Nobel Prize in Physics 2022: Entangled Photons and Bell Inequality Tests

This note covers the Nobel Prize in Physics 2022: who won it, what quantum entanglement and Bell inequalities mean, how Alain Aspect, John Clauser and Anton Zeilinger tested and used entangled photons, how the discovery unfolded from 1935 to the 1990s, why it matters for quantum technology, and quick facts for exams.

What was the Nobel Prize in Physics 2022 awarded for?

The Royal Swedish Academy of Sciences awarded the prize "for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science". This is the official citation, and the prize's full official name is the Nobel Prize in Physics.

In plain words, the three laureates built real experiments to test a strange prediction of quantum mechanics: that two tiny particles can be linked, or entangled, so that measuring one instantly tells you about the other, no matter how far apart they are.

Their results showed that this linking is real and cannot be explained by any particle secretly carrying hidden instructions.

This finding later became the basis of a new field called quantum information science, which includes quantum computers and secure quantum communication.

Who are the laureates?

Alain Aspect

Alain Aspect was born on 15 June 1947 in Agen, France. At the time of the award he was affiliated with the Institut d'Optique Graduate School-Université Paris-Saclay and the École Polytechnique, both in France, and received one third of the prize.

In 1981 and 1982, working with French collaborators, Aspect refined the entangled-photon experiment and closed an important gap called the locality loophole, by switching the measurement settings while the photons were still travelling, so that the setting at one detector could not have affected the particle's behaviour at its source.

John Clauser

John F. Clauser was born on 1 December 1942 in Pasadena, California, USA. At the time of the award he was affiliated with J.F. Clauser & Assoc., in Walnut Creek, California, and received one third of the prize.

In 1972, with Stuart Freedman, a doctoral student supervised by Eugene Commins, Clauser built the first practical experiment to test a Bell inequality using entangled photons from calcium atoms, and found a clear violation that matched quantum mechanics rather than a theory with hidden variables.

Anton Zeilinger

Anton Zeilinger was born on 20 May 1945 in Ried im Innkreis, Austria. At the time of the award he was affiliated with the University of Vienna and the Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences, and received one third of the prize.

Using refined tools, Zeilinger's group demonstrated quantum teleportation in 1997 and entanglement swapping in 1998, techniques that let quantum information move between particles that never met.

What problem were the laureates trying to solve?

Quantum mechanics predicts that two particles can share a single combined state, called an entangled state, even after they separate. Measuring one particle seems to instantly decide the outcome for the other.

Albert Einstein found this troubling; he and colleagues Boris Podolsky and Nathan Rosen set out their doubts in a 1935 paper, which came to be called the EPR paradox.

Erwin Schrödinger, discussing the same puzzle in 1935, called entanglement the characteristic trait of quantum mechanics.

The underlying question was whether particles secretly carry hidden variables, instructions that fix in advance what result they will give when measured, or whether, as quantum mechanics says, the outcome is genuinely undecided until the moment of measurement.

In 1964 the physicist John Stewart Bell, who worked at CERN, worked out a mathematical rule called Bell's inequality.

This rule states that if hidden variables exist, the correlation between results of many repeated measurements can never go above a certain limit. Quantum mechanics, however, predicts that certain experiments will break this limit.

The three laureates built the actual experiments that could tell the two possibilities apart.

How does an entangled-photon experiment work?

The basic test uses pairs of photons, particles of light, that are emitted together so that their polarisation (the direction in which their light wave vibrates) is linked.

Each photon in a pair is sent in an opposite direction towards its own filter, which only lets through light polarised in a chosen direction, similar to the way polarised sunglasses block certain reflected light.

1. A source emits two entangled photons at the same moment, travelling in opposite directions.
2. Each photon meets a polarising filter that can be set at any angle, with one filter for each observer.
3. Detectors on each side record whether the photon passes through its filter or is blocked.
4. The experiment is repeated many times with the filters set at different combinations of angles.
5. The pattern of results is compared against the limit set by Bell's inequality.

When the filters are set at certain skewed angles, quantum mechanics predicts a stronger correlation between the two sides than any hidden-variable theory would allow. Clauser's 1972 experiment, with Stuart Freedman, a doctoral student supervised by Eugene Commins, found exactly this stronger correlation, a clear violation of a Bell inequality that matched quantum mechanics.

Draw a source in the centre emitting two entangled photons in opposite directions. On each side draw a filter that can be rotated to a chosen angle, then a detector behind it.

Label the angle between the two filters, and show that the rate at which both photons pass through depends on this angle.

Definition: Entanglement. A state in which two or more particles are linked so that a measurement on one of them instantly fixes the outcome of an equivalent measurement on the other, even when they are far apart.

How did Aspect and Zeilinger take the experiment further?

Clauser's result still left gaps, called loopholes, that a sceptic could use to argue the experiment had not fully ruled out hidden variables.

One concern was that the filter settings were fixed before the photons were even released, so information about the settings could, in principle, have reached the source in time to affect it.

Aspect closed this gap in the early 1980s by building a setup that could switch the measurement angle after an entangled pair had already left its source, with the filters six metres away and the switch happening in a few billionths of a second.

This meant the setting used at one detector could not have influenced how the photon was emitted or what happened at the other detector. His results again clearly broke a Bell inequality, strongly supporting quantum mechanics over hidden-variable theories.

Zeilinger's group later pushed the experiments much further, separating observers by 400 metres and using other refinements to keep the measurements independent of each other.

Beyond repeating Bell tests, Zeilinger's team showed in 1997 that an unknown quantum state could be transferred from one particle to another distant particle, a process called quantum teleportation, and in 1998 that two particles which had never interacted could still become entangled through a related trick called entanglement swapping.

These techniques showed that entanglement could be treated as a genuine tool, not just a philosophical puzzle.

What has this work made possible?

The Nobel Committee for Physics, through its chair Anders Irbäck, said: "It has become increasingly clear that a new kind of quantum technology is emerging." The press release described the laureates' experiments as clearing the way for technology based on quantum information, naming quantum computers, quantum networks and secure quantum encrypted communication as the fields that benefit.

The source material also records that entangled photons have since been sent through optical fibres covering tens of kilometres, and that entanglement has been demonstrated between a satellite and a station on the ground, showing how laboratory results have grown into working long-distance systems.

A quantum network can use entangled photons as signals travelling along optical fibres, and entanglement swapping allows the usable distance of such a network to be stretched further than a single fibre run would normally allow.

This matters because an ordinary light amplifier, of the kind used in everyday internet cables, cannot simply boost an entangled signal: it would have to measure the light first, and that measurement destroys the entanglement. Entanglement swapping offers a way around this limit.

Taken together, the three laureates' experiments turned entanglement from a topic of philosophical argument into a practical resource, supporting the entire field now called quantum information science.

How did the discovery unfold?

Why does this matter for everyday technology?

The press release explained that these experiments cleared the way for new technology, part of a large field of research that now spans quantum computers, quantum networks and secure quantum encrypted communication.

Entangled photons can be sent through optical fibres to act as signals in a quantum network, and entanglement swapping allows the effective distance of such networks to be extended, because ordinary light amplifiers cannot be used without destroying the fragile entangled state.

The source material also notes that researchers have since demonstrated entanglement between photons sent through tens of kilometres of optical fibre, and between a satellite and a ground station, showing how far these ideas have travelled from the original laboratory tests.

The popular science page compared this shift to an earlier transformation: the first quantum revolution gave the world transistors and lasers, while the laureates' work is said to open a new era built on directly manipulating entangled particles.

At a more fundamental level, the work also matters because it settled a long scientific argument.

It showed, with strong experimental evidence, that nature does not hide secret information inside particles in the way Einstein had hoped; instead, the strange uncertainty of quantum mechanics is a genuine feature of reality, not a sign of an incomplete theory.

How does this connect to what you study?

This prize connects directly to the chapter on dual nature of radiation and matter and basic quantum ideas in senior school physics, where students first meet the idea that particles such as photons and electrons do not behave like everyday objects.

The idea of polarisation of light, used in every experiment described here, also appears in the optics portion of the physics syllabus, where polarising filters and their angle-dependent transmission are introduced; the same effect is used in polarised sunglasses that block light reflected off water.

The laureates' experiments used exactly this property: a filter lets through light polarised in one chosen direction, and rotating two such filters relative to each other changes how often paired photons pass through together.

Understanding entanglement itself is not required at school level, but the polarisation and photon concepts that make these experiments possible are part of standard physics teaching, and they give a concrete, testable example of how light behaves as discrete particles rather than only as a continuous wave.

Students who study the photoelectric effect and the particle nature of light are, in effect, meeting the building blocks that later let physicists design experiments sensitive enough to test deep questions about reality itself.

Quick facts for exams

The Nobel Prize in Physics 2022 was awarded jointly to Alain Aspect, John F. Clauser and Anton Zeilinger, each receiving one third of the prize, for experiments with entangled photons that established the violation of Bell inequalities and pioneered quantum information science.

The prize was announced by the Royal Swedish Academy of Sciences on 4 October 2022, with a total value of 10 million Swedish kronor. Aspect was born in France and worked at French institutions;

Clauser was born and worked in the United States; Zeilinger was born and worked in Austria.

Their work, stretching from Clauser's 1972 experiment to Zeilinger's 1997 to 1998 teleportation and swapping results, underlies modern research on quantum computers and quantum communication.

Note: Source. The prize facts in this note are from the Nobel Prize's official site, nobelprize.org.

Glossary

• Entanglement — a shared quantum state linking two or more particles so that measuring one instantly fixes the outcome for the other.
• Bell inequality — a mathematical limit on the correlation between measurements that any hidden-variable theory must obey.
• Hidden variables — hypothetical secret instructions that would tell a particle in advance what result it must give in an experiment.
• EPR paradox — the 1935 argument by Einstein, Podolsky and Rosen that quantum mechanics might not fully describe reality.
• Polarisation — the direction in which the electric field of a light wave vibrates.
• Quantum teleportation — moving the unknown quantum properties of one particle onto another, distant particle, with the original particle's state destroyed in the process.
• Entanglement swapping — making two particles that never interacted become entangled, by entangling each with a partner that is then jointly measured.
• Locality loophole — a gap in early Bell tests where the measurement setting was fixed before the particles left the source.
• Quantum information science — the study of storing, transferring and processing information using quantum states.
• Photon — a particle of light.
• Quantum network — a system of linked nodes that exchange information carried by quantum states such as entangled photons.
• Quantum encrypted communication — a way of sharing a secret key where any eavesdropping disturbs the entangled correlation and can be detected.

Common errors and misconceptions

• Misconception: Entangled particles send a signal to each other faster than light. Correct: The sources describe entanglement as a correlation, not a signal; nothing is described as travelling between the particles faster than light.
• Misconception: John Clauser alone proved Bell's inequality is violated for all time with no loopholes. Correct: Clauser's 1972 experiment showed a clear violation, but Aspect later closed a further loophole that Clauser's setup had left open.
• Misconception: Quantum teleportation means moving matter instantly from place to place. Correct: The source describes it as transferring a quantum state from one particle to another, with the original destroyed, not moving physical matter.
• Misconception: The three laureates worked together as one team. Correct: Each conducted separate experiments, in different decades and countries, building on each other's work; Clauser in the 1970s, Aspect in the early 1980s, and Zeilinger from the late 1990s.
• Misconception: Bell's inequality is a law of quantum mechanics. Correct: It is a mathematical limit that applies to theories with hidden variables; quantum mechanics is the theory that predicts this limit can be broken.
• Misconception: The prize was for inventing quantum computers. Correct: The citation credits the laureates with experiments on entangled photons and pioneering quantum information science, which later enabled work such as quantum computing, rather than building a computer themselves.

Exam-style questions with model answers

Q1. State the official citation for the Nobel Prize in Physics 2022. [2 marks]

1. The prize was awarded "for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science".

Q2. Name the three laureates of the Nobel Prize in Physics 2022 and their countries of birth. [2 marks]

1. Alain Aspect was born in France, John F. Clauser was born in the USA, and Anton Zeilinger was born in Austria.

Q3. Explain what is meant by a hidden-variable theory and why Bell's inequality was important for testing it. [4 marks]

1. A hidden-variable theory proposes that particles secretly carry fixed instructions deciding the result of any future measurement, so that the outcome is determined in advance rather than being genuinely random.
2. In 1964 John Stewart Bell derived a mathematical inequality showing that any such hidden-variable theory must keep the correlation between repeated measurements below a specific limit.
3. Quantum mechanics, by contrast, predicts that certain experimental setups will produce a correlation that exceeds this limit, a violation of Bell's inequality.
4. This gave physicists, for the first time, an experiment that could distinguish between the two competing explanations rather than leaving the debate purely philosophical.

Q4. Describe how Alain Aspect's experiment improved on John Clauser's original test. [4 marks]

1. Clauser's 1972 experiment, with Stuart Freedman, used fixed filter angles set before the photons were emitted, which left open the possibility that the source could somehow have been influenced by the planned settings.
2. In 1981 and 1982 Aspect redesigned the apparatus so that the measurement angle could be switched after the entangled pair had already left the source.
3. With filters placed six metres away, the switch happened in a few billionths of a second, too fast for information about the setting to travel between the two sides at the speed of light.
4. This closed an important loophole and gave a clearer demonstration that quantum mechanics, not a hidden-variable theory, correctly describes the results.

Q5. Discuss the contributions of all three laureates and why the Nobel Committee linked their work to future quantum technology. [6 marks]

1. John Clauser built the first practical experiment, with Stuart Freedman, a doctoral student supervised by Eugene Commins, to test a Bell inequality using entangled photons from calcium atoms in 1972, finding a clear violation that supported quantum mechanics over hidden-variable theories.
2. Alain Aspect, working in France in 1981 and 1982, refined this experiment and closed the locality loophole by switching measurement settings after the photons had left their source, giving an even clearer violation of Bell's inequality.
3. Anton Zeilinger, based at the University of Vienna, pushed the experiments further, separating observers over longer distances and, with his research group, demonstrated quantum teleportation in 1997 and entanglement swapping in 1998.
4. Together, these experiments moved entanglement from a philosophical puzzle to a usable physical resource.
5. The press release stated that their results cleared the way for new technology based on quantum information, including quantum computers, quantum networks and secure quantum encrypted communication.
6. The Nobel Committee for Physics chair, Anders Irbäck, said the laureates' work with entangled states carried great importance beyond the basic questions about how quantum mechanics should be interpreted.

Q6. What is quantum teleportation, and who first demonstrated it experimentally? [3 marks]

1. Quantum teleportation is a process that transfers an unknown quantum state from one particle to another, distant particle, with the original particle's state destroyed in the process.
2. It relies on a shared entangled pair of particles together with a classical message sent between the two locations.
3. Anton Zeilinger's research group performed the first experimental demonstration of quantum teleportation in 1997.

Q7. Why can ordinary signal amplifiers not be used to extend entangled quantum networks over long distances? [2 marks]

1. An ordinary amplifier works by capturing and measuring the light signal before boosting it, but measuring an entangled photon in this way destroys its entanglement.

Q8. What did John Stewart Bell propose in 1964, and where did he work? [2 marks]

1. Bell developed the mathematical inequality named after him, setting a limit on correlations allowed by hidden-variable theories, while working at CERN, the European particle physics laboratory.

Key takeaways

• The 2022 Physics prize honoured Alain Aspect, John F. Clauser and Anton Zeilinger for experiments on entangled photons.
• Their work established that quantum mechanics violates Bell's inequality, ruling out simple hidden-variable explanations.
• Clauser performed the first practical Bell test in 1972 with Stuart Freedman.
• Aspect closed an important experimental loophole in 1981 to 1982 by switching settings after emission.
• Zeilinger's group demonstrated quantum teleportation in 1997 and entanglement swapping in 1998.
• The findings underlie modern quantum computers, quantum networks and quantum encrypted communication.
• The prize of 10 million Swedish kronor was shared equally among the three laureates.
• The citation credits them with "pioneering quantum information science".

Test yourself

Who shared the Nobel Prize in Physics 2022?

Alain Aspect, John F. Clauser and Anton Zeilinger shared the prize equally, one third each, for experiments on entangled photons.

What does entanglement mean in quantum mechanics?

It means two or more particles share a linked state, so measuring one instantly fixes the outcome for the other, even far apart.

What did John Clauser achieve in 1972?

John Clauser, with Stuart Freedman, performed the first practical experiment showing a clear violation of a Bell inequality using entangled photons.

How did Alain Aspect close an important loophole?

Alain Aspect switched the measurement filter settings after the entangled photons had already left their source, within a few billionths of a second.

What new technique did Anton Zeilinger's group demonstrate in 1997?

Anton Zeilinger's group demonstrated quantum teleportation, transferring an unknown quantum state from one particle to a distant particle.

What is a Bell inequality used for?

It is used to test whether measurement correlations can be explained by hidden variables, or whether they require quantum mechanics.

Where was Anton Zeilinger based at the time of the award?

Anton Zeilinger was affiliated with the University of Vienna and the Austrian Academy of Sciences in Vienna, Austria.

What practical fields did the Nobel Committee link to this prize?

The committee linked the work to quantum computers, quantum networks and secure quantum encrypted communication.