M. Koniorczyk

Wigner-function description of quantum teleportation in arbitrary dimensions and a continuous limit

We present a unified approach to quantum teleportation in arbitrary dimensions based on the Wigner-function formalism. This approach provides us with a clear picture of all manipulations performed in the teleportation protocol. In addition within the framework of the Wigner-function formalism all the imperfections of the manipulations can be easily taken into account.

Optimization of periodic single-photon sources

We introduce a theoretical framework which is suitable for the description of all spatial and time-multiplexed periodic single-photon sources realized or proposed thus far. Our model takes into account all possibly relevant loss mechanisms. This statistical analysis of the known schemes shows that multiplexing systems can be optimized in order to produce maximal single-photon probability for various sets of loss parameters by the appropriate choice of the number of multiplexed units of spatial multiplexers or multiplexed time intervals and the input mean photon pair number, and reveals the physical reasons of the existence of the optimum. We propose a novel time-multiplexed scheme to be realized in bulk optics, which, according to the present analysis, would have promising performance when experimentally realized. It could provide a single-photon probability of 85\% with a choice of experimental parameters which are feasible according to the experiments known from the literature.

Direct versus measurement-assisted bipartite entanglement in multiqubit systems and their dynamical generation in spin systems

We consider multiqubit systems and relate quantitatively the problems of generating cluster states with high values of concurrence of assistance, and of generating states with maximal bipartite entanglement. We prove an upper bound for the concurrence of assistance. We consider dynamics of spin-1 / 2 systems that model qubits, with different couplings and possible presence of magnetic field, to investigate the appearance of the discussed entanglement properties. We find that states with maximal bipartite entanglement can be generated by an isotropic XY Hamiltonian, and their generation can be controlled by the initial state of one of the spins. The same Hamiltonian is capable of creating states with high concurrence of assistance from a suitably chosen initial state. We show that the production of graph states using the Ising Hamiltonian is controllable via a single-qubit rotation of one spin-1 / 2 subsystem in the initial multiqubit state. We show that the property of Ising dynamics to convert a product-state basis into a special maximally entangled basis is temporally enhanced by the application of a suitable magnetic field. Similar basis transformations are found to be feasible in the case of isotropic XY couplings with a magnetic field.

Teleportation: From probability distributions to quantum states

The role of the off-diagonal density-matrix elements of the entangled pair is investigated in the quantum teleportation of a qubit. The dependence between these elements and the off-diagonal elements of the teleported density matrix is shown to be linear. In this way ideal quantum teleportation is related to a completely classical communication protocol: the one-time pad cypher. The latter can be regarded as the classical counterpart of Bennetts quantum-teleportation scheme. The quantum-to-classical transition is demonstrated by a gedankenexperiment.

Entropy rate of message sources driven by quantum walks

Institute of Mathematics and Informatics, University of P´ecs, Ifju´sa´g u´tja 6, H-7624 P´ecs, Hungary(Dated: March 5, 2014)The amount of information generated by a discrete time stochastic processes in a single step canbe quantified by the entropy rate. We investigate the differences between two discrete time walkmodels, the discrete time quantum walk and the classical random walk in terms of entropy rate.We develop analytical methods to calculate and approximate it. This allows us to draw conclusionsabout the differences between classical stochastic and quantum processes in terms of the classicalinformation theory.

Distinguishing Schrödinger cats in a lossy environment

Optical Schrodinger cat states-that is, even and odd coherent states-are considered as possible candidates for forming a computational basis for a coherent state qubit. The distinguishability of the two originally orthogonal states after experiencing loss is quantified in terms of quantum relative entropy. This is a physically instructive quantity related to probabilities of faults in identifying the state. This distinguishability is important for classical communication and for the problem of reading out the result of a quantum computation by a lossy device. It is shown that the distinguishability can significantly increase if the environment is prepared in an appropriately chosen squeezed state.

Optimal universal asymmetric covariant quantum cloning circuits for qubit entanglement manipulation

We consider the entanglement manipulation capabilities of the universal covariant quantum cloner or quantum processor circuit for quantum bits. We investigate its use for cloning a member of a bipartite or a genuine tripartite entangled state of quantum bits. We find that for bipartite pure entangled states a nontrivial behavior of concurrence appears, while for GHZ entangled states a possibility of the partial extraction of bipartite entanglement can be achieved.

One-complex-plane representation: a coherent-state description of entanglement and teleportation

It is shown that any state of two modes of the electromagnetic field (or any other bipartite bosonic system) can be expressed as a coherent superposition of conjugate coherent-state pairs. This representation has a deep connection with both the two-mode squeezing operator and a maximally entangled basis. A description of continuous variable quantum teleportation is presented, as an example of the application of this method.

Coherent-state approach to entanglement and teleportation

Two-mode oscillator quantum states are described via coherent state superpositions. The one-complex-plane coherent-state representation is introduced as a generalization of low-dimensional coherent-state representations of single mode fields. Application to two-mode squeezed states is emphasized, because of their entangled nature and applicability for quantum communication. Continuous variable quantum teleportation is treated in this framework.

Optimization of periodic single-photon sources based on combined multiplexing

We consider periodic single-photon sources with combined multiplexing in which the outputs of several time-multiplexed sources are spatially multiplexed.We give a full statistical description of such systems in order to optimize them with respect to maximal single-photon probability.We carry out the optimization for a particular scenario which can be realized in bulk optics and its expected performance is extremely good at the present state of the art. We find that combined multiplexing outperforms purely spatially or time-multiplexed sources for certain parameters only, and we characterize these cases. Combined multiplexing can have the advantages of possibly using less nonlinear sources, achieving higher repetition rates, and the potential applicability for continuous pumping. We estimate an achievable single-photon probability between 85% and 89%. DOI: 10.1103/