2022 Nobel Prize in Physics Quantum Entanglement Experiments Explained - Aspect, Zeilinger, Clauser.

So this has been a very exciting week for quantum physics as the 2022 Nobel Prize in physics has been awarded to Anton Zeilinger, Alain Aspect and John Klauser for their work. They're very long, protracted and very groundbreaking work on quantum entanglement. Verifying it experimentally in the case of Zeilinger and Aspect and verifying John Klauser being the main theorist behind the so-called CHSH inequality which is a form of the Bell inequality relating quantum correlations with statistical counts that show that there are indeed no local hidden variables which affect the aspect of quantum entanglement which is the effect in which photons or electrons that originate from the same source, the same quantum source from a decay or from a non-linear optics effect will be correlated to each other not by some force but by the very strange nature of quantum entanglement which involves no hidden variables effectively. So this experiment which was carried out by Aspect in 1982 was a way in which using John Klauser's proven inequality that he created with Horn and other physicists in 1974 is a way in which one can count statistically using either Geiger counters or photodetectors correlations and relating those correlations to an inequality of summation effectively at different polarizations, different angles of incidence in effect, angles of polarization of the photon can be used to generate an inequality or a number which violates the famous Bell inequality and so this was carried out first by Aspect and then was carried out more by Anton Zeilinger who really perfected the techniques of using non-linear optics in this field. So an example of the experiment is in the image I'm showing you so the source is creating the entangled photons they're the two blue circles here and these two photons are in what's called a singlet or form a density matrix basically of the different polarization states of the two photons and by putting each photon through a opposite polarization filter and detecting the correlations using coincidence detector between the polarizations horizontally or vertically whichever the case may be because they will be random from the source they won't be determined one can actually generate a inequality based on the coincidence counts that will either violate the Bell inequality Bell's inequality so it will mean if there are local hidden variables Bell's inequality says that s must be less than or equal to 2 but if there are not any local hidden variables then s is equal to 2.7 and all the experiments Aspect's experiment being the first one but then Zeilinger's after found that s is indeed not less than or equal to 2 it's greater than 2 so this shows among other things that there is no local let's say hidden variables now there may be global hidden variables that's something I may talk about in a future video but that's in other theories there are theories of the so-called Bohm-de Broglie wave theory in which there may be global hidden variables that somehow guide the local wave functions of these entangled photons or electrons whichever they may be but the experiments of Aspect and Zeilinger showed that under the CHSH inequality formula that there are no local hidden variables so this was very important work done in the 80s really started in the 1960s by John Bell and I have uploaded a documentary several years ago that actually showcases Bell and Aspect both in a documentary so I'll play a clip from it here. It was John Bell who in 1965 derived a theorem which proved that Einstein's thought experiment could be carried out in practice. John Bell became the judge of the controversy. At the time of Einstein and Bohr it was just a theoretical discussion you could endorse either Einstein's position or Bohr's position and it was just a matter of taste and then there was the great discovery by John Bell in 65 that it is not only a matter of taste because according to the position that you have at a certain point you have predictions which are not the same. If you follow Einstein you have a certain type of predictions that is to say you have results in agreement with what is called Bell's inequalities and if you follow Bohr's position you have another type of prediction which are in contradiction in opposition with Bell's inequalities and this was in 1982 in Paris Einstein's thought experiment was put into practice. Aspect's experiment involved the polarization of photons. Polarization means that light oscillates in specific planes. A polarization filter will only permit light to pass if it oscillates in a specific plane. In Aspect's experiment lasers irradiate the element calcium. This causes two photons to be emitted from the same calcium atom simultaneously. Measuring instruments on both sides register the photons. In order to determine the polarizations polarization filters are inserted. Sometimes the photons pass through but other times they are stopped by the polarization filters. The measuring instruments register the photons that pass through. The polarization filters can be turned so that it is possible to register photons of other polarization directions. Aspect compared the polarizations of the two photons and observed a correlation. If the polarization filters were set identically both photons would always either pass through or be stopped. How could this correlation between the two photons be explained? One possibility was that the polarizations of the two photons might be predetermined from the very moment of emission. This would be entirely in the spirit of Einstein. A reasonable picture in the mind of Einstein was to admit that at the very beginning the two particles have something in common. They bring with them a common property until the last moment and at the last moment when you make the measurement the result of the measurement is due to this property. So you have a property at the beginning then each particle brings a property and then the result of the measurement depends on the property and there is nothing spooky in this picture. This was a picture by Einstein. In order to find out whether the polarizations of the two photons could be predetermined the respective settings of the polarization filters would have to be different. Then one might expect a certain limit to the number of pairs of photons that ought to pass through. This limit was known as Bell's inequality. Aspect performed measurements of many thousands of pairs of photons. A counter connected to the measuring instruments recorded when both photons passed through. The setting of the polarization filters was varied in many directions. However the results of the measurements showed that it was impossible for the photons to have predetermined polarizations at the moment of emission. If you believe that the property belong to the photon from the beginning from the emission to the source then the result of measurements must obey to a certain inequality that you call Bell's inequalities. And we have found experimental results violating Bell's inequalities. So we are compelled to reject the idea that the polarization of the photon was already existing just after the emission. The experiment thus pointed to the peculiar fact that the photons chose a common polarization at some moment or other after their emission from the calcium atom. Could it be conceivable that an unknown signal would have time to inform the photons about the setting of the polarization filters? Einstein's theory of relativity insists that no signal can be transmitted faster than the speed of light, namely 186 000 miles per second. Aspect was obliged to improve his experiment. You have to change something on this side fast enough in order to change it before any information has been transmitted to the other side of the experiment. There are 12 meters between both ends of the experiment. It takes 40 nanoseconds for the light to travel from this end to the other end. If I change something in less than 40 nanoseconds then what I do here will not be known by the apparatus on the other side. On both sides Aspect now placed two polarization filters with different polarization directions and in between them a specially developed optical switch. The optical switch now sends the photon out to one of the two polarization filters. It cannot know to which. The switch changes direction in a millionth of a second. Everything now took place so fast that even an unknown signal traveling at the speed of light would not have time to inform the other photon. And when you switch here this optical switch from the up to the down direction everything is equivalent to a single polarizer that you turn from the first direction to the second direction and all this system is equivalent to a single polarizer that you switch very fast within a few nanoseconds from one direction to another direction. This was a trick used for the experiment. Once again Aspect made measurements of thousands of pairs of photons and he was still able to observe a correlation between the two photons. Either both photons would pass through or they would both be stopped. This decisive version of the experiment established the paradoxical fact that the photons did not decide on the same polarization until the actual moment of measurement when they're 12 meters apart. A distance that could have been infinitely greater. We have found results in perfect agreement with the quantum mechanical predictions. There is absolutely no deviation and quantum mechanics predicts that this correlation will not change when you go very far apart. Even if you go to cosmological distances. The conclusion of the experiment supported Niels Bohr's contention that the properties of the two particles are not determined until the moment of measurement. This was what Einstein in 1935 found absurd and described as spooky actions at a distance. And a few decades later and shortly thereafter as Zeilinger in Austria was able to and I'll let him speak in a minute. The basic idea was he was able to use non-linear optics at this stage to provide a high fidelity teleportation of photons. So quantum teleportation is different to aspects experiment and it's more like a gate-based operation. But nevertheless this was a huge achievement for its time. Quantum teleportation was made possible initially over the river Danube as he'll describe now. But since then it's been done between telescopes on the Canary Islands and even in space. And of course the other person who won the prize. Yes John Klauser developed the Klauser inequality. Klauser and Horn's inequality. The CHSH inequality. So Klauser, Horn, Shimony and Holt. And that effectively is an equation in which one can basically using different polarization states in a channel in a quantum channel can violate or determine Bell's inequality. In the case of the experiments of course as I mentioned the aspect experiments in 1982 and later on showed that the experiments violate Bell's inequality or the CHSH inequality. If the polarization states for A and B and A prime and B prime are 0 degrees, 45 degrees, 22.5 degrees and 67.5 degrees in this exact position in a practice chosen so-called Bell test angles. So that's the angle on which it violates the inequality. And of course this has been improved on and done further. And one application though I won't dwell on too much is of course entanglement based key distribution where the classical polarization states are shared, are known. However the actual information that's being transmitted is quantum information and only determined on the measurement of the particle. So it's a way to share quantum keys and that's why it's attracting industry for instance. But one interesting thing at least from my point of view is a so-called Clouser Horn CHSH games. So in fact there are you can actually make games, classical game theory, various classical game theories or classical optimization strategies that you can make quantum using the quantum inequalities discovered by John Clouser. These lead to very interesting outcomes, one of which is an outcome in the Prisoner's Dilemma game. So a few years ago I actually wrote an article on this on my blog back in 2015, Spooky Quantum Strategies and Game Theory. And I basically go through the Prisoner's Dilemma game with Bonnie and Clyde, two thieves that are captured and interviewed by a detective. And they choose to remain silent, that's a variable x, or to confess that's a variable y. And so the goal of the detective is to find well the best strategy for guessing that they both correlate with confessing basically. And you can make a game, a table, in a kind of a classical game theory sort of way. And if you do this you end up with a very standard result in which they have a 75% chance of winning. However you can construct a bell state, so that's the payoff table, same effectively logic as an AND gate, 75% chance of winning if they only have classical physics at their disposal. However if they were to share an quantum entangled state of particles, basically a density matrix or singlet, they would have amplitudes of this signal, singlet, and it will correspond to two entangled qubits in the following bell states that are in a superposition. And they will have coordinates on a block sphere. If they are measurable by Bonnie and Clyde it will correspond to some sort of angle which one can input into the CHSH inequality. And this gives a payoff which is not classical, it's 85.36%, much better than 75% using the classical strategy. So we have a 10.36% advantage over the classical method. So this is very interesting and it's one of many reasons why I'm personally interested in quantum entanglement, quantum networks in general. To plug something, I'm actually developing technology based on using nonlinear optics, beta-baryon borate optics, also periodically polled nonlinear crystals to generate basically entangled states which one can, using polarization controllers and fiber optics, generate remote states that are prepared with a distance from each other. Again it could be in a laboratory or it could be over from country to country, town to town, city to city, or it could be from a satellite to a ground station and using free space optics and so forth. But the main idea is of course is to transmit information using a quantum channel, using effectively a similar setup to the one that Alan Aspect used by measuring quantum correlations and seeing if they violate Bell's inequality, but also to see if we can transmit a density matrix over a quantum channel that's not connected by any classical means and perhaps using a classical channel to assist in polarization control if we wanted to teleport, for example, a quantum key. However, the main interest is to see if it's possible to transmit it in the first place using technology that we have today, but also looking forward and seeing if it could be done using RF microwave frequencies using new technology such as Josephson junctions, superconducting elements that can emit microwaves, basically entangled microwaves. So all of this is a very interesting time to work in this field and definitely the Nobel Prize today has encouraged me to share more information about developments that's occurring and that hopefully I'll be able to participate in in the near future. So that was a long and a short of the Nobel Prize of this year, 2022. And very interesting, it kind of fits into the Nobel Prize given last year, which was on using complex system science, which actually borrowed a lot of techniques from quantum computing, quantum optimization type problems to model climate, weather, even ocean currents. And one video I made a few, about a year ago or so, was actually trying to see if there is concepts that unify complexity theory to quantum entanglement, quantum theory. And there are, there are quite a lot of them. In fact, there's theories of metaheuristics and synchronization in particularly that seem to offer a unification between these two separate fields. The world of the very complex, the very large, the very dynamic, to the world of the quantum, which is very small, very precise, but also very random and very probabilistic. The quantum world, if it can be defined by anything other than entanglement, it's that it's a world of probabilities where things exist in wave functions of probabilities. So thank you very much for listening and I hope this encourages you to look up a bit more about quantum entanglement. It's definitely a technology and an area of physics that's well worth investing the time and energy in studying, as it's really fascinating just how is a principle in which unifies the very small aspect of the quantum world to, again, these kind of large, larger scale setups, experiments where we can see quantum weirdness at play. Okay, thank you very much for listening, and have a good one.