Mário Ziman

Thermalizing Quantum Machines: Dissipation and Entanglement

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 family of thermalizing machines. The model shows a tight link between dissipation, fluctuations, and the maximal entanglement that can be generated by the machines. The interplay of quantum and classical information processes that give rise to practical irreversibility is discussed.

Towards quantum-based privacy and voting

The privacy of communicating participants is often of paramount importance, but in some situations it is an essential condition. A typical example is a fair (secret) voting. We analyze in detail communication privacy based on quantum resources, and we propose new quantum protocols. Possible generalizations that would lead to voting schemes are discussed.

Probabilistic implementation of universal quantum processors

We present a probabilistic quantum processor for qudits on a single qudit of dimension N. The processor itself is represented by a fixed array of gates. The input of the processor consists of two registers. In the programregister the set of instructions (program) is encoded. This program is applied to the data register. The processor can perform any operation on a single qudit of dimension N with a certain probability. For a general unitary operation, the probability is 1/N 2 , but for more restricted sets of operators the probability can he higher. In fact, this probability can be independent of the dimension of the qudit Hilhert space of the qudit under some conditions.

Concurrence versus purity : Influence of local channels on Bell states of two qubits

We analyze how a maximally entangled state of two qubits (e.g., the singlet

Process positive-operator-valued measure: A mathematical framework for the description of process tomography experiments

psi_s

All (qubit) decoherences: Complete characterization and physical implementation

) is affected by the action of local channels described by completely positive maps

Toward protocols for quantum-ensured privacy and secure voting

E

Entanglement, purity and energy: Two qubits vs Two modes

. We analyze the concurrence and the purity of states

Description of Quantum Dynamics of Open Systems Based on Collision-Like Models

rho_E=E \otimes I_psi_s

Simulation of indivisible qubit channels in collision models

. Using the concurrence-versus-purity phase diagram we characterize local channels