By Professor Matt Leifer
The Institute for Quantum Studies congratulates Alain Aspect, John F. Clauser and Anton Zeilinger for receiving the 2022 Nobel Prize in Physics https://www.nobelprize.org/prizes/physics/2022/summary/. The citation says that they are awarded the prize “for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science”
Personally, I am very happy about this prize, as I was quoted in the media saying that this kind of experiment would win a Nobel prize back in 2015:
“I wouldn’t be surprised if in the next few years we see one of the authors of this paper, along with some of the older experiments, Aspect’s and others, named on a Nobel prize,” says Matthew Leifer, a quantum physicist at the Perimeter Institute for Theoretical Physics in Waterloo, Canada. “It’s that exciting.”
What are now called Bell inequalities, were first discovered by the Northern Irish physicist John Bell in 1964. Violation of Bell inequalities by entangled quantum particles is one of the most important foundational concepts in quantum mechanics. The mathematics behind Bell inequalities is quite simple, but how we should interpret the violation is still the subject of intense debate. It would be impossible for me to describe what it means in a few sentences without annoying at least some of my colleagues, but here goes anyway. My take is that, subject to background assumptions, the violation shows that faster-than-light influences exist in nature. This is seemingly in conflict with Einstein’s relativity, which sets the speed of light as the ultimate speed limit of nature, but this tension is mitigated by the fact that these faster-than-light influences cannot be used to send a message faster-than-light. The background assumptions include: measurements can only have a single outcome (no splitting into multiple universes), correlations need to have a causal explanation, there are no influences that travel backwards in time, … Each of the background assumptions is denied by at least a small group of experts in quantum mechanics, who therefore think that there are no faster-than-light influences. Despite all of these subtleties, the physics community has decided to call the violation of Bell inequalities “quantum nonlocality”. Think of this as a useful technical shorthand for the conceptual soup of possibilities compatible with Bell inequality violations.
This years Nobel prize was awarded, in part, for experiments that verified that Bell inequalities are actually violated in nature. The first experiments were conducted by Clauser and Freedman in the 1970’s. But these experiments were subject to “loopholes”, meaning that, strictly speaking, a local explanation of their experiments is possible. The two most important loopholes are called the “locality loophole” and the “detection loophole”. Bell experiments involve two locations at which measurements must be made in such a way that there is no possibility of a signal travelling at the speed of light or slower between the two wings on each run of the experiment. This means that the measurement settings need to be switched very rapidly and randomly. In the Clauser-Freedman experiments, the measurement settings were kept the same for long periods of time before switching. This is the locality loophole. Clauser and Freedman’s experiments were done with photons (particles of light) and the efficiency of photon detectors was not very high in the 1970’s. In fact, most of the photons were lost. This opens up the detection loophole. In order to have a local explanation, the measurements would have to violate the predictions of quantum mechanics on some runs of the experiment. We can imagine that the photons “decide” not to be detected whenever they encounter a situation in which they would have to violate the predictions.
Aspect and Zeilinger were responsible for experiments that led the way to closing these loopholes. In the 1980’s, Aspect made progress towards closing the locality loophole by switching the measurement settings rapidly. However, the settings were still switched in a periodic pattern rather than randomly, so this did not fully close the loophole. In the 1990’s, Zeilinger’s group managed to close both loopholes, but in different experiments. The locality loophole was closed by using a quantum random number generator to rapidly switch the measurement settings. The detection loophole was closed by doing the experiment with trapped ions rather than photons. This experiment had a much higher detection efficiency, but there was no question of closing the locality loophole as the ions need to be very close together. Note that, at this point, to retain a local explanation, we would have to imagine that nature exploits the detection loophole when measuring photons, but the locality loophole when measuring ions. Despite the implausibility of this, closing both loopholes in the same experiment, dubbed a “loophole-free” experiment, remained a holy grail.
The first loophole-free experiments were finally performed in 2015. The first to be published was by the group of Ronald Hanson at Delft in the Netherlands. Their experiment used a clever technique called “entanglement swapping” to entangle two remotely located nitrogen vacancies in diamond without the two pieces of diamond ever being in the same location and interacting. The entanglement was transferred from photons, with which we can close the locality loophole, to the nitrogen vacancies, which have a high detection efficiency. Interestingly, Zeilinger’s group were one of the first to demonstrate entanglement swapping, but not in the context of Bell inequality violations.
The Delft experiment was quickly followed by two experiments, one from Zeilinger’s group in Vienna and another from the National Institute of Standards and Technology (NIST) in the USA. Both experiments were more conventional, being based only on entangled photons. They simply exploited improvements in detector efficiency that had occurred in the past two decades, as well as Zeilinger’s method of closing the detection loophole.
By rights, I believe that Hanson deserves a share of the prize, being the first to publish the results of a loophole-free experiment, but here we run into the Nobel prize’s rule that each award can have at most 3 recipients. It is also worth remarking that all of these experiments involved teams of people in the lab, people doing theory calculations, and people doing data analysis. It is only right that we also recognize the achievements of the grad students, postdocs, and faculty who were involved in these experiments.
As well as their fundamental significance, violations of Bell inequalities can be used to generate secure keys for cryptography (sending messages secretly) that are very secure. They would be secure even if quantum mechanics is wrong, provided it remains impossible to send a signal faster than light. This is called “device independent” quantum cryptography. The loophole free experiments pave the way towards this becoming a practical technology.
Bell inequalities are important to much of our work here at IQS. Here are some examples:
- Matt Leifer’s “Science on Tap” about Bell’s theorem: https://youtu.be/ERaF-MDDl4A
- New Scientist article about Cai Waegell and Kelvin McQueen’s work on Bell’s theorem: https://www.newscientist.com/article/2213756-a-classic-quantum-theorem-may-prove-there-are-many-parallel-universes/