Putting Quantum Physics to the Test: The Story Behind the 2022 Nobel Prize in Physics

Science finds the reality in dreams, and the utility in reality.

Scepticism is a key tenet of science. Big ideas of science, especially theoretical physics, can often initially seem indistinguishable from fantasy. If you move fast enough you will time travel into the future! The centre of our galaxy is a supermassive black hole which we are moving around at a speed of 800 000 km/h! All of space is permeated by mysterious dark energy that is pulling the universe apart! These are all accepted as scientific facts only because they have withstood all attempts – by argument and experiment – to show them to be false. When we question established scientific theories, good scientists are not offended or concerned by our scepticism because it is only a fraction of that it has already been subjected to by other scientists.

Quantum physics is a great example of this. In being introduced to quantum physics we are often told such remarkable claims as that cats can be both alive and dead at the same time, and that just looking at waves transforms them into particles. It is natural to question these claims and wonder whether perhaps these quantum effects do not really happen but rather the appearance of them just shows we need a better theory. This doubt is not naïve; it is exactly the sceptical approach that a scientist should take. In fact, it was the view taken by Albert Einstein. We should not be expected to accept the seemingly incredible implications of quantum physics just because we are told to by a physicist, or even because it is implied by some sophisticated mathematics. It is reasonable to expect experiments that show that it cannot be any other way.

The 2022 Nobel Prize in Physics, announced this week, honours the scientists who performed exactly these experiments. It will be awarded to Alain Aspect, John Clauser and Anton Zeilinger “for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science.” They demonstrated the reality of the quantum mechanical effect of entanglement and that this effect cannot be explained by more intuitive, classical theories of nature. Their work was the culmination of a great story of scientific scepticism by which a reductio ad absurdum intended to demonstrate the limitations of quantum physics was turned into a testable hypothesis and then into an established fact that confirms the reality of quantum effects.

Einstein’s Scepticism

In quantum physics, particles are described by mathematical objects called quantum wavefunctions. Quantum wavefunctions are strange objects. For example, two different properties we can measure about a particle are its position and its speed. However, there is no quantum wavefunction that describes a particle that has both a specific position and a specific speed. If we have a particle that we know to be moving with a particular speed, this is necessarily described by a wavefunction that does not correspond to any single position. If we make a measurement to determine where such a particle is, its wavefunction changes into a different one that describes a particle in a particular position (which we find out), but now with with no specific speed.

In 1935, Einstein argued that there are two possible ways of interpreting this. The first possibility was that the quantum wavefunction is a true description of particles and so the particle did not truly have any real position until we measured it. The alternative is that the quantum wavefunction is just an incomplete approximation to the real particle and that if we had a deeper understanding of physics then we would find that the particle had a real position all along and that our measurement changed only our understanding of it, not the physical particle.

Statue of Einstein in Washington, D.C. with some of his most famous equations depicted on the page. Among these is his 1905 equation for the photoelectric effect by which he played a pivotal role in the initiation of quantum physics and for which he was awarded the 1921 Nobel Prize in Physics. Image Source: David, CC BY 2.0, via Flickr.

Einstein argued for the second possibility – that quantum physics is incomplete – by a thought experiment intended to show the absurdity of the first possibility. Specifically, he showed that quantum physics predicts that it is possible to take two particles and interact them so that they become entangled. This means that a measurement of one particle does not only change the wavefunction of that particle, but also the wavefunction of the other particle. This happens instantly regardless of how far apart the two particles are! If the quantum wavefunction is a true description of the particles, this means that measuring one particle instantly changes the other even if it is on the other side of the universe! Einstein later called this spooky actions at a distance. To Einstein, it was patently absurd, especially since his own theory of relativity dictated that instantaneous communication is impossible.

He therefore concluded that quantum mechanics could not be a complete theory of physical reality. Instead, he believed there must be a hidden description of particles (later called a hidden variable) which would encode the position, speed and all other properties of particles even when they were not specified by the quantum wavefunction, and that would be uncovered by an as-yet undiscovered deeper theory of physics. With this deeper theory, quantum effects such as action at a distance through entanglement would be explained as mere approximations.

Testing the Nature of Reality

In 1964, John Bell found a way to bring this speculation firmly into the realm of science and test it. He showed that quantum physics makes predictions about how entanglement could be used that are inconsistent with any theory of the type Einstein described. Specifically, he found that if nature is truly described by a local hidden variable theory – that is, a theory where instantaneous action at a distance does not occur and all the properties of particles are well-defined even when we have not measured them – then it would be impossible to pass a certain type of test, called a Bell test. Therefore, if an experiment could demonstrate that such a test was passed, then Einstein’s belief must be wrong, and the counterintuitive effects of quantum physics must be real.  

John Bell posing in front of a board depicting a schematic of a Bell test. In his original formulation, local hidden variable theories (the type advocated by Einstein) could not achieve a score above 2, while quantum mechanics could achieve up to 2 times the square root of 2 (approximately 2.8). Image Source: CERN PhotoLab, CC BY 4.0, via CERN Document Server.

One way to present a Bell test is as a game, called the CHSH game. The game is as follows. There are two players who play as a team and win or lose together. They are allowed to plan a strategy together before the game starts but are then separated so that they cannot communicate any more during the game. In each round of the game, each player flips a coin. They then choose to press either a blue button or a red button. If at least one of their coins shows tails then they win only if they press the same colour button. However, if both of their coins show heads then they win only if they press opposite colours. They play many rounds of this game, and their score is the percentage of the time that they win.

If you try playing this game with a range of possible strategies, you will find that the best score you can get (after a large number of rounds) is 75%.¹ In fact, Bell proved that if nature really is described by a local hidden variable model (the type of theory Einstein believed in) then 75% is the highest possible score, provided the players are separated in a way that makes communication between them impossible.² On the other hand, if quantum physics is correct, he showed that you could create a device that used quantum entanglement that would make it possible to achieve a higher score (up to 85%)! Scoring over 75% in the CHSH game is therefore an example of a Bell test. If it can be demonstrated in an experiment, it would prove that effects like quantum entanglement are not just approximations to a deeper physical theory. It would show that quantum physics must really describe reality!

Demonstrating the Reality, and Usefulness, of the Quantum World

In 1972, John Clauser and his colleague Stuart Freedman did this experiment. They prepared entangled pairs of photons – particles of light – and showed that these pairs could pass a Bell test. This was immensely important, but not conclusive, because they had not arranged their experiment such that it was impossible for some form of communication to be taking place between the photons. Ten years later, Alain Aspect led a team that rectified this by using automated devices that, though only separated by twelve metres, were also only given ten nanoseconds to answer. Since it takes forty nanoseconds to cross this distance at the speed of light, and no information can travel faster than the speed of light, this ensured that communication between the devices was impossible. The Bell test was still passed! These experiments directly demonstrate that the counterintuitive effects described by quantum physics are not just artefacts of an incomplete theory; they really occur.

Today, entanglement is not just a demonstrated physical effect, it is increasingly becoming a useful practical tool through the field of quantum information science. For example, in 1999, Anton Zeilinger and his team demonstrated quantum key distribution using entangled photons. Quantum key distribution is the most secure scalable form of encryption ever developed. Provided that the sender and receiver’s computers are both secure, it allows messages to be sent that it is impossible for an interceptor to ever read without requiring the sender and receiver to ever meet in person. Today, entangled photons can be used to send encrypted messages in this way across thousands of kilometres and are even being trialled for private use. In 1997, Zeilinger also led the first team to demonstrate quantum teleportation (again using entangled photons) which is invaluable to the emerging technology of quantum computing.

A driving motivation of science is Galileo’s call to “measure what is measurable, and make measurable what is not so”. In 1935, Einstein imagined entanglement as a hypothetical absurdity. Decades later, Bell made it measurable, and Clauser and Aspect measured it. Now, through the work of Zeilinger and others, entanglement has become a tool being harnessed for human progress. The work for which the 2022 Nobel Prize in Physics is to be awarded can therefore be seen as the continuation of a great tradition. Science finds the reality in dreams, and the utility in reality.

1. You can prove this by trying all possible strategies. Since the players can only see their own coins, their strategy must consist of instructions of what each will do based on the outcome of their own coin flip. For example: one strategy could be: Player 1 will press blue if her coin shows heads and red if her coin shows tails, Player 2 will press red if his coin shows heads and red if his coin shows tails. You can show that with this strategy they win 75% of the time, since they will win all rounds except those where Player 1’s coin shows tails and Player 2’s shows heads. By testing all sixteen possible strategies (i.e., all possible combinations of blue/red in the italicised positions in the above strategy), you can prove that 75% is the best score that any strategy achieves.

2. Bell actually used a different Bell test in his proofs. The CHSH game is a later adaptation of his work.