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Wootters [Phys. Rev. Lett. 80, 2245 (1998)] has given an explicit formula for the entanglement of formation of two qubits in terms of what he calls the concurrence of the joint density operator. Wootters’s concurrence is defined with the help of the superoperator that flips the spin of a qubit. We generalize the spin-flip superoperator to a “universal… (More)

- Vladimír R. Buzek, Mark Hillery
- QCQC
- 1998

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)

- Mário Ziman, Peter Stelmachovic, Vladimír R. Buzek
- Open Syst. Inform. Dynam.
- 2005

- Vladimír Buzek, Miguel Orszag, Marián Rosko
- Physical review letters
- 2005

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)

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)

- Valerio Scarani, Mário Ziman, Peter Stelmachovic, Nicolas Gisin, Vladimír Buzek
- Physical review letters
- 2002

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)

- Jan Bouda, Vladimír R. Buzek
- Int. J. Found. Comput. Sci.
- 2003

described as a vector in an 2-dimensional Hilbert spaceHa0 spanned by two orthonormal basis vectors |0〉a0 and |1〉a0 . The complex amplitudes αi are normalized to unity, i.e. |α0| + |α1| = 1. Simultaneously Bob has a qubit initially prepared in a specific (i.e., known) state |0〉a1 which is a vector in the Hilbert space Ha1 . From the general rules of quantum… (More)

- Vladimír R. Buzek, Mark Hillery, Mário Ziman, Marián Rosko
- Quantum Information Processing
- 2006

- Martin Plesch, Mário Ziman, Vladimír R. Buzek, Peter Stelmachovic
- Open Syst. Inform. Dynam.
- 2006

When standard methods of process (black-box) estimation are applied straightforwardly then it may happen that some sets of experimental data result in unphysical estimations of the corresponding channels (maps) describing the process. To prevent this problem, one can use the method of maximum likelihood (MML), which provides an efficient scheme for… (More)