physical objects, whether or not they are conscious or macroscopic. Any "measurement event" is seen simply as an ordinary physical interaction, an establishment of the sort of correlation discussed above. Thus the physical content of the theory has to do not with objects themselves, but the relations between them.[32][33]

QBism, which originally stood for "quantum Bayesianism", is an interpretation of quantum mechanics that takes an agent's actions and experiences as the central concerns of the theory. This interpretation is distinguished by its use of a subjective Bayesian account of probabilities to understand the quantum mechanical Born rule as a normative addition to good decision-making. QBism draws from the fields of quantum information and Bayesian probability and aims to eliminate the interpretational conundrums that have beset quantum theory.

QBism deals with common questions in the interpretation of quantum theory about the nature of wavefunction superposition, quantum measurement, and entanglement.[34][35] According to QBism, many, but not all, aspects of the quantum formalism are subjective in nature. For example, in this interpretation, a quantum state is not an element of realityinstead it represents the degrees of belief an agent has about the possible outcomes of measurements. For this reason, some philosophers of science have deemed QBism a form of anti-realism.[36][37] The originators of the interpretation disagree with this characterization, proposing instead that the theory more properly aligns with a kind of realism they call "participatory realism", wherein reality consists of more than can be captured by any putative third-person account of it.[38][39]

The consistent histories interpretation generalizes the conventional Copenhagen interpretation and attempts to provide a natural interpretation of quantum cosmology. The theory is based on a consistency criterion that allows the history of a system to be described so that the probabilities for each history obey the additive rules of classical probability. It is claimed to be consistent with the Schrdinger equation.

According to this interpretation, the purpose of a quantum-mechanical theory is to predict the relative probabilities of various alternative histories (for example, of a particle).

The ensemble interpretation, also called the statistical interpretation, can be viewed as a minimalist interpretation. That is, it claims to make the fewest assumptions associated with the standard mathematics. It takes the statistical interpretation of Born to the fullest extent. The interpretation states that the wave function does not apply to an individual system for example, a single particle but is an abstract statistical quantity that only applies to an ensemble (a vast multitude) of similarly prepared systems or particles. In the words of Einstein:

The attempt to conceive the quantum-theoretical description as the complete description of the individual systems leads to unnatural theoretical interpretations, which become immediately unnecessary if one accepts the interpretation that the description refers to ensembles of systems and not to individual systems.

Einstein in Albert Einstein: Philosopher-Scientist, ed. P.A. Schilpp (Harper & Row, New York)

The most prominent current advocate of the ensemble interpretation is Leslie E. Ballentine, professor at Simon Fraser University, author of the text book Quantum Mechanics, A Modern Development.

The de BroglieBohm theory of quantum mechanics (also known as the pilot wave theory) is a theory by Louis de Broglie and extended later by David Bohm to include measurements. Particles, which always have positions, are guided by the wavefunction. The wavefunction evolves according to the Schrdinger wave equation, and the wavefunction never collapses. The theory takes place in a single spacetime, is non-local, and is deterministic. The simultaneous determination of a particle's position and velocity is subject to the usual uncertainty principle constraint. The theory is considered to be a hidden-variable theory, and by embracing non-locality it satisfies Bell's inequality. The measurement problem is resolved, since the particles have definite positions at all times.[40] Collapse is explained as phenomenological.[41]

Quantum Darwinism is a theory meant to explain the emergence of the classical world from the quantum world as due to a process of Darwinian natural selection induced by the environment interacting with the quantum system; where the many possible quantum states are selected against in favor of a stable pointer state. It was proposed in 2003 by Wojciech Zurek and a group of collaborators including Ollivier, Poulin, Paz and Blume-Kohout. The development of the theory is due to the integration of a number of Zurek's research topics pursued over the course of twenty-five years including: pointer states, einselection and decoherence.

The transactional interpretation of quantum mechanics (TIQM) by John G. Cramer is an interpretation of quantum mechanics inspired by the WheelerFeynman absorber theory.[42] It describes the collapse of the wave function as resulting from a time-symmetric transaction between a possibility wave from the source to the receiver (the wave function) and a possibility wave from the receiver to source (the complex conjugate of the wave function). This interpretation of quantum mechanics is unique in that it not only views the wave function as a real entity, but the complex conjugate of the wave function, which appears in the Born rule for calculating the expected value for an observable, as also real.

Objective-collapse theories differ from the Copenhagen interpretation by regarding both the wave function and the process of collapse as ontologically objective (meaning these exist and occur independent of the observer). In objective theories, collapse occurs either randomly ("spontaneous localization") or when some physical threshold is reached, with observers having no special role. Thus, objective-collapse theories are realistic, indeterministic, no-hidden-variables theories. Standard quantum mechanics does not specify any mechanism of collapse; QM would need to be extended if objective collapse is correct. The requirement for an extension to QM means that objective collapse is more of a theory than an interpretation. Examples include

In his treatise The Mathematical Foundations of Quantum Mechanics, John von Neumann deeply analyzed the so-called measurement problem. He concluded that the entire physical universe could be made subject to the Schrdinger equation (the universal wave function). He also described how measurement could cause a collapse of the wave function.[44] This point of view was prominently expanded on by Eugene Wigner, who argued that human experimenter consciousness (or maybe even dog consciousness) was critical for the collapse, but he later abandoned this interpretation.[45][46]

Quantum logic can be regarded as a kind of propositional logic suitable for understanding the apparent anomalies regarding quantum measurement, most notably those concerning composition of measurement operations of complementary variables. This research area and its name originated in the 1936 paper by Garrett Birkhoff and John von Neumann, who attempted to reconcile some of the apparent inconsistencies of classical boolean logic with the facts related to measurement and observation in quantum mechanics.

Modal interpretations of quantum mechanics were first conceived of in 1972 by Bas van Fraassen, in his paper "A formal approach to the philosophy of science". Van Fraassen introduced a distinction between a dynamical state, which describes what might be true about a system and which always evolves according to the Schrdinger equation, and a value state, which indicates what is actually true about a system at a given time. The term "modal interpretation" now is used to describe a larger set of models that grew out of this approach. The Stanford Encyclopedia of Philosophy describes several versions, including proposals by Kochen, Dieks, Clifton, Dickson, and Bub.[47] According to Michel Bitbol, Schrdinger's views on how to interpret quantum mechanics progressed through as many as four stages, ending with a non-collapse view that in respects resembles the interpretations of Everett and van Fraassen. Because Schrdinger subscribed to a kind of post-Machian neutral monism, in which "matter" and "mind" are only different aspects or arrangements of the same common elements, treating the wavefunction as ontic and treating it as epistemic became interchangeable.[48]

Time-symmetric interpretations of quantum mechanics were first suggested by Walter Schottky in 1921.[49][50] Several theories have been proposed which modify the equations of quantum mechanics to be symmetric with respect to time reversal.[51][52][53][54][55][56] (See WheelerFeynman time-symmetric theory.) This creates retrocausality: events in the future can affect ones in the past, exactly as events in the past can affect ones in the future. In these theories, a single measurement cannot fully determine the state of a system (making them a type of hidden-variables theory), but given two measurements performed at different times, it is possible to calculate the exact state of the system at all intermediate times. The collapse of the wavefunction is therefore not a physical change to the system, just a change in our knowledge of it due to the second measurement. Similarly, they explain entanglement as not being a true physical state but just an illusion created by ignoring retrocausality. The point where two particles appear to "become entangled" is simply a point where each particle is being influenced by events that occur to the other particle in the future.

Not all advocates of time-symmetric causality favour modifying the unitary dynamics of standard quantum mechanics. Thus a leading exponent of the two-state vector formalism, Lev Vaidman, states that the two-state vector formalism dovetails well with Hugh Everett's many-worlds interpretation.[57]

As well as the mainstream interpretations discussed above, a number of other interpretations have been proposed which have not made a significant scientific impact for whatever reason. These range from proposals by mainstream physicists to the more occult ideas of quantum mysticism.

The most common interpretations are summarized in the table below. The values shown in the cells of the table are not without controversy, for the precise meanings of some of the concepts involved are unclear and, in fact, are themselves at the center of the controversy surrounding the given interpretation. For another table comparing interpretations of quantum theory, see reference.[58]

No experimental evidence exists that distinguishes among these interpretations. To that extent, the physical theory stands, and is consistent with itself and with reality; difficulties arise only when one attempts to "interpret" the theory. Nevertheless, designing experiments which would test the various interpretations is the subject of active research.

Most of these interpretations have variants. For example, it is difficult to get a precise definition of the Copenhagen interpretation as it was developed and argued about by many people.

Although interpretational opinions are openly and widely discussed today, that was not always the case. A notable exponent of a tendency of silence was Paul Dirac who once wrote: "The interpretation of quantum mechanics has been dealt with by many authors, and I do not want to discuss it here. I want to deal with more fundamental things."[67] This position is not uncommon among practitioners of quantum mechanics.[68] Others, like Nico van Kampen and Willis Lamb, have openly criticized non-orthodox interpretations of quantum mechanics.[69][70]

Almost all authors below are professional physicists.

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