1935, at the Institute for Advanced Studies in Princeton, Einstein, postdoctoral fellow Rosen and researcher Podolski collaborated to complete the paper "Can the quantum mechanical description of physical reality be considered complete? And published this paper in the May issue of Physical Review. This is the first paper to discuss the counterintuitive prediction of strongly correlated systems with quantum mechanics theory. In this paper, they expounded the EPR paradox and tried to discuss the incompleteness of quantum mechanics through a thinking experiment. They did not further study the properties of quantum entanglement. [2]
After reading the EPR paper, Schrodinger has many ideas. He wrote a letter to Einstein in German, in which he used the term "reverse time" for the first time? Nkung (he translated it as "entanglement") describes the relationship between two temporarily coupled particles in EPR thought experiments, which remains after they are no longer coupled. Soon after, Schrodinger published an important paper, defined the term "quantum entanglement" and explored related concepts. Schrodinger realized the importance of this concept. He showed that quantum entanglement is not only a very interesting property of quantum mechanics, but also a characteristic property of quantum mechanics. Quantum entanglement makes a complete cut between quantum mechanics and classical thought. Like Einstein, Schrodinger is not satisfied with the concept of quantum entanglement, because quantum entanglement seems to violate the speed limit set for information transmission in relativity. Later, Einstein even laughed at quantum entanglement as a ghostly action at a distance.
EPR papers have obviously aroused the interest of many physicists and inspired them to explore the basic theory of quantum mechanics. But beyond that, physicists believe that this topic has nothing to do with modern quantum mechanics. After a long time, the physics circle did not pay special attention to this topic, nor did it find any major defects in EPR papers. EPR thesis attempts to establish the theory of localized hidden variables to replace the theory of quantum mechanics. 1964, john bell's paper shows that the prediction of quantum mechanics for EPR thought experiment is obviously different from the theory of localized hidden variables. Generally speaking, if the spins of two particles along different axes are measured, the statistical correlation obtained by quantum mechanics is much stronger than that obtained by the theory of localized hidden variables. Bell inequality gives this difference qualitatively, which should be detected by experiments. So physicists have done many experiments to test Bell's inequality.
The first quantum entanglement image
1972, John krause and Stuart Freedman first completed this test. 1982 Alan Aspe's doctoral thesis is about this kind of test experiment. The experimental results obtained by them are consistent with the predictions of quantum mechanics, but inconsistent with the predictions of the theory of localized hidden variables, thus confirming that the theory of localized hidden variables is not valid. However, there are loopholes in every related experiment, which leads to the correctness of the experiment being questioned. More accurate experiments need to be completed before making a summary.
Over the years, many research results have helped to apply these super correlations to the possibility of transmitting information, which has led to the successful development of quantum cryptography. The most famous ones are BB84 protocol invented by Charles Bennett and Gilles Blasart and E9 1 protocol invented by Artur eckert.
On June 6, 20 17, the quantum science experimental satellite Mozi was successfully realized for the first time. Two quantum entangled photons can still maintain their quantum entangled state when they are distributed over a distance of 1200 km.
On April 25th, 20 18, the experimental team led by Mika Sillanp, a professor at Aalto University in Finland, successfully entangled two separate vibrating eardrums. The width of each eardrum is only 15 micron, which is about the width of hair. It consists of 10 aluminum atoms. Through the superconducting microwave circuit, the interaction between the two eardrums lasted about 30 minutes at near absolute temperature (-273. 15 degrees Celsius). This experiment demonstrates macroscopic quantum entanglement.
Suppose a zero spin neutral π meson decays into an electron and a positron. These two decay products move in opposite directions. When the electron moves to region A, the observer "Alice" will observe the spin of the electron along a certain axis; The positron moves to region B, where the observer "Bob" will also observe the spin of the positron along the same axis. Before measurement, these two entangled particles * * * together form an "entangled state" with zero spin, which is the superposition of two product states, and is represented by Dirac symbol [3].
Among them, it indicates that the spin of particles is upward spin or downward spin, respectively.
The first term in brackets indicates that the spin of electrons is topspin, and only if the spin of positrons is downspin; The second term indicates that the electron spin is downward if and only if the positron spin is upward. The two situations are superimposed, and each situation can happen. Not sure what will happen. So electrons and positrons are entangled to form entangled states. Without measurement, it is impossible to know the spin of either of these two particles. According to Copenhagen's explanation, this attribute does not exist. The two particles in this singlet state are inversely related, and the spins of the two particles are measured separately. If the spin of an electron is upward, the spin of a positron is downward, and vice versa. If the electron spins down, the positron spins up, and vice versa. Quantum mechanics can't predict which set of values it is, but the probability of getting any set of values is 50%.
The spins of particles along different axes are incompatible with each other, and it is a basic theory of quantum mechanics that the measurements of these incompatible observables must not get clear results at the same time. In classical mechanics, this basic theory is meaningless. Theoretically, the properties of any particle can be measured to any accuracy. Bell theorem refers to a fact that has been tested by experiments, that is, the results obtained by measuring two incompatible observables do not obey Bell inequality. Therefore, basically, quantum entanglement is a non-classical phenomenon. [4]
The maintenance of uncertainty principle must rely on quantum entanglement mechanism. For example, imagine a previous case of zero-spin neutral π meson decay. The two decay products move in opposite directions, and the position of electrons and the momentum of positrons are measured respectively. If the quantum entanglement mechanism does not exist, the position and momentum of two particles can be predicted by conservation law, which violates the uncertainty principle. Due to the quantum entanglement mechanism, the position and momentum of particles obey the uncertainty principle.
The physical properties of two entangled particles are measured from two reference frames moving at relativistic speed. Although the time sequence of two particles measured in each reference frame is different, the experimental data still violate Bell inequality, and the quantum correlation of two entangled particles can still be reliably reproduced.