Quantum physics

Death by experiment for local realism

A fundamental scientific assumption called local realism conflicts with certain predictions of quantum mechanics. Those predictions have now been verified, with none of the loopholes that have compromised earlier tests. See Letter p.682

The world is made up of real stuff, existing in space and changing only through local interactions — this local-realism hypothesis is about the most intuitive scientific postulate imaginable. But quantum mechanics implies that it is false, as has been known for more than 50 years1. However, brilliantly successful though quantum mechanics has been, it is still only a theory, and no definitive experiment has disproved the local-realism hypothesis — until now. On page 682 of this issue, Hensen et al.2 report the first violation of a constraint called a Bell inequality, under conditions that prevent alternative explanations of the experimental data. Their findings therefore rigorously reject local realism, for the first time.

Bell inequalities are named after John Bell, the physicist who discovered in 1964 that the predictions of quantum mechanics are incompatible with the local-realism hypothesis1. There are many different ways to make this hypothesis precise3, but Hensen and colleagues' exposition basically follows Bell's original formulation, which states it as the conjunction of two other hypotheses: realism (which Bell called predetermination), essentially meaning that measurements reveal pre-existing physical properties of the world; and locality, roughly meaning that any change enacted at one place cannot have an effect at another place unless there would have been time for a light signal to get from the first place to the second place. The speed of light is relevant because, according to Einstein's theory of relativity, no causal influence can travel faster than light.

A Bell inequality is a mathematical relationship regarding the statistics of measurement outcomes obtained by two or more parties, and also involving the measurement settings chosen by those parties. Suppose that the parties are in well-separated laboratories, and that the measurement settings are chosen and implemented, and the outcomes obtained, in a sufficiently short time that the only way the choice of setting by any party could affect the outcome of any other party would be through a faster-than-light influence. Then, by definition, all Bell inequalities will be satisfied by all local-realistic theories. An experiment violating a Bell inequality therefore implies that either locality or realism is false. Bell's theorem is that, according to quantum mechanics, such an experiment is possible if the parties share particles prepared in a suitable entangled state. Entanglement is a holistic property of a system of quantum particles that can persist even when the particles are far apart.

Bell inequalities have been violated experimentally many times before4,5,6,7,8,9. However, all of these experiments had loopholes. Either the parties were not far enough apart, given how long it took for the processes involved (randomly choosing a setting, adjusting the apparatus appropriately and obtaining an outcome), or the measurements were inefficient, so that quite often no outcome at all was registered. The inefficiency is relevant because it can allow the existence of local realistic theories — albeit highly contrived ones — that exploit the existence of null outcomes to simulate the correlations of quantum mechanics.

Several groups worldwide have been racing to perform the first Bell experiment that combines large separation, efficient detection and fast operation of the apparatus. Hensen et al. have won the race by using a new scheme. Previously, the leading approach was to prepare an entangled state of two photons, send one to one laboratory — conventionally called Alice's — and the other to a second laboratory, Bob's. By contrast, Hensen and colleagues' experiment should be regarded as using a three-party Bell inequality.

In this three-party approach (Fig. 1), Alice and Bob each prepare an entangled state of a photon and an electron, keep their electrons in a diamond lattice and send their photons to Juanita, as I'll call her. Alice and Bob then each choose a setting and measure their electrons, which can be done efficiently, while Juanita performs a joint measurement on the two photons. Alice's and Bob's outcomes will be purely random unless Juanita gets a rare 'successful' result, in which case the outcomes indicate entanglement between Alice's and Bob's electrons. Unlike Alice and Bob, Juanita always makes the same measurement, and so its inefficiency does not open a loophole.

Figure 1: Violation of a three-party Bell inequality.

A Bell inequality is a mathematical relationship regarding the statistics of measurement outcomes obtained by two or more parties. Under certain physical conditions relating to the timing of events, a violation of a Bell inequality proves that local realism — a hypothesis satisfied in all of science except quantum mechanics — is false. Hensen et al.2 have violated a Bell inequality in such a way that the requisite physical conditions were satisfied for the first time, using the scheme shown in this cartoon. a, At separate locations, Alice and Bob create entangled states of an electron and a photon, then send the photons to Juanita's laboratory. b, Alice and Bob randomly choose a setting for measurements of their respective electrons. c, They obtain their measurement outcomes, and Juanita performs a joint measurement of the photons. Alice's and Bob's outcomes are purely random unless Juanita gets a rare successful outcome (as shown here) that indicates entanglement between Alice's and Bob's electrons. By collating the results over many runs, Hensen et al. showed that a Bell inequality had been violated by a statistically significant amount.

Hensen and co-workers' experiment was made possible only by combining state-of-the art quantum technologies — it was performed in the Netherlands, but used diamond substrates prepared in the United Kingdom and fast random-number generators developed in Spain. Maintaining optimal operation of all the devices during the experiments was extremely challenging, and the rate of events (defined as Juanita getting a successful outcome) was only about one per hour. As a consequence, only 245 such events were recorded, and the statistical uncertainty in the reported Bell-inequality violation is comparatively large. Nevertheless, from a careful analysis of the entire data set, including runs in which Juanita did not get the desired outcome, Hensen et al. reject the local-realism null hypothesis at a confidence level conventionally considered to be statistically significant. It is to be hoped that more data will be generated soon.

The authors' approach might allow them to implement quantum-information protocols enabling secure communication, even when the devices used are not trusted by the users. For this to be practical, the event rate would have to be massively increased above its current level. However, the basic technology and the scheme (involving joint measurements by the intermediary Juanita) are promising.

The immediate significance of the reported experiment, however, is in hammering the final nail in the coffin of local realism. Some almost metaphysical loopholes remain open — if the results can be replicated with humans, rather than machines, freely choosing the measurement settings and consciously registering the outcomes, then the coffin will have been interred and buried. That experiment, however, is for many years hence. For the moment, we should celebrate Hensen and colleagues' landmark achievement in physics. Footnote 1


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Correspondence to Howard Wiseman.

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Wiseman, H. Death by experiment for local realism. Nature 526, 649–650 (2015). https://doi.org/10.1038/nature15631

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