We show that the strategy of ghost imaging can be successfully adapted to the estimation of parameters of open systems. Two entangled qubits, so that only one of them interacts with a depolarizing sample, may lead to precision in the estimation of the depolarizing parameter better than the one corresponding to sending the pair, entangled or not, through the sample. As opposed to ghost imaging, entanglement plays an essential role in this case, as does the entangling detection procedure, consisting in projecting the final state on a Bell-state basis. For isotropic depolarization, quantum advantage is obtained only when the final state, after interaction with the sample, is not entangled anymore.
We demonstrate, using an all-optical setup, the difference between local and global dynamics of entangled
quantum systems coupled to independent environments. Even when the environment-induced decay of each
system is asymptotic, quantum entanglement may suddenly disappear.
We discuss the emergence of the classical description from quantum mechanics for chaotic systems. We consider a specific model, which corresponds to an ion trapped in a harmonic potential and submitted to a sequence of laser pulses: the kicked harmonic oscillator, under conditions that lead to chaotic behavior for the classical system. We show that noise plays an essential role in the transition from quantum to classical behavior. This transition is described in terms of the separation between the classical phase space distribution and the Wigner function corresponding to the quantum system, obtained by integrating over the whole phase space the magnitude of the difference between the two distributions. It is shown that, in the semiclassical limit, this separation is governed by a single parameter, which depends on the diffusion coefficient, the Lamb-Dicke parameter, and the kick strength. The Lamb-Dicke parameter plays the role of a dimensioneless Planck constant. As this parameter goes to zero, we show that both distributions remain close together for all times.
We show that the technique of Ramsey interferometry may be used to completely determine the quantum state of an electromagnetic field in a cavity. Our method, which allows the direct measurement of the Wigner function of the field, is a useful tool for probing the quantum-classical transition and, in particular, the decoherence between coherent superpositions of distinguishable coherent states of the electromagnetic field in the cavity. We also show that the realization of a controlled-not gate within framework of cavity QED is a special case of our proposal, and leads to the measurement of a negative value for the Wigner function of an electromagnetic field.
Conference Committee Involvement (1)
Noise and Fluctuations in Photonics, Quantum Optics, and Communications
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