Historic moment! Nobel Prize awarded for the first time to quantum information science

On October 4, the Royal Swedish Academy of Sciences announced the winners of the 2022 Nobel Prize in Physics: Alain Aspect, John Clauser, and Anton Zeilinger [1] for their "experiments on entangled photons, establishing violations of Bell's inequality and pioneering quantum information science "[2] for their outstanding achievements. Among them, Zeilinger is the doctoral supervisor of Pan Jianwei, a member of the Chinese Academy of Sciences. This is the first time that a Nobel Prize has been awarded to quantum information science.

The three laureates have each conducted groundbreaking experiments using entangled quantum states in which two particles behave as one even when separated. Their results have cleared the way for new technologies based on quantum information.

01The foundation of quantum information science: entangled states

Research in quantum information science includes quantum computers, quantum networks, and secure quantum-encrypted communications.

A key element of this development is how quantum mechanics allows two, or more, particles to exist in an entangled state: what happens to one particle in an entangled pair determines what happens to the other particle, even if they are far apart.

Does color exist when no one is watching? An entangled pair in quantum mechanics can be compared to a machine that throws balls of opposite colors in opposite directions. When Bob catches a ball and sees that it is black, he immediately knows that Alice has caught a white one. In a theory that uses hidden variables, the balls always contain hidden information about what color is being displayed. However, quantum mechanics says that the balls are gray until someone looks at them when one of them randomly turns white and the other turns black. Bell's inequality suggests that there are experiments that can distinguish between these cases. Such experiments have proven the description of quantum mechanics to be correct.

The question has long been whether this correlation is due to the fact that the particles in an entangled pair contain hidden variables, i.e. instructions that tell them what result they should give in the experiment. In the 1960s, John Stewart Bell proposed the mathematical inequality that bears his name. This states that if there are hidden variables, the correlation between a large number of measurements will never exceed a certain value. However, quantum mechanics predicts that some type of experiment will violate Bell's inequality, leading to stronger correlations than would otherwise be the case.

John Clauser developed John Bell's idea and performed an actual experiment. When he measured it, they supported quantum mechanics by explicitly violating Bell's inequality. This means that quantum mechanics cannot be replaced by a theory that uses hidden variables.

Experimenting with Bell's Inequality

After John Clauser's experiments, there were still some holes. alain Aspect developed this setup and used it to close an important loophole. He was able to switch the measurement settings after the entangled pair left its source, so that the settings present when they were fired would not affect the results.

Two pairs of entangled particles are emitted from different sources. One particle of each pair is brought together in a special way that causes them to become entangled. The other two particles (1 and 4 in the figure) are also entangled. In this way, two particles that have never been in contact can become entangled.

Using refined tools and a long series of experiments, Anton Zeilinger set out to use entangled quantum states. Among other things, his group, of which Pan is a member, has demonstrated a phenomenon called "quantum invisible transfer," which makes it possible to move a quantum state from particle to particle at a distance.

"It is becoming increasingly clear that a new kind of quantum technology is emerging." Anders Irbäck, chairman of the Nobel Committee in Physics, said, "We can see that the work of the laureates on the entangled state is of great importance, even beyond the fundamental questions about the interpretation of quantum mechanics."

02Three pioneers of quantum information science

The three academics who received the award are Alain Aspect, John F. Clauser and Anton Zeilinger, who will share equally the prize of 10 million Swedish kronor. in 2010, they also jointly received the Wolf Prize for Physics in Israel [4].

The Royal Swedish Academy of Sciences had this to say about the three scientists: "This year's two laureates, John F. Clauser and Alain Aspect, are being honored for pioneering a new era of work that has opened the eyes of the physics community to the importance of entanglement and provided the means to create, process and measure Bell pairs in more complex and incredibly technology. The experimental work of the third recipient, Anton Zeilinger, stands out for its innovative use of entanglement and Bell pairs, both in curiosity-driven fundamental research and in applications such as quantum cryptography."

Alain Aspect is a French physicist known for his experimental work on quantum entanglement. born in 1947 in Ardenne, France, he received his PhD from the University of Paris-Sud in Orsay, France, in 1983; he is now a member of the French Academy of Sciences and the French Institute of Technology, and a professor at the Ecole Polytechnique in Paris. In the early 1980s, while completing his PhD thesis, Aspect performed Bell test experiments; after completing his work on Bell inequalities, Aspect turned to the study of laser cooling of neutral atoms and was mainly involved in experiments related to Bose-Einstein condensation.

John F. Clauser was born in Pasadena, California, in 1942 and received his Ph.D. from Columbia University, New York, USA, in 1969. clauser is known for his contributions to the foundations of quantum mechanics, in particular the Clauser-Horne-Shimony -Holt inequality (CHSH inequality).He worked mainly at Lawrence Berkeley National Laboratory, Lawrence Livermore National Laboratory, and the University of California, Berkeley, from 1969 to 1996; in 1972, in collaboration with Stuart Freedman, he performed the first experimental prediction of the CHSH- Bell theorem test: this was the first experimental observation of a violation of Bell's inequality; in 1974, in collaboration with Michael Horne, he showed for the first time that the generalization of Bell's theorem provides strict constraints on all local realistic natural theories (also known as objective local theories); in the same year, he observed for the first time the sub-Poisson statistics of light, thus proving for the first time that photons have a definite particle-like character.

Anton Zeilinger is an Austrian quantum physicist, currently Professor of Physics at the University of Vienna and Senior Scientist at the IQOQI Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences, where his research deals with fundamental aspects and applications of quantum entanglement. -Newton Medal (2007) and the King Faisal International Prize (2005), and he is a member of seven academies of science.

Zeilinger is involved in fundamental research in quantum mechanics. Together with Daniel Greenberger and Michael Horne, he discovered new counter-intuitive features of three- and four-particle states: he and his team were the first to implement these experimentally, which opened up the field of multi-particle interference and multi-particle quantum correlations. Using the method developed on this occasion, he achieved the first quantum transport of independent quantum bits, followed by entangled interchanges: entangled states were transmitted.

This work was followed by extensive testing of Bell inequalities, including the cosmic Bell test. Other fundamental experiments include Leggett's theory of nonlocal reality, and experiments on nonlocal Schrödinger with entangled states.

Many of these results became relevant in the development of quantum information technology, and he was also conducting groundbreaking experiments. His quantum dense coding experiments were the first to use entanglement to prove a primitive, impossible in classical physics. He also implemented the first entanglement-based quantum coding experiments and later quantum communication over ever greater distances and, to achieve higher dimensional states with ever greater information capacity. Possible applications also include one-way quantum computing and blind quantum computing.

Reference links：

[1]https://www.nobelprize.org/prizes/physics/2022/press-release/

[2]https://www.nobelprize.org/uploads/2022/10/advanced-physicsprize2022.pdf