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Physical Review A

Entanglement of atomic beams: Tests of complementarity and other applications

Pál Bogár, János A. Bergou

Published Jan 1, 1996• 24 citations
Publisher

Abstract

It is shown that distinct atomic beams can be entangled when they interact with quantum superpositions of macroscopically separated micromaser fields. Experimentally feasible tests of complementarity are proposed, detecting Ramsey interference (or not) in one and ``Welcher Weg'' information (or not) in the other entangled beam. Available information and fringe contrast can be manipulated using classical and quantum fields. The ``quantum eraser'' is realized in the former case, while it is only a special feature in the latter one. Other applications of entangled atoms are also suggested. \textcopyright{} 1996 The American Physical Society.

DOI
10.1103/physreva.53.49

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It is shown that distinct atomic beams can be entangled when they interact with quantum superpositions of macroscopically separated micromaser fields. Experimentally feasible tests of complementarity are proposed, detecting Ramsey interference (or not) in one and Welcher Weg'' information (or not) in the other entangled beam. Available information and fringe contrast can be manipulated using classical and quantum fields. The quantum eraser'' is realized in the former case, while it is only a special feature in the latter one. Other applications of entangled atoms are also suggested. \textcopyright{} 1996 The American Physical Society.
A scheme for an atomic beam quantum self-eraser is presented. The proposal is based on time reversal invariance on a quantum optical Ramsey fringes experiment, where a realization of complementarity for atomic coherence can be achieved. It consists of two high finesse resonators that are pumped and probed by the same atom. This property relates quantum erasing with time reversal symmetry, allowing for a full quantum erasing of the which-way information stored in the cavity fields. The outlined scheme also prepares and observes a non-local state in the fields of the resonators: a coherent superposition between correlated states of macroscopically separated quantum systems. The proposed scheme emphasizes the role of entanglement swapping in delayed-choice experiments. Finally, we show that the quantum self-eraser violates temporal Bell inequalities and analyze the relation between this violation and the erasability of which-way information.
After they have interacted, quantum particles generally behave as a single nonseparable entangled system. The concept of entanglement plays an essential role in quantum physics. We have performed entanglement experiments with Rydberg atoms and microwave photons in a cavity and tested quantum mechanics in situations of increasing complexity. Entanglement resulted either from a resonant exchange of energy between atoms and the cavity field or from dispersive energy shifts affecting atoms and photons when they were not resonant. With two entangled particles (two atoms or one atom and a photon), we have realized new versions of the Einstein-Podolsky-Rosen situation. The detection of one particle projected the other, at a distance, in a correlated state. This process could be viewed as an elementary measurement, one particle being a ``meter'' measuring the other. We have performed a ``quantum nondemolition'' measurement of a single photon, which we detected repeatedly without destroying it. Entanglement is also essential to understand decoherence, the process accounting for the classical appearance of the macroscopic world. A mesoscopic superposition of states (``Schr\"odinger cat'') gets rapidly entangled with its environment, losing its quantum coherence. We have prepared a Schr\"odinger cat made of a few photons and studied the dynamics of its decoherence, in an experiment which constitutes a glimpse at the quantum/classical boundary. We have also investigated entanglement as a resource for the processing of quantum information. By using quantum two-state systems (qubits) instead of classical bits of information, one can perform logical operations exploiting quantum interferences and taking advantage of the properties of entanglement. Manipulating as qubits atoms and photons in a cavity, we have operated a quantum gate and applied it to the generation of a complex three-particle entangled state. We finally discuss the perspectives opened by these experiments for further fundamental studies.
After they have interacted, quantum particles generally behave as a single nonseparable entangled system. The concept of entanglement plays an essential role in quantum physics. We have performed entanglement experiments with Rydberg atoms and microwave photons in a cavity and tested quantum mechanics in situations of increasing complexity. Entanglement resulted either from a resonant exchange of energy between atoms and the cavity field or from dispersive energy shifts affecting atoms and photons when they were not resonant. With two entangled particles (two atoms or one atom and a photon), we have realized new versions of the Einstein-Podolsky-Rosen situation. The detection of one particle projected the other, at a distance, in a correlated state. This process could be viewed as an elementary measurement, one particle being a ‘‘meter’’ measuring the other. We have performed a ‘‘quantum nondemolition’’ measurement of a single photon, which we detected repeatedly without destroying it. Entanglement is also essential to understand decoherence, the process accounting for the classical appearance of the macroscopic world. A mesoscopic superposition of states (‘‘Schrodinger cat’’) gets rapidly entangled with its environment, losing its quantum coherence. We have prepared a Schrodinger cat made of a few photons and studied the dynamics of its decoherence, in an experiment which constitutes a glimpse at the quantum/classical boundary. We have also investigated entanglement as a resource for the processing of quantum information. By using quantum two-state systems (qubits) instead of classical bits of information, one can perform logical operations exploiting quantum interferences and taking advantage of the properties of entanglement. Manipulating as qubits atoms and photons in a cavity, we have operated a quantum gate and applied it to the generation of a complex three-particle entangled state. We finally discuss the perspectives opened by these experiments for further fundamental studies.
Summary form only given. The transition between quantum and classical systems is usually ascribed to decoherence effects, which become increasingly important for larger size systems and tend to obscure the quantum behavior. Decoherence effects also come into play in the famous thought 'Gedanken' experiments on the wave-particle duality of matter. For example, in a double slit experiment with electrons, a wavepacket passes simultaneously through both slits and forms an interference pattern. However, if one attempts to keep track of the electron path, e.g., by scattering a photon from the electron, the resulting decoherence destroys the interference pattern and the particle behavior takes over. We report on the extension of such a which-path experiment to an arrangement with more than two interfering paths, which is realized using the interference of four internal states of the cesium atom. This two-qubits arrangement represents a more complex quantum system than a two-beam interferometer, which represents a single qubit system. While in a two-beam interference experiment the observation of a path will inevitable destroy the fringe signal, we demonstrate that the scattering of a photon on one of the path of a multiple path arrangement cannot only lead to a decrease of the contrast, but, under certain conditions, also lead to an increase of the fringe contrast.