The predictions of quantum mechanics are sometimes difficult to relate to ideas about the classical world. While the position and momentum of a classical particle can be measured simultaneously, in the quantum case, you can only know the probability of finding a particle in one state or another. Moreover, quantum theory states that when two systems are entangled, measuring the state of one of them instantly affects the other. In 2015, three groups of physicists made significant progress in understanding the nature of quantum entanglement and teleportation. Physics Today and Lenta.ru talk about the achievements of scientists.
Albert Einstein disagreed with the probabilistic interpretation of quantum mechanics. It was in this connection that he said that "God does not play dice" (to which the Danish physicist Niels Bohr later replied that it was not for Einstein to decide what to do with God). The German scientist did not accept the uncertainty inherent in the microworld, and considered classical determinism to be correct. The creator of the general theory of relativity believed that when describing the microworld, quantum mechanics does not take into account some hidden variables, without which the quantum theory itself is incomplete. The scientist suggested looking for hidden parameters when measuring a quantum state with a classical device: this process involves a change in the first by the second, and Einstein considered it possible to experiment where there is no such change.
Since then, scientists have been trying to determine if hidden variables exist in quantum mechanics or if it was Einstein's invention. The problem of hidden variables was formalized in 1964 by the British theoretical physicist John Bell. He proposed the idea of an experiment in which the presence of any hidden parameter in the system can be found out by conducting a statistical analysis of a series of special experiments. The experiment was like this. An atom was placed in an external field, simultaneously emitting a pair of photons, which scattered in opposite directions. The task of the experimenters is to carry out multiple measurements of the direction of the photon spins.
This would make it possible to collect the necessary statistics and, using Bell's inequalities, which are a mathematical description of the presence of hidden parameters in quantum mechanics, check Einstein's point of view. The main difficulty lay in the practical implementation of the experiment, which later physicists managed to reproduce. The researchers have shown that there are most likely no hidden parameters in quantum mechanics. In the meantime, there were two loopholes in theory (location and detection) that could prove Einstein was right. In general, there are more loopholes. The experiments of 2015 closed them down and confirmed that there is most likely no local realism in the microcosm.
"Spooky action" between Bob and Alice
Image: JPL-Caltech / NASA
We are talking about the experiments of three groups of physicists: from the Delft Technical University in the Netherlands, the National Institute of Standards and Technology in the USA and the University of Vienna in Austria. Scientists' experiments not only confirmed the completeness of quantum mechanics and the absence of hidden parameters in it, but also opened up new possibilities of quantum cryptography - a method of encrypting information (protecting it) using quantum entanglement using quantum protocols - and led to the creation of yet unbreakable algorithms for generating random numbers.
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Quantum entanglement is a phenomenon in which the quantum states of particles (for example, the spin of an electron or the polarization of a photon), separated by a distance from each other, cannot be described independently. The procedure for measuring the state of one particle leads to a change in the state of another. In a typical quantum entanglement experiment, spaced apart interacting agents - Alice and Bob - each possess one particle (photons or electrons) from a pair of entangled ones. Measurement of a particle by one of the agents, for example, Alice, correlates with the state of the other, although Alice and Bob do not know in advance about each other's manipulations.
This means that the particles somehow store information about each other, and do not exchange it, say, at light speed using some fundamental interaction known to science. Albert Einstein called it "spooky action at a distance." Entangled particles violate the principle of locality, according to which the state of an object can only be influenced by its immediate environment. This contradiction is associated with the Einstein-Podolsky-Rosen paradox (assuming the above-mentioned incompleteness of quantum mechanics and the presence of hidden parameters) and constitutes one of the main conceptual difficulties (which, however, is no longer considered a paradox) of quantum mechanics (at least in its Copenhagen interpretation).
Scheme of the experiment of the Dutch scientists
Photo: arXiv.org
Proponents of local realism argue that only local variables can affect particles, and the correlation between Alice and Bob's particles is carried out using some hidden method that scientists still do not know. The task of scientists was to refute this possibility experimentally, in particular to prevent the propagation of a hidden signal from one agent to another (assuming it moves at the speed of light in a vacuum - the maximum possible in nature), and thus show that a change in the quantum state of the second particle has occurred before the latent signal from the first particle could reach the second.
In practice, this means placing Bob and Alice at a considerable distance from each other (at least tens of meters). This prevents the propagation of any signal about a change in the state of one of the particles before measuring the state of the other (location trap). Meanwhile, the imperfection of detecting the quantum state of single particles (especially photons) leaves room for a sampling (or detection) loophole. For the first time, physicists at the Delft University of Technology managed to avoid two difficulties at once.
In the experiment, we used a pair of diamond detectors with a signal separator between them. Scientists took a pair of non-entangled photons and scattered them into different spaces. Then each of the electrons was entangled with a pair of photons, which were then moved to third space. In the course of experiments, it was possible to observe that a change in the state (spin) of one of the electrons affected the other. In just 220 hours (over 18 days), physicists have tested Bell's inequality 245 times. The observed quantities of electrons were measured using laser beams.
The experiment was able to measure the quantum states of particles separated by a distance of about 1.3 kilometers and to show the validity of Bell's inequality (that is, the validity of quantum theory and the fallacy of the concept of local realism). The results of this study are published in the journal Nature. Its authors are predicted to have a Nobel Prize in physics.
Position of the detectors in the Dutch experiment
Photo: arXiv.org
Teams from the United States and Austria have experimented with photons. So, scientists from the National Institute of Standards and Technology were able to break the record for the distance of quantum teleportation (transmission of the quantum state of a system over a distance) over a fiber-optic cable, carrying it out at a distance of 102 kilometers. To do this, the scientists used four single-photon detectors created at the same institute on the basis of superconducting nanowires (cooled to minus 272 degrees Celsius) from siliceous molybdenum. Only one percent of the photons traveled a distance of 102 kilometers. The previous record for the distance of quantum teleportation over fiber was 25 kilometers (for comparison: the record for the distance of quantum teleportation over the air was 144 kilometers).
Austrian scientists used more efficient sensors than American ones, but the temporal resolution in the experiments of physicists from the USA is much higher. Unlike the Dutch physicists, whose setup recorded about one event per hour, scientists from the United States and Austria were able to conduct more than a thousand tests per second, which virtually eliminates any chance correlation in the experimental results.
Scientists are currently trying to improve the efficiency of experiments - they carry particles to ever greater distances and increase the measurement frequency. Unfortunately, lengthening the optical channel leads to a loss in the fraction of detected particles and again actualizes the danger of a detection loophole. Scientists at the National Institute of Standards and Technology are trying to combat this by using a quantum random number generator in experiments. In this case, there is no need to carry photons over long distances, and the created technology will be useful in quantum cryptography.
Andrey Borisov