Optimal cloning of qubits given by arbitrary axisymmetric distribution on Bloch sphere

We find an optimal quantum cloning machine, which clones qubits of arbitrary symmetrical distribution around the Bloch vector with the highest fidelity. The process is referred to as phase-independent cloning in contrast to the standard phase-covariant cloning for which an input qubit state is a priori better known. We assume that the information about the input state is encoded in an arbitrary axisymmetric distribution (phase function) on the Bloch sphere of the cloned qubits. We find analytical expressions describing the optimal cloning transformation and fidelity of the clones. As an illustration, we analyze cloning of qubit state described by the von Mises-Fisher and Brosseau distributions. Moreover, we show that the optimal phase-independent cloning machine can be implemented by modifying the mirror phase-covariant cloning machine for which quantum circuits are known.Comment: 8 pages, 6 figure

Experimental characterization of photon-number noise in Rarity-Tapster type interferometers

In this paper, we develop a simple model describing inherent photon-number noise in Rarity-Tapster type interferometers. This noise is caused by generating photon pairs in the process of spontaneous parametric down-conversion and adding a third photon by attenuating fundamental laser mode to single-photon level. We experimentally verify our model and present resulting signal to noise ratios as well as obtained three-photon generation rates as functions of various setup parameters. Subsequently we evaluate impact of this particular source of noise on quantum teleportation which is a key quantum information protocol using this interferometric configuration.Comment: 7 pages, 5 figure

Resource-efficient linear-optical quantum router

All-linear-optical scheme for fully featured quantum router is presented. This device directs the signal photonic qubit according to the state of one control photonic qubit. In the introduction we formulate the list of requirements imposed on a fully quantum router. Then we describe our proposal showing the exact principle of operation on a linear-optical scheme. Subsequently we provide generalization of the scheme in order to optimize the success probability by means of a tunable controlled-phase gate. At the end, we show how one can modify the device to route multiple signal qubits using the same control qubit.Comment: 7 pages, 6 figure