Nobel Prize in Physics 2022: Proving and Using the Peculiar Quantum Nature of Reality

Aside from the fact they are all septuagenarians and have already shared the 2010 Wolf Prize in Physics, what links this year’s three Nobel Prize in Physics recipients John Clauser, Alain Aspect and Anton Zeilinger is a 1964 paper written in an obscure physics journal by Northern Irish physicist John Bell. In it, Bell proposed a thought experiment that would settle a debate that had raged between the likes of Albert Einstein, Niels Bohr and Erwin Schrödinger since the birth of quantum mechanics –­ whether quantum theory was deficient in describing reality.

Bell focused on entanglement, a fundamental yet counterintuitive concept in quantum mechanics where two widely separated particles appear to share information despite having no conceivable way of communicating. He derived a mathematical constraint (later named the Bell inequality and extended to a group of related constraints collectively named Bell inequalities) that said that the correlation between particle properties should never exceed a certain value for repeated experiments on two separated particles.

If the inequality was satisfied, it proved Einstein’s position that the universe was fundamentally classical and ‘local hidden variables’ underpin a more classical universe. In contrast, if the inequality was violated and the correlation was stronger – as quantum theory predicted – the uncertainties in quantum theory’s predictions are not due to ignorance or the unknowability of these properties: the universe is fundamentally probabilistic and quantum mechanical just as Bohr claimed.

Testing Quantum Mechanics to the Limit

With Bell’s ideas, the debate on quantum mechanics’ deficiency or otherwise changed dramatically. It finally became possible to settle the question by experiment. In the late 1960s, Clauser set out to do just that, refining Bell’s theorem in order to check it against real-world data. Initially, the likes of Richard Feynman (1965 Nobel Prize in Physics) discouraged Clauser from pursuing the work, arguing that quantum mechanics needed no further experimental proof. But Bell and Charles Townes (1964 Nobel Prize in Physics) backed Clauser – who was joined by Stuart Freedman (now deceased) at the University of California, Berkeley – to perform a ‘Bell test’; a real-world experiment on one of the Bell inequalities.

Their setup used calcium atoms that emitted two entangled photons at a time, each towards a filter that tested their polarisation. In 1972, results of their experiment showed a clear violation of a Bell inequality and agreed with the predictions of quantum mechanics. “I was very sad to see that my own experiment had proven Einstein wrong,” said Clauser in a 2022 interview with the California Institute of Technology.

There were, however, several ‘loopholes’ in this experiment that left the door open to the possibility that hidden-variable theories could still be correct. It was argued that perhaps the photons detected were not a fair sample of all photons emitted by the source (the detection loophole) or that elements of the experiment thought to be independent were somehow causally connected (the locality loophole).

Aspect and colleagues at the Université Paris-Sud in Orsay, France, designed an experiment that made substantial inroads into closing both loopholes. By using improved techniques and novel instruments, Aspect’s team developed a new way of exciting the atoms so they emitted entangled photons at a higher rate. They could also switch between different settings, so the system would not contain any advance information that could affect the results. With this setup, Aspect managed to establish violations of a Bell inequality with very high precision. But with the distance between the entangled photons being a mere 12 metres and for various other technical reasons, Aspect’s experiments were not ideal – the locality loophole still lurked in the background.

Zeilinger and colleagues at the University of Innsbruck, Austria, managed to get a handle on this loophole in 1998. Among a number of improvements on Clauser and Aspect’s experiments, the researchers used two fully independent quantum random-number generators to set the directions of the photon measurements, which were made 400 m apart across the Innsbruck University science campus. This meant they could fully enforce the condition of locality for the first time. Unsurprisingly, the researchers observed a strong violation of Bell’s inequality. Since then, Zeilinger has conducted several loophole-free tests, each one more ingenious than the last, that have put hidden-variables theories to bed.

Beyond Entangled Pairs

Alongside improving upon Clauser and Aspect’s Bell tests, Zeilinger was one of the first researchers to realise that entanglement could be used as a quantum resource for conducting evermore complex quantum physics experiments and developing practical applications in quantum information science.

In 1997, his group and another led by Francesco De Martini were the first to demonstrate quantum teleportation. Quantum teleportation uses entanglement to transfer information carried by an object over to another place where the object is reconstituted. In the experiment, Zeilinger’s group transferred the polarisation direction of one particle over to another without ever learning the polarisation direction that was transported.

Just a year later, Zeilinger and colleagues demonstrated another useful quantum process: entanglement swapping. In the experiment, the researchers swapped entanglement between photons that had never been in contact with each other. Next, in 1999 Zeilinger followed up work he had conducted alongside Daniel Greenberger and Michael Horne a decade earlier that revealed multiparticle entangled states could exist in theory by creating a three-photon entangled state in the laboratory.

More recently, Zeilinger has been at the forefront of research into quantum communication and cryptography. In 2006, his group used an entanglement-based quantum key distribution (QKD) protocol and an optical free space link to establish a secure key (a secret message that gets destroyed by any attempted interception) between the two Canary Islands of La Palma and Tenerife that are separated by 144 km. And in 2017, he collaborated with former student Jian-Wei Pan in wielding China’s Micius quantum-communications satellite to share a QKD key between Beijing and Vienna, 7400 km apart.

Clauser, Aspect and Zeilinger’s pioneering experiments on entanglement have opened the door to new technology based on quantum information, with applications such as quantum computers, quantum networks and secure quantum-encrypted communication now within reach. But perhaps most importantly, they have definitively shown that reality is quantum mechanical. It may be hard to grasp how our seemingly ordered universe is dictated by the randomness of individual quantum events, but knowing that it is sets us on a path down which a deeper understanding of the machinations of reality can be sought.