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Preparation and reconstruction of non-classical field states in a cavity QED experiment

I. Dotsenko, J. Bernu, S. Deléglise, C. Sayrin, M. Brune, J. M. Raimond, S. Haroche

Published May 4, 2008• 0 citations
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Abstract

We prepare Schrodinger cats and other non-classical photonic states in a high-Q cavity. We reconstruct them using maximum entropy principle from an incomplete set of quantum non-demolition measurements performed with Rydberg atoms crossing the cavity.

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We perform quantum non-demolition (QND) photon counting on a microwave field trapped in a very high Q superconducting cavity, employing circular Rydberg atoms as non-absorbing light probes. Beyond realizing fundamental tests of quantum measurement theory, we use this QND method to prepare non-classical Fock and Schrodinger cat states of the field and to reconstruct their Wigner functions. Monitoring the evolution of these functions provides a direct observation of the decoherence process. Quantum feedback procedures will enable us to steer the field towards target states and to protect them against decoherence.
After a brief review of experiments realized on optical fields, the author will focus on recent cavity quantum electrodynamics (cavity QED) studies performed on a microwave field trapped between highly reflecting superconducting mirrors and probed by Rydberg atoms. Exotic Fock and Schrodinger cat states of radiation are generated, kept in a cavity for an observable time and reconstructed in details. The decoherence of these states - i.e. their progressive transformation into mundane classical states under the effect of their coupling to the environment - is recorded in real time, realizing a detailed exploration of the quantum to classical boundary.
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.
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.
Application of the quantum nondemolition (QND) method to Schrodinger cat states is described. Phase cat states of trapped light is generated by QND measurement of the photon number parity of a coherent field initially injected into the cavity. The measured Wigner function of a cat state at four different delays between preparation and measurement is presented. The Wigner function exhibits two positive, ideally Gaussian, peaks corresponding to the two classical components of the state. Quantitative analysis of the decoherence process is also presented. Generalization of this experiment to the preparation of non-local cat states located at the same time between two different cavities is discussed.