Viewpoint: A Closer Connection Between Entanglement and Nonlocality

Serge Massar and Stefano Pironio, Laboratoire d'Information Quantique, CP 225, Université libre de Bruxelles, Av. F. D. Roosevelt 50, B-1050 Bruxelles, Belgium

A generalization of one of the most famous experiments in quantum foundations provides a powerful new unifying concept.

Figure 1: S distributes the two parts of an entangled state ρ A B to two measuring devices. The measuring devices are provided with instructions on what measurement to perform. The measuring device then provides a classical output, symbolized here by which light bulb—green or red—lights up. (a) In the standard Bell scenario, the instructions to the measurement devices are classical [ σ A B . This scenario enables the "activation" of hidden nonlocality. (c) In the semiquantum scenario introduced by Buscemi [ i and j fed to the detectors are encoded in quantum systems described by quantum states σ A i and σ B j . Appropriate choice of the states σ A i and σ B j can produce nonlocal correlations for all entangled states ρ A B . Different experiments can be used to test nonlocality. In each case, a sourcedistributes the two parts of an entangled stateto two measuring devices. The measuring devices are provided with instructions on what measurement to perform. The measuring device then provides a classical output, symbolized here by which light bulb—green or red—lights up. (a) In the standard Bell scenario, the instructions to the measurement devices are classical [ 3 ]. The state exhibits nonlocality if the correlations between measurement settings and measurement results violate a Bell inequality. (b) In the scenario considered by Masanes, Liang, and Doherty [ 8 ], the instructions are classical, as in the standard Bell scenario, but the measurement apparatuses have access to an auxiliary entangled state. This scenario enables the "activation" of hidden nonlocality. (c) In the semiquantum scenario introduced by Buscemi [ 4 ], the instructionsandfed to the detectors are encoded in quantum systems described by quantum statesand. Appropriate choice of the statesandcan produce nonlocal correlations for all entangled states Different experiments can be used to test nonlocality. In each case, a source S distributes the two parts of an entangled state ρ A B to two measuring devices. The measuring devices are provided with instructions on what measurement to perform. The mea... Show more

Figure 1: Different experiments can be used to test nonlocality. In each case, a source S distributes the two parts of an entangled state ρ A B to two measuring devices. The measuring devices are provided with instructions on what measurement to perform. The measuring device then provides a classical output, symbolized here by which light bulb—green or red—lights up. (a) In the standard Bell scenario, the instructions to the measurement devices are classical [3]. The state exhibits nonlocality if the correlations between measurement settings and measurement results violate a Bell inequality. (b) In the scenario considered by Masanes, Liang, and Doherty [8], the instructions are classical, as in the standard Bell scenario, but the measurement apparatuses have access to an auxiliary entangled state σ A B . This scenario enables the "activation" of hidden nonlocality. (c) In the semiquantum scenario introduced by Buscemi [4], the instructions i and j fed to the detectors are encoded in quantum systems described by quantum states σ A i and σ B j . Appropriate choice of the states σ A i and σ B j can produce nonlocal correlations for all entangled states ρ A B . [Credit: ] ×

Entanglement, first introduced by Einstein, Podolsky, and Rosen [1], and Schrödinger [2] in 1935, can arise when two quantum systems are produced from a common source, for instance, when two particles are produced with opposite spin in a decay process. Mathematically, this means that the state of the quantum system cannot be written as a mixture of product states of its constituent subsystems. As shown in 1964 [3], such states can violate a set of relations now called Bell inequalities, implying that quantum theory exhibits a form of nonlocality. That is, entangled quantum systems behave as if they can affect each other instantaneously, even when they are extremely remote from each other. Entanglement and nonlocality are two of the main concepts studied in the quantum information sciences. Although it is immediately clear that entanglement is necessary for nonlocality, a detailed quantitative relation between these two concepts is not yet established. Writing in Physical Review Letters, Francesco Buscemi of the University of Nagoya, Japan, makes important progress in this direction [4].

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