Generation of Einstein-Podolsky-Rosen Pairs of Atoms

Abstract

Pairs of atoms have been prepared in an entangled state of the Einstein-Podolsky-Rosen ( EPR) type. They were produced by the exchange of a single photon between the atoms in a high Q cavity. The atoms, entangled in a superposition involving two different circular Rydberg states, were separated by a distance of the order of 1 cm. At variance with most previous EPR experiments, this one involves massive particles. It can be generalized to three or more atoms and opens the way to new tests of nonlocality in mesoscopic quantum systems.[S0031-9007 (97)03502-3] PACS numbers: 03.65. -w, 32.80. -t, 42.50. -pOne of the most puzzling aspects of quantum mechanics, its nonseparability, is illustrated vividly by the famous Einstein-Podolsky-Rosen (EPR) paradox [1]. A pair of particles flying apart from each other is predicted by quantum mechanics to yield measurement results incompatible with our intuitive conceptions about locality and reality. Such a nonclassical behavior is expected from any system made of two parts whose wave function cannot be written, in any basis, as a direct product of independent substates. The system parts are then said to be entangled. The study of entanglement has been given a firm conceptual ground by Bell who derived inequalities that Nature should obey if locality and reality were respected and which are violated by quantum mechanics [2]. Many experiments since Bell's paper have demonstrated violations of these inequalities and have vindicated quantum theory [3][4][5][6][7].In most EPR experiments so far [3,4,6,7], pairs of photons flying apart are created in a correlated state by a radiative process (spontaneous emission cascade in an atom or down-conversion in a nonlinear medium). Entangled protons have also been studied in an early experiment [5]. All these studies have dealt with very simple elementary particle systems, in which the entanglement mechanism is imposed by spontaneous processes.Entangling more complex systems in a controlled way is a challenging goal, which has been discussed in many recent proposals. The generation of EPR pairs of massive atoms instead of massless photons has been considered [8][9][10][11]. Ideas to generalize entanglement to larger numbers of particles have also been analyzed [8,10,12].The "manipulation" of entanglement is another important aspect of the new EPR experiment proposals. The idea is to apply a set of well-controlled interactions to the particles of the system in order to bring them into a "tailored" entangled state. In this context, the physics of entanglement meets the theory of quantum information processing. Teleportation of quantum states could in principle be achieved [13] as well as quantum cryptography [14]. Simple quantum computation steps could also be carried out. Particles can then be viewed as carriers of quantum bits of information and the realization of "engineered" entanglement is closely related to the building of gates acting on these bits [15].We describe here an experiment in which we have entangled two initially independent a...

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Nonclassicality and information exchange in deterministic entanglement formation

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