Vladimír R. Buzek

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We show that the basic dynamical rules of quantum physics can be derived from its static properties and the condition that superluminal communication is forbidden. More precisely, the fact that the dynamics has to be described by linear completely positive maps on density matrices is derived from the following assumptions: (1) physical states are described(More)
We review our recent work on the universal (i.e. input state independent) optimal quantum copying (cloning) of qubits. We present unitary transformations which describe the optimal cloning of a qubit, and we present the corresponding quantum logic network. We also present a network for an optimal quantum copying " machine " (transformation) which produces N(More)
V. Bužek, 1,2 V. Vedral, 1 M. B. Plenio, 1 P. L. Knight, 1 and M. Hillery 3 Optics Section, The Blackett Laboratory, Imperial College, London SW7 2BZ, England Institute of Physics, Slovak Academy of Sciences, Dubravská cesta 9, 842 28 Bratislava, Slovakia Department of Physics and Astronomy, Hunter College, City University of New York, 695 Park Avenue, New(More)
We study the relaxation of a quantum system towards the thermal equilibrium using tools developed within the context of quantum information theory. We consider a model in which the system is a qubit, and reaches equilibrium after several successive two-qubit interactions (thermalizing machines) with qubits of a reservoir. We characterize completely the(More)
Using tools of quantum information theory we show that the ground state of the Dicke model exhibits an infinite sequence of instabilities (quantum-phase-like transitions). These transitions are characterized by abrupt changes of the bi-partite entanglement between atoms at critical values kappa(j) of the atom-field coupling parameter kappa and are(More)
In classical computation, a 'bit' of information can be flipped (that is, changed in value from zero to one and vice versa) using a logical NOT gate; but the quantum analogue of this process is much more complicated. A quantum bit (qubit) can exist simultaneously in a superposition of two logical states with complex amplitudes, and it is impossible to find(More)