User:Johnjbarton/sandbox/quantum entanglement concept

History

Background: History of quantum mechanics

Article headline regarding the Einstein–Podolsky–Rosen (EPR) paradox paper, in the May 4, 1935 issue of The New York Times

Albert Einstein and Niels Bohr engaged in a long-running collegial dispute about the meaning of quantum mechanics, now known as the Bohr–Einstein debates. During these debates, Einstein introduced a thought experiment about a box that emits a photon. He noted that the experimenter's choice of what measurement to make upon the box will change what can be predicted about the photon, even if the photon is very far away. This argument, which Einstein had formulated by 1931, was an early recognition of the phenomenon that would later be called entanglement. That same year, Hermann Weyl observed in his textbook on group theory and quantum mechanics that quantum systems made of multiple interacting pieces exhibit a kind of Gestalt. In 1932, Erwin Schrödinger wrote down the defining equations of quantum entnglement but set them aside, unpublished. In 1935, Einstein, Boris Podolsky and Nathan Rosen published a paper on what is now known as the Einstein–Podolsky–Rosen (EPR) paradox, a thought experiment that attempted to show that "the quantum-mechanical description of physical reality given by wave functions is not complete." Their concept had two systems interact, then separate, and they showed that afterwards quantum mechanics cannot describe the two systems individually.

Shortly after this paper appeared, Erwin Schrödinger wrote a letter to Einstein in German in which he used the word Verschränkung (translated by himself as entanglement) "to describe the correlations between two particles that interact and then separate, as in the EPR experiment." Schrödinger followed up with a full paper defining and discussing the notion of entanglement, saying "I would not call [entanglement] one but rather the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought." Like Einstein, Schrödinger was dissatisfied with the concept of entanglement, reasoning that "we do not have a quantum mechanics that takes into account relativity theory". Einstein later famously derided entanglement as "spukhafte Fernwirkung" or "spooky action at a distance."

In 1946, John Archibald Wheeler suggested studying the polarization of pairs of gamma-ray photons produced by electron-positron annihilation. Chien-Shiung Wu and I. Shaknov carried out this experiment in 1949, thereby demonstrating that the entangled particle pairs considered by EPR could be created in the laboratory.

Despite Schrödinger's claim of its importance, little work on entanglement was published for decades after his paper was published. In 1964 John S. Bell showed that quantum entanglement predicted a different kind of correlation, one not discussed in the EPR paper, and this correlation cannot be reproduced by any local hidden variable theory alternative to quantum mechanics. His results became known as Bell's inequality and it has been subjected to numerous experimental tests. An early experimental breakthrough was due to Carl Kocher, who already in 1967 presented an apparatus in which two photons successively emitted from a calcium atom were shown to be entangled – the first case of entangled visible light. Stuart Freedman and John Clauser built on Kocher's experiment and found a violation of the local realism limit in 1972. Alain Aspect and his team improved upon this test in 1982, and many other experiments followed.

While Bell actively discouraged students from pursuing work like his as too esoteric, after a talk at Oxford a student named Artur Ekert suggested that these entanglement correlations could be used as a resource for communication. Ekert followed up by publishing a quantum key distribution protocol called E91 that uses the violation of a Bell's inequality as a proof of security.

In 1992, the entanglement concept was leveraged to propose quantum teleportation, an effect that was realized experimentally in 1997.

Beginning in the mid-1990's, Anton Zeilinger used the generation of entanglement via parametric down-conversion to develop entanglement swapping. and demonstrate quantum cryptography with entangled photons.

In 2022, the Nobel Prize in Physics was awarded to Aspect, Clauser, and Zeilinger "for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science".

Concept

Wavefunctions

The wavefunction for two non-interacting particles can be written as the product of two one-particle wavefunctions: $$\Psi (\mathbf {r} _{1},\mathbf {r} _{2},t)=\Psi _{a}(\mathbf {r} _{1},t)\Psi _{b}(\mathbf {r} _{2},t)$$ In this case particle 1 can be said to be in state a and particle 2 in state b. Other wavefunctions built up from linear combinations (also known as superpositions) of single-particle wavefunctions cannot be factored as a product of states, for example: $$\Psi '(\mathbf {r} _{1},\mathbf {r} _{2},t)={\frac {4}{5}}\Psi _{a}(\mathbf {r} _{1},t)\Psi _{b}(\mathbf {r} _{2},t)+{\frac {3}{5}}\Psi _{c}(\mathbf {r} _{1},t)\Psi _{d}(\mathbf {r} _{2},t)$$ These are entangled states, that is to say, they do not represent individual particles but an inseparable whole. Measurements on such a system results in characteristics of only one of the terms in the sum, in proportion to the square of its weight in the combination. For the above state, the predicted energy measurements can be summarized as:

Predicted energy measurements

Probability	Measurement 1	Measurement 2
36%	$E_{a}$	$E_{b}$
64%	$E_{c}$	$E_{d}$

The first measurement gives one of the values randomly, but the second measurement is correlated with the first one. In entanglement, measurements of properties become correlated.

Spin systems

Entanglement is often introduced using spin states. Descriptions often use two experimentalists, Alice and Bob, each with their own lab and spin-measurement equipment. Working independently, Alice and Bob might measure a spin in their labs as spin up or spin down, giving a total of four outcomes or degrees of freedom. A composite space of states describes systems that include the spins in the two labs together. Using the bra–ket notation, Alice measuring spin up while Bob measures spin down in the same experiment could be written as $|\uparrow \downarrow \rangle$. When the states in the two labs interact before measurement, new non-classical entangled states arise that cannot be described as two independent spins. The total quantum space has six degrees of freedom. The entangled singlet state: $${\frac {1}{\sqrt {2}}}\left(|\uparrow \downarrow \rangle -|\downarrow \uparrow \rangle \right)$$ is an example of a state of the composite system that has no analog among the states that represent results in the two labs independently. This singlet state is an example of a maximally entangled state. The entanglement means Alice and Bob will measure correlated spin values: if Bob measures spin up in his lab on Alpha Centauri, Alice in Palo Alto measures spin down. However, nothing happens to Alice's model immediately after Bob's measurement. Alice's measurements are correlated with Bob's but they have no knowledge of the correlation until they communicate with each other.

Bell states

Bell states are four entangled basis states that describe a two particle system: $$|\psi ^{\pm }\rangle ={\frac {1}{\sqrt {2}}}\left(|0\rangle |1|\rangle \pm |1\rangle |0|\rangle \right)$$ $$|\phi ^{\pm }\rangle ={\frac {1}{\sqrt {2}}}\left(|0\rangle |0|\rangle \pm |1\rangle |1|\rangle \right)$$ Also called EPR states, these states are maximally entangled and a measurement on these states is equally likely to find a subsystem in $|0\rangle$ as to find $|1\rangle$. Bell states play an important role in quantum information and communication theory.

Multipartite systems

An entangled state of three particles called the Greenberger–Horne–Zeilinger state allows succinct, deterministic evidence against any local hidden-variable theory. This state can be written as a wavefunction: $$\psi =f(\mathbf {r} _{1},\mathbf {r} _{2},\mathbf {r} _{3}){\frac {|1,1,1\rangle +|-1,-1,-1\rangle }{\sqrt {2}}}$$ where the function f represents the physical separation of three measurements on the state and the numbers in angle brackets represent spin states identified by their eigenvalue along the z axis. At each location 1, 2, or 3, spin operators along other axes x and y have these effects: $$\sigma _{x}|1\rangle =|-1\rangle \ \ \ \sigma _{y}|1\rangle =i|-1\rangle$$ $$\sigma _{x}|-1\rangle =|1\rangle \ \ \ \sigma _{y}|-1\rangle =-i|-1\rangle$$ If measurements are along the y axis are applied at locations 2 and 3 and along x at location 1, the wavefunction is an eigenstate of the combined operators eigenvalue 1: $$\sigma _{1x}\sigma _{2y}\sigma _{3y}\psi =\psi$$ If however all three measurements are the x axis, the eigenvalue is -1: $$\sigma _{1x}\sigma _{2x}\sigma _{3x}\psi =-\psi .$$ A model for this state as independent particles with inherent spin properties would predict that the measurement at location 1 would depend only the the particle, not on the measurements at the other 2 locations. Such a model would predict a +1, exactly the opposite the observed value.

Mermin's three state thought experiment device

N. David Mermin demonstrated the non-classical quantum entanglement results from a three particle state with a thought experiment that used three identical detectors. Each detector flashed red or green depending on the eigenvalue (1, -1); each detector has a switch corresponding to the axis measured (x, y). A source in the middle emits particles into all three detectors. Mermin shows how the results that correspond to quantum mechanics cannot be predicted if you assume that particles from the source contain instructions on how to respond to the switch settings.

Multipartite states are useful in quantum teleportation and entanglement swapping, and are used in quantum computing systems.

Quantum teleportation and entanglement swapping

If Alice and Bob share a Bell state, Alice can tell Bob over a telephone call how to reproduce a quantum state, $|\Psi \rangle$, she has in her lab. Alice performs measurements on the $|\Psi \rangle$ quantum state using the Bell state and tells Bob the results. Bob operates on his Bell state to create a state that matches Alice's $|\Psi \rangle$. The $|\Psi \rangle$ is said to be "quantum teleported" through the Bell state quantum channel with the understanding that classical communications limited to the speed of light are required.

Entanglement states from independent sources can become entangled through Bell state analysis.

Entanglement swapping is a form of quantum teleportation. Four particles are involved, arriving at locations labeled Alice, Bob, Clare, and David. Two independent and separate sources of entanglement each send two particle states with one particle from each source, for Bob and Clare, entering a Bell state analysis. Together Bob and Clare project onto a Bell state basis and tell Alice and Bob the results. Alice and Bob they can operate on their states to find that their particles from independent sources are entangled.

Practical complications

Simple descriptions of entanglement use pure states. A general theoretical analysis also involves

• mixed states described by a density matrix,
• exchange symmetry required by indistinguishable particles, and
• consideration of the effects of relativity

Paradox

Two quantum paradoxes relate to entanglement. These arise from a misuse of quantum concepts.

The first paradox is known as the EPR paradox. A singlet state, as discussed in the spin example but with the component states marked A for Alice and B for Bob, $${\frac {1}{\sqrt {2}}}\left(|\uparrow _{A}\downarrow _{B}\rangle -|\downarrow _{A}\uparrow _{B}\rangle \right)$$ shows that if Alice measures spin up, Bob will measure spin down: their results are correlated. Since the particles Alice and Bob measure came from the same source, Einstein, Podolsky, and Rosen argued that origin of the correlation lies inside those particles. John Stewart Bell showed that the EPR reasoning meant that the particles would have simultaneous properties of spin along other axes, properties that cannot be known according to quantum mechanics. Experiments measure correlations between Alice's and Bob's measurements of spin along these other axes, correlations that the EPR reasoning do not predict. The intuitive EPR reasoning is not correct. Another way to say this is that the entangled singlet state completely specifies the composite system but contains no information about the components. There are no hidden variables to explain the correlations. The component information only appears after a measurement.

Without local hidden variables to explain the correlation, the alternative would seem to be "spooky action at a distance". In this model, Alice's measurement seems to causes action that instantaneously alters the state even if Bob is far away. However, the quantum mechanics model does not account for the principle of relativity and this instantaneous change is an illusion. The results on Alice's end have no effect on Bob's knowledge. Alice's results are not known to Bob. Nothing at his distant location has changed. His only options are to measure the state himself or wait to hear from Alice using communications at the speed of light.

Emergent time

Main article: Problem of time

There is a fundamental conflict, referred to as the problem of time, between the way the concept of time is used in quantum mechanics, and the role it plays in general relativity. In standard quantum theories time acts as an independent background through which states evolve, while general relativity treats time as a dynamical variable which relates directly with matter. Part of the effort to reconcile these approaches to time results in the Wheeler–DeWitt equation, which predicts the state of the universe is timeless or static, contrary to ordinary experience. Work started by Don Page and William Wootters suggests that the universe appears to evolve for observers on the inside because of energy entanglement between an evolving system and a clock system, both within the universe. In this way the overall system can remain timeless while parts experience time via entanglement. The issue remains an open question closely related to attempts at theories of quantum gravity.

Emergent gravity

In general relativity gravity arises from the curvature of spacetime and that curvature derives from the distribution of matter. However, matter is governed by quantum mechanics. Integration of these two theories faces many problems. In an (unrealistic) model space called the anti-de Sitter space, the AdS/CFT correspondence allows a quantum gravitational system to be related to a quantum field theory without gravity. Using this correspondence, Mark Van Raamsdonk suggested that spacetime arises as an emergent phenomenon of the quantum degrees of freedom that are entangled and live in the boundary of the spacetime.