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European Quantum Electronics Conference (EQEC)

Quantum field manipulation in cavity QED experiments

J. M. Raimond, M. Brune, Ferdinand Schmidt‐Kaler, S. Haroche

Published Jan 1, 1994• 0 citations
Publisher

Abstract

The dispersive interaction between a circular Rydberg atom and a superconducting millimeter-wave cavity creates an entangled quantum state of these two systems. Due to the long lifetime of the atomic levels and of the field, this correlation survives a long time after the atom has crossed the cavity mode. This situation is reminiscent of the EPR paradox, with non-local correlations between two subsystems. A quantum measurement performed on one part of this entangled state yields thus information on the other part.

DOI
10.1109/eqec.1994.698201

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The dispersive interaction between a circular Rydberg atom and a superconducting millimeter-wave cavity creates an entangled quantum state of these two systems. Due to the long lifetime of the atomic levels and of the field, this correlation survives a long time after the atom has crossed the cavity mode. This situation is reminiscent of the EPR paradox, with non-local correlations between two subsystems. A quantum measurement performed on one part of this entangled state yields thus information on the other part.
Summary form only given. The preparation of simple entangled quantum systems and the demonstration of their nonclassical properties has been a challenging goal since the early days of quantum mechanics and the famous Einstein Poldolsky Rosen (ERR) paper. Using circular Rydberg atoms strongly interacting with a zero or one-photon field stored in a microwave superconducting cavity, we have been able to prepare a new kind of EPR pair involving two entangled massive particles. The entanglement originates from the Rabi oscillation of a single excited atom emitting a photon in an initially empty cavity on a transition between two adjacent Rydberg levels.
We have performed multiparticle entanglement experiments with circular Rydberg atoms crossing one at a time a high Q superconducting microwave cavity. Two-level atoms and a zero or one photon field stored in the cavity act as qubits carrying quantum information. Controlled qubit entanglement is produced by the quantum Rabi oscillation coupling the atom to the cavity field. Qubit state superpositions are produced and analyzed by classical microwave pulses before and after the atom cross the high Q cavity, using a Ramsey interferometer arrangement. We have demonstrated the coherent operation of a quantum phase gate and used it to perform for the first time a quantum nondestructive measurement of a single photon. Combining this gate with quantum Rabi oscillations of various durations, we have entangled step by step three subsystems—two atom and one field mode—by a controlled succession of one and two qubit operations. Once some limitations of our experiment are overcome, it will be generalized to larger number of particles, opening the way to the study of even more complex entangled states.
A single circular Rydberg atom is used to prepare in a high quality superconducting cavity a “Schrödinger cat” state, quantum superposition of two coherent fields with different classical phases. A second atom probes the cavity state after a tunable delay. The decay of the quantum correlations between the two atoms reveals the evolution of the field state from an initial quantum superposition into a mere statistical mixture. The time scale of this decay decreases when the separation between the two components increases. This is the first experimental observation of the dynamics of decoherence, lying at the heart of the quantum measurement process.
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.
Various experiments on entangled atomcavity states in cavity QED will be presented. Rydberg atoms can be coupled to a superconducting cavity either resonantly by emitting and absorbing photons, or nonresonantly, by producing a macroscopic phase shift on a coherent field injected in the cavity (single atom dispersive index effect). By sending atoms one at a time across the cavity, it is possible to use these resonant and dispersive effects to entangle atoms with each other or with the cavity field.