The quantum physical phenomenon of entanglement forms the basis of all quantum technologies - from quantum computers to quantum communication. This year's Nobel Prize in Physics goes to three physicists who have made significant contributions to the experimental study of quantum entanglement and its practical application. US physicist John Clauser and his French colleague Alain Aspect developed a test for photon entanglement. Anton Zeilinger from the University of Vienna was the first to succeed in transferring entanglements - and thus created the prerequisites for quantum communication.
Quantum computing and quantum communications allow for the rapid solving of complex problems and the use of "unbreakable" encrypted information. This is based on quantum physical phenomena such as superposition and entanglement, which determine the behavior of particles such as photons, ions or atoms. Albert Einstein called the phenomenon of entanglement "spooky action at a distance" and Erwin Schrödinger called it the most important feature of quantum mechanics. Accordingly, two entangled particles remain in an undifferentiated state of superposition until the state of one of the two is measured. Only then does the other decide automatically and at the same time for its state.
This can be compared to two balls, one white and one black, whose state in flight can only be recognized as gray - they are in a superposition of their states. Only when one of the balls is caught does its final coloring become apparent. At the same time, it is also clear that the other ball must have the opposite color. But the big question is: how do the balls know what color to match? Is their color purely coincidental when measured, or do they possibly contain hidden information that determines their later color in advance? In the 1960s, the physicist John Stewart Bell developed a theoretical possibility of answering this question experimentally. According to this, a true entanglement without hidden variables would have to show a certain degree of correlation when the measurements were repeated countless times. But how this should be measured in practice remained unclear.
John Clauser and Alain Aspect: The Bell test becomes practical
This is where the first prizewinner, the US physicist John Clauser, came in: He was the first to develop an experiment with which the violation of Bell's inequality and thus the nature of quantum entanglement could be demonstrated. To do this, the researcher generated pairs of photons that were entangled with each other via their polarization. By sending these photons through different polarization filters, Clauser was able to determine how often which combination occurred. It turned out that the entangled photons actually violated Bell's inequality. The magnitude of the correlations could not be explained by a predetermined state or hidden variables. Instead, it was actually a "spooky action at a distance" - the state of the second particle is only determined by measuring the state of the first and thus canceling the superposition.
However, the experiment developed by Clauser and his team had the disadvantage that it was still very inefficient: Only a small proportion of the photons generated could be detected via the filter and were therefore suitable for the measurement. This is where the second laureate, French physicist Alain Aspect, comes in. He further developed the experiment and managed to direct the entangled photons through two different polarizers, which improved the measurement possibilities.
Anton Zeilinger: Teleportation and quantum amplification
The third prizewinner, Anton Zeilinger from the University of Vienna, has solved a fundamental problem in quantum communication that is closely related to entanglement: If optical information is sent over long distances, for example in a fiber optic cable, the light signal weakens and thus limits the range. On average, every second photon is lost over a distance of ten kilometers. With normal optical signals, this is compensated for by intermediate amplifiers. However, this is not possible with entangled photons: because the amplifier first has to read out the signal in order to intensify it, this would break the entanglement and thus destroy the quantum signal. Zeilinger and his team provided the solution to this dilemma in 1998 with quantum teleportation. This is based on the knowledge that an entangled pair of photons can transfer its entanglement to another pair.
A quantum amplifier only has to ensure that both pairs of photons come into contact with each other under the right circumstances in order to transfer the entanglement and the quantum information it contains from the old pair to a new, fresh pair. Only this discovery makes it possible to transmit quantum signals over long distances in fiber optic cables.
Pioneer of quantum technology
Together, the three winners of the Physics Nobel Prize 2022 laid the foundation for quantum technologies to become practically usable. “The laureates' work with entangled states is of great importance. Because their results paved the way for the new technology based on quantum information," says the Nobel Prize Foundation's award announcement.
Source: Nobelprize.org