# 2022 Nobel Prize in Physics: Experimental Efforts Towards Quantum Entanglement

## Assoc Professor Kwek Leong Chuan | Co-Director, Quantum Science and Engineering Centre, NTU

Image source: https://www.nobelprize.org/prizes/physics/2022/summary/

**1 Preamble**

I have only met John Clauser once, and that incident was nearly two decades ago. And I don’t think he remembered me at all, as I was listed as a participant from Nanyang Technological University, China (the country was wrongly listed). It was a conference organised by Anton Zeilinger, in honour of John Bell: “Quantum (Un)Speakables: Conference in Commemoration of John S. Bell”, where many people associated with Bell inequality like Roman Jackiw, Roger Penrose, Ed Fry, Reinhardt Bertlmann, Bernard d’Espagnat, Abner Shimony, Michael Horne, Artur Ekert, Marek Zukowski, Gerardus ’t Hooft [BZ02]. I think John even joined us for a dinner organised by Artur Ekert and Lev Vaidman in Vienna. My lasting impression of John Clauser was that he was always carrying a old typewriter everywhere.

Of course, Alain Aspect was also at the conference and so was Anton (organiser). At that conference, I do not remember interacting a lot with Alain, though we got to know each other better later on when he visited Singapore several times. Alain was fond of good food and I brought him to a Mod-Sin restaurant, started by Willin Low, one of my former students. Mod-Sin refers to “Modern Singaporean” cuisine, coined by Willin. Of course, Alain thoroughly enjoyed the dinner.

Anton was the organiser of the conference and naturally he was everywhere. He was actually our tour guide for a trip to look at the statue of the Venus of Willendorf on the banks of the Danube River. In one of his papers [JSW^{+}00], Anton had previously used a large key generated through the Bennett-Brassard 84 protocol [BB84] to transmit an image of the statue.

Images of the Venus of Willendorf figurine. It is estimated to have been made around 25,000–30,000 years ago. This image is taken from https://en.wikipedia.org/wiki/Venus of Willendorf.

**2 The Prize**

On the official Nobel Prize website, the Nobel Prize in Physics 2022 was 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”. Entangled photons were studied a long time ago, as early as 1949, when Wu and Shaknov [WS50], and later by Kocher and Commins [KC67], conducted experiments on correlation of photons. Wu and Shaknov bombarded a sheet of copper with deuterons. This generates an unstable isotope, Cu 64, as a source of positrons -- the antimatter. When a positron and an electron collide, they annihilate each other, releasing two photons that fly apart in opposite directions (paired production). The resulting photons would be polarized in orthogonal polarizations, in congruence with Wheeler’s theory.

**3 Quantum entanglement**

In 1935, Albert Einstein, Boris Podolsky and Nathan Rosen published a paper titled “Can Quantum- Mechanical Description of Physical Reality be Considered Complete?” [EPR35]. The paper was written mainly by Boris Podolsky and Einstein was not completely happy with the published version. In the same journal, Bohr rebuked with another paper with the same title [Boh35]. Stripped of its pedantry, the paper essentially describes the possibility for wave function of two particles to be correlated in such a way that when the momentum of one particle is measured, the momentum of the other particle can be predicted precisely. After all, if the two particles fly apart in opposite directions, their momentum is conserved. Also, when the position of one particle is measured, the position of the other particle is also known. So one can measure the position of one particle, say particle A, and the momentum of the other particle, say particle B. Since the momentum of particle B is known, so is the momentum of particle A. In this way, the position and momentum of particle A can be determined precisely in contradiction to Heisenberg uncertainty principle. This seemingly profound paradox is now popularly called the Einstein-Podolsky-Rosen or EPR paradox.

Einstein believed that Nature is either local or realistic or both. By locality, they referred to the fact that results obtained at one location are independent of any actions performed at another space-like separated site. By realism, they meant that measurement results are determined by properties the particles carry prior to, and independent of, the measurement. Yet, in quantum mechanics, whenever we make a measurement, the wave function collapses and, the single-shot measurement result is random. It is in this context that the authors argued that quantum mechanics as a theory is therefore incomplete.

Experimental realisation using polarisation entangled pairs of photons, a key was created, and it was used to actually transmit the image of the famous Venus von Willendorf sculpture.

**4 Bell inequalities**

In the early sixties, John Bell formulated an inequality that provides constraint on the statistical correlations in experiments involving two subsystems, a so-called bipartite system^{1}. Essentially Bell’s inequality allows the experimentalist to decides if a state shared between two parties is separable or entangled through the correlations.

**5 Experiments**

Bell’s inequality is a testable proposition. We have mentioned that Wu and Shaknov as well as Kocher and Commins were among the first to test for correlation of particles and photons [KC67]. But John Clauser, as a graduate student in Columbia, stumbled on Bell’s paper in the library in 1967 and together with Mike Horne, Abner Shimony and Richard Holt, they proposed a modified Bell inequality, now called the Clauser-Horne-Shimony-Holt (CHSH) inequality [CHSH69] that could put Bell inequality to an experimental test. And John, together with his graduate student, Stuart Freedman went on to prove experimentally that two spatially separated particles (about 10 feet apart) can violate the CHSH inequality [FC72]. Their experiments were in some sense similar to the earlier ones by Kocher and Commins except that in experiments performed by Kocher and Commins, the angles were fixed at 0^{◦}* *and 90* ^{◦}*. These experiments led to many other experimental confirmations [Cla74, Fry73, HP74]. Many years later, John, on why he did the experiments, said: “I found Einstein’s ideas to be very clear. I found Bohr’s rather muddy and difficult to understand.”

^{2}So, that was a reason for doing an experiment.

We need to remember that in the sixties and early seventies, most physicists shunned the subject of local realistic description of Nature. After all, Richard Feynman even told Clauser that testing Bell inequalities is equivalent to a disbelief in quantum theory, which he and many other researchers believe to be right. It was also in this atmosphere that John Bell seminal work in 1964 on Bell’s theorem was actually relegated to obscure journals, like Physics [Bel64], and an underground physics newspaper, Epistemological Letters [Bel85]. Yet, when Clauser wrote to Bell, John Bell actually encouraged Clauser to pursue the experiment on his version of the Bell inequality.

But the first generation of experiments performed by John Clauser and other researchers were not sufficiently convincing for the philosophically minded. Aside from low signal to noise ratios, these experiments suffered from many extra assumptions needed for the test to hold. These assumptions form the main force behind the need to pluck the loopholes in Bell tests. Two most well-known loopholes are the “locality” loophole and the “efficiency” loophole. The early experiments were of course not immune to the loophole of a hypothetically unknown interaction between the two polarisers, which were far away from each other. This is called the locality loophole and Alain Aspect knew about it.

Alain Aspect had the privilege of getting a permanent position as a maître** **assistant at École Normale Supérieure at Cachan. When he approached his PhD supervisor Christian Imbert for a topic, Christian passed him a box of papers on the foundation of quantum mechanics. Among the various papers in the box, there were three papers that caught Alain’s attention: John Bell paper on the inequality [Bel64], the original EPR paper [EPR35] and Bell’s review paper in the review of Modern Physics [Bel66]. He got absorbed into those papers, and he approached Bell that he would like to do experiments that once and for all vindicate quantum theory through his (John Bell’s) inequality. John as before encouraged him to do so, but then he also asked Alain if he had a permanent position[PD23]. In his interview with Bill Phillips and Jean Dalibard, Alain also said: “he (John Bell) explained that I would be considered by a majority of physicists as a crackpot. I didn’t know the word, so I asked him to explain the word ‘crackpot.’ So he told me that most of the physicists would consider in the best case that it was a waste of time, and more often that I was just crazy to do so[*sic*].” [PD23]

Alain started his first experiment in collaboration with Aspect’s undergraduate student, Philippe Grangier, and a research engineer, G´erard Roger[AGR81]. In the second experiment, still with Grangier and Roger as collaborators, Aspect used two-channel polarisers to have a straightforward transposition of the EPR Gedankenexperiment [AGR82]. In all these experiments, they were able to show how one could overcome some of the loopholes in the theory. In fact, Aspect’s experiments were performed using fast-switching acoustic-optic modulators, and the Bell inequality was violated to 5 standard deviations [ADR82]. Also, in the Aspect’s experiment, the source was separated from the polarizer by 6.5 m.

But this distance is going to be increased. In the eighties and nineties, there were a furry of beautiful experiments [OM88, OPKP92, KESC94, KSC93] involving nonlinear splitting of ultraviolet photons to produce pairs of correlated EPR photons. These EPR pairs allow discrete variables measurements such as polarisation or spin components, as considered by Bell, or continuous variables measurements of the type originally considered by Einstein, Podolsky and Rosen [Asp99].

In 1998, Anton Zeilinger and his team improved on Aspect’s 1982 experiment with time-varying analyzers [WJS^{+}98]. Their experiment tightened the locality loophole. They switched to analysers that could satisfy the need for random settings. And, their detectors were separated by 400 m, and yet they violate Bell’s inequality impressively by more than 30 standard deviations. By 2015, there were several experiments that attempted to close the locality and the detection (fair-sampling) loopholes. These experiments were performed in at the University of Vienna led by Anton Zeilinger [GVW^{+}15], at NIST in Boulder, Colorado by Lynden Shalm [SMSC^{+}15], and at Delft University of Technology by Ronald Hanson[HBD^{+}15]. In particular, the last experiment extends the distance betwen two electron spins to 1.3 km. Zeilinger’s and Shalm’s teams used the state-of-the-art detectors with efficiency above 90%. Entangled photon pairs were produced through parametric down conversion and they installed new random number generators capable of changing the alignments of the polarisers more efficiently. Hanson’s team used electron spins in solid state system and developed an advanced strategy to prevent the fair-sampling loophole. The spatial separation of the detectors for the violation of Bell inequalities has also been extended to longer and longer distances [PYB^{+}05, TBZG98, YCL^{+}17]. By 2015, this distance has already extended to more than 100 km through satellite connection. There were even proposals and tests of photons violating Bell inequalities from distant quasar sources [GFK14, RHH^{+}18].

###### 6 Quantum Entanglement as a Resource

But Anton Zeilinger is also a master of quantum entanglement. By the nineties, several schemes using quantum entanglement as a resource were proposed: the teleportation of an unknown quantum state between two distant observers [BB84], the application of Bell inequality to quantum key distribution [Eke91], a quantum communication scheme, known as quantum dense coding, enabling the transmission of more than one bit of classical information with the transmission of a single particle [BW92] and so forth. In each case, Anton led beautiful experiments that convincingly demonstrated the viability [BPM^{+}97, JSW^{+}00, PFU^{+}04, MWKZ96]. Perhaps, what is even more important is the fact that one has gone through ostensibly “charlatan” physics to real serious theoretical work to experimental verification and future applications.

Watch the video on the 2022 Nobel Prize In Physics

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