{"id":179,"date":"2022-10-06T00:36:19","date_gmt":"2022-10-06T00:36:19","guid":{"rendered":"https:\/\/blogschapman.wpenginepowered.com\/the-institute-for-quantum-studies\/2022\/10\/06\/this-years-nobel-prize-in-physics\/"},"modified":"2022-10-06T00:36:19","modified_gmt":"2022-10-06T00:36:19","slug":"this-years-nobel-prize-in-physics","status":"publish","type":"post","link":"https:\/\/blogs.chapman.edu\/the-institute-for-quantum-studies\/2022\/10\/06\/this-years-nobel-prize-in-physics\/","title":{"rendered":"This year's Nobel Prize in Physics"},"content":{"rendered":"

By Professor Matt Leifer<\/strong><\/p>\n

The Institute for Quantum Studies congratulates Alain Aspect, John F. Clauser and Anton Zeilinger for receiving the 2022 Nobel Prize in Physics\u00a0 https:\/\/www.nobelprize.org\/prizes\/physics\/2022\/summary\/<\/a>.\u00a0 The citation says that they are awarded the prize \u201cfor experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science\u201d\"Matt_Leifer\"<\/p>\n

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:<\/p>\n

\u201cI wouldn\u2019t 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\u2019s and others, named on a Nobel prize,\u201d says Matthew Leifer, a quantum physicist at the Perimeter Institute for Theoretical Physics in Waterloo, Canada. \u201cIt\u2019s that exciting.\u201d<\/p>\n

https:\/\/www.nature.com\/articles\/nature.2015.18255<\/a><\/p>\n

What are now called Bell inequalities, were first discovered by the Northern Irish physicist John Bell in 1964.\u00a0 Violation of Bell inequalities by entangled quantum particles is one of the most important foundational concepts in quantum mechanics.\u00a0 The mathematics behind Bell inequalities is quite simple, but how we should interpret the violation is still the subject of intense debate.\u00a0 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.\u00a0 My take is that, subject to background assumptions, the violation shows that faster-than-light influences exist in nature.\u00a0 This is seemingly in conflict with Einstein\u2019s 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.\u00a0 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, \u2026\u00a0 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.\u00a0 Despite all of these subtleties, the physics community has decided to call the violation of Bell inequalities \u201cquantum nonlocality\u201d.\u00a0 Think of this as a useful technical shorthand for the conceptual soup of possibilities compatible with Bell inequality violations.<\/p>\n

This years Nobel prize was awarded, in part, for experiments that verified that Bell inequalities are actually violated in nature.\u00a0 The first experiments were conducted by Clauser and Freedman in the 1970\u2019s.\u00a0 But these experiments were subject to \u201cloopholes\u201d, meaning that, strictly speaking, a local explanation of their experiments is possible.\u00a0 The two most important loopholes are called the \u201clocality loophole\u201d and the \u201cdetection loophole\u201d.\u00a0 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.\u00a0 This means that the measurement settings need to be switched very rapidly and randomly.\u00a0 In the Clauser-Freedman experiments, the measurement settings were kept the same for long periods of time before switching.\u00a0 This is the locality loophole.\u00a0 Clauser and Freedman\u2019s experiments were done with photons (particles of light) and the efficiency of photon detectors was not very high in the 1970\u2019s.\u00a0 In fact, most of the photons were lost.\u00a0 This opens up the detection loophole.\u00a0 In order to have a local explanation, the measurements would have to violate the predictions of quantum mechanics on some runs of the experiment.\u00a0 We can imagine that the photons \u201cdecide\u201d not to be detected whenever they encounter a situation in which they would have to violate the predictions.<\/p>\n

Aspect and Zeilinger were responsible for experiments that led the way to closing these loopholes.\u00a0 In the 1980\u2019s, Aspect made progress towards closing the locality loophole by switching the measurement settings rapidly.\u00a0 However, the settings were still switched in a periodic pattern rather than randomly, so this did not fully close the loophole.\u00a0 In the 1990\u2019s, Zeilinger\u2019s group managed to close both loopholes, but in different experiments.\u00a0 The locality loophole was closed by using a quantum random number generator to rapidly switch the measurement settings.\u00a0 The detection loophole was closed by doing the experiment with trapped ions rather than photons.\u00a0 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.\u00a0 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.\u00a0 Despite the implausibility of this, closing both loopholes in the same experiment, dubbed a \u201cloophole-free\u201d experiment, remained a holy grail.<\/p>\n

The first loophole-free experiments were finally performed in 2015.\u00a0 The first to be published was by the group of Ronald Hanson at Delft in the Netherlands.\u00a0 Their experiment used a clever technique called \u201centanglement swapping\u201d to entangle two remotely located nitrogen vacancies in diamond without the two pieces of diamond ever being in the same location and interacting.\u00a0 The entanglement was transferred from photons, with which we can close the locality loophole, to the nitrogen vacancies, which have a high detection efficiency.\u00a0 Interestingly, Zeilinger\u2019s group were one of the first to demonstrate entanglement swapping, but not in the context of Bell inequality violations.<\/p>\n

The Delft experiment was quickly followed by two experiments, one from Zeilinger\u2019s group in Vienna and another from the National Institute of Standards and Technology (NIST) in the USA.\u00a0 Both experiments were more conventional, being based only on entangled photons.\u00a0 They simply exploited improvements in detector efficiency that had occurred in the past two decades, as well as Zeilinger\u2019s method of closing the detection loophole.<\/p>\n

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\u2019s rule that each award can have at most 3 recipients.\u00a0 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.\u00a0 It is only right that we also recognize the achievements of the grad students, postdocs, and faculty who were involved in these experiments.<\/p>\n

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.\u00a0 They would be secure even if quantum mechanics is wrong, provided it remains impossible to send a signal faster than light.\u00a0 This is called \u201cdevice independent\u201d quantum cryptography.\u00a0 The loophole free experiments pave the way towards this becoming a practical technology.<\/p>\n

Bell inequalities are important to much of our work here at IQS.\u00a0 Here are some examples:<\/p>\n