Bell-Test
Bell's Theorem states that a "Bell inequality" must be obeyed under any local hidden variable theory but is violated under quantum mechanics (QM). The term "Bell inequality" can mean any one of a number of inequalities — in practice, in real experiments, the CHSH or CH74 inequality, not the original one derived by John Bell. It places restrictions on the statistical results of experiments on pairs of particles that have taken part in a quantum-mechanical interaction and then separated. A Bell test experiment is one designed to test whether or not the real world obeys a Bell inequality. Such experiments fall into two classes, depending on whether the analyser used has one or two output channels.
Conduct of Bell test experiments
A typical CHSH (two-channel) experiment
The source S produces pairs of "photons", sent in opposite directions. Each photon encounters a two-channel polariser whose orientation can be set by the experimenter. Emerging signals from each channel are detected and coincidences counted by the coincidence monitor CM.
In practice most actual experiments have used light, assumed to be emitted in the form of particle-like photons, rather than the atoms that Bell originally had in mind. The property of interest is, in the best known experiments (Aspect, 1981, 1982a,b), the polarisation direction, though other properties can be used. The diagram shows a typical optical experiment of the two-channel kind for which Alain Aspect set a precedent in 1982 (Aspect, 1982a). Coincidences (simultaneous detections) are recorded, the results being categorised as '++', '+−', '−+' or '−−' and corresponding counts accumulated.
Four separate subexperiments are conducted, corresponding to the four terms E(a, b) in the test statistic S ((2) below). The settings a, a′, b and b′ are generally in practice chosen to be 0, 45°, 22.5° and 67.5° respectively — the "Bell test angles" — these being the ones for which the QM formula gives the greatest violation of the inequality.
For each selected value of a and b, the numbers of coincidences in each category (N++, N--, N+- and N-+) are recorded. The experimental estimate for E(a, b) is then calculated as:
(1) E = (N++ + N-- − N+- − N-+)/(N++ + N-- + N+- + N-+).
Once all four E’s have been estimated, an experimental estimate of the test statistic
(2) S = E(a, b) − E(a, b′) + E(a′, b) + E(a′ b′)
can be found. If S is numerically greater than 2 it has infringed the CHSH inequality. The experiment is declared to have supported the QM prediction and ruled out all local hidden variable theories.
A strong assumption has had to be made, however, to justify use of expression (2). It has been assumed that the sample of detected pairs is representative of the pairs emitted by the source. That this assumption may not be true comprises the fair sampling loophole. No absolute check on its validity is feasible.
The derivation of the inequality is given in the CHSH Bell test page.
A typical CH74 (single-channel) experiment
The source S produces pairs of "photons", sent in opposite directions. Each photon encounters a single channel (e.g. "pile of plates") polariser whose orientation can be set by the experimenter. Emerging signals are detected and coincidences counted by the coincidence monitor CM.
Prior to 1982 all actual Bell tests used "single-channel" polarisers and variations on an inequality designed for this setup. The latter is described in Clauser, Horne, Shimony and Holt's much-cited 1969 article (Clauser, 1969) as being the one suitable for practical use. As with the CHSH test, there are four subexperiments in which each polariser takes one of two possible settings, but in addition there are other subexperiments in which one or other polariser or both are absent. Counts are taken as before and used to estimate the test statistic
(3) S = (N(a, b) − N(a, b′) + N(a′, b) + N(a′, b′) − N(a′, ∞) − N(∞, b)) / N(∞, ∞),
where the symbol ∞ indicates absence of a polariser.
If S exceeds 0 then the experiment is declared to have infringed Bell's inequality and hence to have "refuted local realism".
The only theoretical assumption (other than Bell's basic ones of the existence of local hidden variables) that has been made in deriving (3) is that when a polariser is inserted the probability of detection of any given photon is never increased: there is "no enhancement". The derivation of this inequality is given in the page on Clauser and Horne's 1974 Bell test.
Experimental assumptions
In addition to the theoretical assumptions made, there are practical ones. There may, for example, be a number of "accidental coincidences" in addition to those of interest. It is assumed that no bias is introduced by subtracting their estimated number before calculating S, but that this is so is not obvious (Thompson, 2003) and needs to be justified in every instance. There may be synchronisation problems — ambiguity in recognising pairs due to the fact that in practice they will not be detected at exactly the same time.
Nevertheless, despite all these deficences of the actual experiments, one striking fact emerges: the results are exactly what quantum mechanics predicts. Thus, if imperfect experiments give us such excellent overlap with quantum predictions, a reasonable bet would be to predict that once a perfect Bell test is done, the Bell inequalities would be still violated. This attitude dominates among working quantum physicists, and it has lead to the emergence of a new sub-field of physics which is now known as quantum information theory. One of the main achievements of this new branch of physics is showning that violation of Bell's inequalities leads to the possiblity of a secure information transfer, which utilizes the so-called quantum crytography (involving entangled states of pairs of particles).
Notable experiments
There have now been a great number of Bell test experiments conducted, almost all claimed to have confirmed quantum theory and shown effects that cannot be explained under local hidden variable theories. The later experiments have often been more of the nature applications of quantum entanglement, though, the Bell test being conducted simply in order to confirm that the particular setup is producing entangled pairs. There is no pretence in these of any serious attempt to search for alternative explanations.
Freedman and Clauser, 1972
- This was the first actual Bell test, using Freedmans' inequality, a variant on the CH74 inequality. A moderate violation was found. It is possible that the test was biased due to synchronisation problems, as the time window used for coincidences was short.
Aspect, 1981-2
- Aspect and his team at Orsay, Paris, conducted three Bell tests using calcium cascade sources. The first and last used the CH74 inequality, the first being notable as one of the few for which some of the raw data is available. The second was the first application of the CHSH inequality, the third the famous one (originally suggested by John Bell) in which the choice between the two settings on each side was made during the flight of the photons. It is claimed to have blocked the "locality" loophole. All three experiments had large numbers of "accidentals", the subtraction of which is of debatable validity. Aspect defended his decision on these in 1985 (Lettere al Nuovo Cimento 43, 345). C. H. Thompson has analysed the 1981 data and reported the results in (Thompson, 2003). Without the subtraction of accidentals the data does not violate the inequality. The status of the experiments, however, remains unshaken: they are generally accepted as the definitive evidence for quantum entanglement.
Tittel and the Geneva group, 1998
- The Geneva 1998 Bell test experiments showed that distance did not destroy the "entanglement". Light was sent in fibre optic cables over distances of several kilometers before it was analysed. As with almost all Bell tests since about 1985, a "parametric down-conversion" source was used. In interpreting these experiments the possible loopholes to be born in mind include the subtraction of accidentals in some (Thompson, 2002), the fair sampling loophole in others.
Weihs' experiment under "strict Einstein locality" conditions
In 1998 Gregor Weihs and a team at Innsbruck conducted an ingenious experiment that closed the "locality" loophole, improving on Aspect's of 1992. The choice of detector was made using a quantum process to ensure that is was random. The Bell test used, however, was the CHSH one, with its associated requirement to assume fair sampling. The experiment may also illustrate a few other interesting loopholes. See the "Bell test loopholes" page.
References
- Aspect, 1981: A. Aspect et al., Experimental Tests of Realistic Local Theories via Bell's Theorem, Phys. Rev. Lett. 47, 460 (1981)
- Aspect, 1982a: A. Aspect et al., Experimental Realization of Einstein-Podolsky-Rosen-Bohm Gedankenexperiment: A New Violation of Bell's Inequalities, Phys. Rev. Lett. 49, 91 (1982), available at http://fangio.magnet.fsu.edu/~vlad/pr100/
- Aspect, 1982b: A. Aspect et al., Experimental Test of Bell's Inequalities Using Time-Varying Analyzers, Phys. Rev. Lett. 49, 1804 (1982), available at http://fangio.magnet.fsu.edu/~vlad/pr100/
- Clauser, 1969: J. F. Clauser, M.A. Horne, A. Shimony and R. A. Holt, Proposed experiment to test local hidden-variable theories, Phys. Rev. Lett. 23, 880-884 (1969), available at http://fangio.magnet.fsu.edu/~vlad/pr100/
- Freedman, 1972: S. J. Freedman and J. F. Clauser, Experimental test of local hidden-variable theories, Phys. Rev. Lett. 28, 938 (1972)
- Thompson, 2003: C. H. Thompson, Subtraction of ‘accidentals’ and the validity of Bell tests, Galilean Electrodynamics 14 (3), 43-50 (2003)
- Tittel, 1998: W. Tittel et al., Experimental demonstration of quantum-correlations over more than 10 kilometers, Physical Review A 57, 3229 (1998); Violation of Bell inequalities by photons more than 10 km apart, Physical Review Letters 81, 3563 (1998)
- Weihs, 1998: G. Weihs, et al., Violation of Bell’s inequality under strict Einstein locality conditions, Phys. Rev. Lett. 81, 5039 (1998)