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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Generation and reconstruction of Schrodinger cat states of light in a cavity by quantum nondemolition measurements

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

Published Jun 1, 2009• 0 citations
Publisher

Abstract

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.

DOI
10.1109/cleoe-eqec.2009.5192498

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Physica Scripta•Dec 1, 2009•1 citation
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.
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In 1935, Erwin Schrödinger put forward his famous gedanken experiment in which a microscopic degree of freedom - the emission state of a radioactive atom - and a macroscopic, classical state - the life or death of a cat - are entangled [1]. The resulting state is dubbed a Schrodinger-cat state. According to quantum theory, a measurement of the atom in a superposition basis projects the cat into a macroscopic superposition of being alive and dead. Here, we use coherent laser fields with different optical phases as a macroscopic model system and generate entangled Schrodinger-cat states in a deterministic way [3]. We reflect the coherent pulses from an optical cavity that contains a single strongly-coupled atom. This entangles the spin of the atom with the optical phase of the light field [2]. By manipulating and detecting the atomic state, we create a plethora of flying optical coherent-state superpositions. We reconstruct the corresponding Wigner functions (see Fig. 1) to demonstrate the tunability and non-classicality of the produced light state.
We propose a technique for quantum nondemolition (QND) measurement and heralded preparation of Fock states by dynamics of electromagnetically induced transparency (EIT). An atomic ensemble trapped in an optical cavity is driven by two external continuous-wave classical fields to form EIT in steady state. As soon as a weak coherent field is injected into the cavity, the EIT system departs from steady state, falls into transient state dynamics by the dispersive coupling between cavity injected photons and atoms. Because the imaginary part of time-dependent linear susceptibility Im[X(t)] of the atomic medium explicitly depends on the number n of photons during the process of transient state dynamics, the measurement on the change of transmission of the probe field can be used for QND measurement of small photon number, and thus create the photon Fock states in particular single-photon state in a heralded way.
Higher-generation Schrödinger cat states of the quantized electromagnetic field can be produced in a high-Q cavity, starting from a coherent state, through the passage of prepared Rydberg atoms interacting dispersively across it. These states are natural generalizations of the even and odd coherent states, the N th-generations corresponding to specific superpositions of 2 N states on a circle in phase space with well defined parity, and present very peculiar properties. Their photon statistics interchange between super- and sub-Poissonian behaviours and the nature of the photon bunching oscillates as the field intensity in the cavity is varied. For higher-generation even states, the minimum value of the Mandel factor almost reaches −1.0 and the state represents the Fock state |2 N ). Squeezing properties and the Wigner function of these higher-generation Schrödinger cat states are also considered.
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.