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Abstract:

The optimal precision of frequency measurements in the presence of decoherence is discussed. We analyze different preparations of n two level systems as well as different measurement procedures. We show that standard Ramsey spectroscopy on uncorrelated atoms and optimal measurements on maximally entangled states provide the same resolution. The best resolution is achieved using partially entangled preparations with a high degree of symmetry.

Abstract:

Photon antibunching defined by two-time correlation functions has hitherto, to our best knowledge, been considered to constitute a unique, well denned effect. We show explicitly that this is by no means the case. We analyze two of the most famous definitions showing that both antibunching and bunching effects according to one definition can be accompanied by arbitrary photon correlation effects according to another. As an example we discuss a model of parametric frequency conversion.

Abstract:

We establish the best possible approximation to a perfect quantum cloning machine which produces two clones out of a single input. We analyze both universal and state-dependent cloners. The maximal fidelity of cloning is shown to be 5/6 for universal cloners. It can be achieved either by a special unitary evolution or by a novel teleportation scheme. We construct the optimal state-dependent cloners operating on any prescribed two non-orthogonal states, discuss their fidelities and the use of auxiliary physical resources in the process of cloning. The optimal universal cloners permit us to derive a new upper bound on the quantum capacity of the depolarizing quantum channel.