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Experimental
Written by Srinjini Mukhopadhyay, Sayan Pal
Historical Background :
Historical Background :
There are mainly 4 thought experiments/ theories that set the background for the experiments conducted by Clauser, Aspect and Zeilinger.
EPR Paradox/ Thought Experiment :
EPR Paradox/ Thought Experiment :
The Heisenberg Uncertainty Principle states that the position and the momentum of an object cannot be measured simultaneously with infinite precision. In 1935, Albert Einstein, Boris Podolsky and Nathan Rosen published the paper, ‘Can Quantum-Mechanical Description of Physical Reality be Considered Complete?’ where they discussed the behaviour of a system of entangled pairs from the classical and quantum mechanical points of view. Quantum Mechanics states that for an entangled particle pair if the position (or any other physical quantity) of the first particle were measured, the result of measuring the position (corresponding physical quantity) of the second particle could be predicted, no matter how far apart they are. From the classical point of view, this suggested that either the measurement of the first particle somehow interacted with the second particle at a speed faster than the speed of light or that the entangled particles had some unmeasured property that pre-determined (deterministic model) their final quantum states before they were separated. They argued that no action taken on the first particle could instantaneously affect the other, since this would require information being transmitted faster than light, not following the theory of relativity and defying ‘locality’ and thus, the second particle must have a definite value of both position/ momentum before either being measured. Therefore, they concluded, quantum mechanics must be incomplete, because it cannot give a complete description of the particle's true physical characteristics. In other words, quantum particles, like electrons and photons, must carry some property or attributes not included in quantum theory, and the uncertainties in quantum theory are due to these unknown properties, later termed the "hidden variables".
Schematic of an EPR Thought Experiment
Schrodinger’s Proposals :
Schrodinger’s Proposals :
In his 1935 article ‘Discussion of probability relations between separated systems’, Erwin Schrödinger stated: “When two systems, represented by their respective functions, enter into temporary physical interaction due to known forces between them, and when after a time of mutual influence the systems separate again, then they can no longer be described in the same way as before, viz. by endowing each of them with a separate function, as the particles have become entangled.”
This, he considered, to be one of the characteristic traits of quantum mechanics, the one that enforces its entire departure from classical lines of thought.
3. Many Worlds Interpretation :
3. Many Worlds Interpretation :
In 1957, Hugh Everett proposed the ‘many-worlds interpretation’ (the possibility of many parallel worlds existing in the same space and time as our own) that pointed to the sharp distinction between the microscopic phenomena described by quantum mechanics that are studied by the macroscopic detectors which are assumed to obey the laws of classical physics. The ‘many-worlds interpretation’ claimed that whenever such a measurement took place, a different world is created and that there is no connection between these different worlds. In this interpretation, Schrödinger’s cat would be alive in one world and dead in another. However, these theories could not put forth any experimentally testable predictions
4. Bell Inequalities :
4. Bell Inequalities :
In 1964, John Bell proved mathematically, by developing a special setup of the EPR thought experiment, that no hidden variable theory would be able to reproduce all the results of quantum mechanics. He also showed mathematical illustrations, proving that all attempts to construct a local realist model of quantum phenomena are doomed to fail. He used the term ‘local’ to mean the impossibility of instantaneous signalling, limited by the finite speed of light, and ‘realist’ to mean that the outcome of any experiment is fully determined by the properties of the system. The thought experiment considered by Bell was not suitable for physical experimental design, as the assumptions made about the detectors were not possible for any real equipment.
Laboratory Experiments :
The next 4 real life experiments demonstrate the violation of Bell's Inequalities, leading to the achievement of the Nobel Prize 2022:
CHSH Experiments :
John Clauser, Michael Horne, Abner Shimony and Richard Holt (CHSH) proposed a variation of the Bell inequality that made it possible to check the inequality by a physical experiment on entangled photons using existing technology. The experiment involved sending a pair of entangled photons in opposite directions. When these particles are emitted, their direction of polarisation is undetermined and all that is certain is that the particles have parallel polarisation. This can be investigated by using a filter that allows photons that are oriented in a particular direction. If both the particles in the experiment are sent towards filters that are oriented in the same plane, such as vertically, and one slips through – then the other one will also go through. If they are at right angles to each other, one will be stopped while the other will go through. The trick is to measure with the filters set in different directions at skewed angles, as then the results can vary: sometimes both slip through, sometimes just one, and sometimes none. How often both particles get through the filter depends on the angle between the filters. Quantum mechanics leads to a correlation between measurements. The likelihood of one particle getting through depends on the angle of the filter that tested its partner’s polarisation on the opposite side of the experimental setup. This means that the results of both measurements, at some angles, violate a Bell inequality and have a stronger correlation than they would if the results were governed by hidden variables and were already predetermined when the particles were emitted. The next 4 real life experiments demonstrate the violation of Bell's Inequalities, leading to the acheivement of the Nobel Prize 2022:
Freedman- Clauser Experiment :
Freedman- Clauser Experiment :
Clauser was quite interested in quantum mechanics. During his postdoctoral research work at UC Berkeley, he carried out experiments on Bell’s Inequality along with a PhD student, Stuart Freedman. He had prior knowledge of the experimental setup developed by Carl Kocher to study the time correlation between pairs of photons originating from a common source. Clauser thought this experimental equipment could be used and improved to test the Bell–CHSH inequality. In Kocher’s experiment, the two photons had a common origin, so they can be shown to be entangled. But, he could not test the Bell-CHSH inequality due to the angle between the polarisers he chose. Clauser was by now well aware that no previous experiment had tested the Bell–CHSH inequality. Clauser and Freedman identified the polarizers to be the weak point in Kocher’s experiment. The polarizers’ inefficiency made it prohibitively time-consuming to carry out the experiment at a number of different angles between the polarizers. So, they used a different kind of polariser. It took two years for Freedman and Clauser to rebuild Kocher’s experiment so that it could be used to test the Bell–CHSH inequality, and about 200 hours to record the data. The experimental data they got, clearly violated the Bell-CHSH identity and agreed with the predictions of quantum mechanics.
The Aspect Experiments :
The Aspect Experiments :
In the years that followed, John Clauser and other physicists continued discussing the experiment and its limitations. One of these was that the experiment was generally inefficient, both when it came to producing and capturing particles. The measurement was also pre-set, with the filters at fixed angles. And questions would remain: what if the experimental setup in some way selected the particles that happened to have a strong correlation, and did not detect the others? In that case, the particles could still be carrying hidden information. Eliminating this particular loophole was difficult because entwined quantum states are fragile and very difficult to manage, making it absolutely necessary to deal with individual photons.
In 1981 and 1982, French doctoral student Alain Aspect along with his collaborators Phillipe Grangier, Gérard Roger and Jean Dalibard, built a new version of the setup that he refined over several iterations. In his experiment, he could register the photons that passed through the filter and those that did not. This meant more photons were detected and the measurements were better. In the final variant of his tests, he was also able to steer photons towards two different filters that were set at different angles. The Finesse was a mechanism that switched the direction of the entangled photons after they had been created and emitted from their source. The filters were just six metres away, so the switch needed to occur in a few billionths of a second. If information about which filter the photon would arrive at, influenced how it was emitted from the source, it would not be arriving at that filter. Nor could information about the filters on one side of the experiment reach the other side and affect the result of the measurement there. In this way, Alain Aspect closed an important loophole and provided a very clear result: quantum mechanics is correct and there are no hidden variables.
Zeilinger's Experiment :
Zeilinger's Experiment :
Clauser and Aspect’s experiments sort of laid the era of quantum mechanics and stirred the thought of practical applications of Quantum Mechanics among physicists. The power to manipulate and manage quantum states and all their layers of properties gives us access to tools with unexpected potential. This is the basis for quantum computation, the transfer and storage of quantum information, and algorithms for quantum encryption. Anton Zeilinger and his colleagues were the first to explore systems with more than two entangled particles that are now in use.
In 1997 Zeilinger and his team performed the first experimental demonstration of quantum teleportation which uses entanglement to allow quantum states to be moved from one particle to another across arbitrary distances and also executed a similar operation called entanglement swapping. Very soon, Zeilinger’s group also entangled multiple photons in what’s now called a GHZ (Greenberger-Horne-Zeilinger) state and proved that it violated the Bell inequalities. They also set up a novel physical experiment that was able to close the major loopholes simultaneously. In 2015, Zeilinger used detectors that clocked photons efficiently enough to ensure that undetected photons couldn’t tilt the scales. The detectors were also spaced sufficiently distant to close the locality loophole more definitively than done by Aspect’s team. Zeilinger, Kaiser, and colleagues also addressed another loophole, dubbed “freedom-of-choice,” by using light from stars to determine the measurements to be performed on pairs of entangled photons. Today quantum teleportation has become central for nascent efforts to build a globe-spanning “quantum Internet.” Zeilinger also worked to create China’s Micius spacecraft, the first quantum communications satellite.
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