The Nobel Prize in Physics 2022 was conferred jointly to Alian Aspect, John F. Clauser and Anton Zeilinger for their ground-breaking experiments with entangled photons. Their experiments proved the profoundly strange quantum nature of reality. They have collectively established the existence of a bizarre quantum phenomenon known as ‘entanglement’, where two widely separated particles appeared to share information, despite having no conceivable way of communicating. These physics laureates have laid the foundation for a new era of quantum technology. These experiments have established the violation of Bell inequalities and pioneering quantum information science.
One of the key factors in this development is how quantum mechanics allows two or more particles to exist in an entangled state. What happens to one of the particles in an entangled pair determines what happens to the other particle, even if they are far apart. The 2022 Nobel laureates in physics conducted innovative experiments using entangled quantum states, where two distant particles behaved like a single particle. Thus, the results have cleared the way for new technology based upon quantum information. The Nobel Prize money of 10 million Swedish kronor has been shared equally between the three laureates. Nobel Prize is awarded every year by the Royal Swedish Academy of Sciences.
Quantum Entanglement
Quantum entanglement is the phenomenon which occurs when a group of particles are generated, interact, or share spatial proximity in such a way that the quantum state of each particle of the group could not be described independently of the state of the others, even when the particles are separated by a large distance. The topic of quantum entanglement is at the heart of the disparity between classical and quantum physics—entanglement is a primary feature of quantum mechanics not present in classical mechanics.
Measurement of Quantum Entanglement
Measurements of physical properties such as position, momentum, spin, and polarisation performed on entangled particles, in some cases, could be found to be perfectly correlated. For example, if a pair of entangled particles is generated such that their total spin is known to be zero, and one particle is found to have clockwise spin on the first axis, then the spin of the other particle, measured on the same axis, is found to be anticlockwise. However, this behaviour gives rise to seemingly paradoxical effects. Any measurement of a particle’s properties results in an irreversible wave function collapse of that particle and changes the original quantum state. With entangled particles, such measurements affect the entangled system as a whole.
Bell’s Theorem
Whether the correlation was because the particles in an entangled pair contained hidden variables or instructions remained unknown for a long period. Presumably, these hidden variables would tell the particles which result they should produce in an experiment. In the 1960s, John Stewart Bell, the North Ireland Physicist, coined the Bell’s theorem regarding hidden variable theories. As per the theorem, if there are hidden variables, the correlation between the results of a large number of measurements would never exceed a certain value. However, quantum mechanics predicts that a certain type of experiment would violate Bell’s inequality. This resulted in a stronger correlation than would otherwise be possible.
Experiments by John F. Clauser, Alian Aspect, and Anton Zeilinger
John F. Clauser, a Research Physicist at J.F. Clauser and Associates, USA, built an apparatus that emitted two entangled photons at a time, each towards a filter that tested their polarisation. He developed John Bell’s ideas, leading to a practical experiment. Measurements taken by him supported the laws of quantum mechanics. The result, thus, was a clear violation of a Bell’s inequality and agreed with the predictions of quantum mechanics.
Clauser’s evidence was not ironclad. His experiment used fixed orientations of the lenses which was a loophole because if the hidden variable that coordinates the photons’ polarisations, somehow, depends on the experimental positioning of the lenses, Einstein could yet be right.
Nobel Laureate Alain Aspect, a Professor at the Institut d’Optique Graduate School, Paris, developed a set-up to close that important loophole. He carried out a series of increasingly stringent Bell tests in Paris, culminating in a devilishly sophisticated experiment in 1982. In that test, the orientation of the lenses would randomly change during the billionths of a second that the photons spent flying from the emitter to the lens. Thus, the initial lens configuration was erased which did not have any influence on any secret process, i.e., setting the polarisation at the moment of their emission. He switched the measurement settings after an entangled pair had left its source, and the result remained unaffected. However, this also had a slimmest loophole.
In a 2017 experiment, Anton Zeilinger, a Professor at University of Vienna, Austria, led a research team that used the colours of photons emitted from distant stars, hundreds of years ago, to determine the settings of the experiment. If some cosmic conspiracy was creating the illusion of entanglement, it would have had to begin centuries before the births of the experimenters.
His research group researched entangled quantum states and demonstrated a phenomenon called quantum teleportation, which makes it possible to move a quantum state from one particle to one at a distance.
Intense research and development are underway to utilise the special properties of individual particle systems to construct quantum computers, improve measurements, build quantum networks, and establish secure quantum encrypted communication.
The Nobel Prizes are awarded to those who confer the greatest benefit to humankind. Instituted after the name of Alfred Nobel, the prizes are given in the field of Physics, Chemistry, Physiology (Medicine), Literature, Peace, and Economic Sciences. Nobel Prizes are widely regarded as the most prestigious awards available in their respective fields. The prize consists of a medal struck in 18 caret gold, plated with 24 caret gold; a diploma; and a cash prize. The first Nobel Prize was awarded in 1901.
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