Maximum efficiency of a linear-optical Bell-state analyzer

In a photonic realization of qubits the implementation of quantum logic is rather difficult due the extremely weak interaction on the few photon level. On the other hand, in these systems interference is available to process the quantum states. We formalize the use of interference by the definition of a simple class of operations which include linear optical elements, auxiliary states and conditional operations. We investigate an important subclass of these tools, namely linear optical elements and auxiliary modes in the vacuum state. For this tools, we are able to extend a previous quantitative result, a no-go theorem for perfect Bell state analyzer on two qubits in polarization entanglement, by a quantitative statement. We show, that within this subclass it is not possible to discriminate unambiguously four equiprobable Bell states with a probability higher than 50 %.Comment: 6 pages, 2 fig

Quantum benchmarks for the storage or transmission of quantum light from minimal resources

We investigate several recently published benchmark criteria for storage or transmission of continuous-variable quantum information. A comparison reveals that criteria based on a Gaussian distribution of coherent states are most resilient to noise. We then address the issue of experimental resources and derive an equally strong benchmark, solely based on three coherent states and homodyne detection. This benchmark is further simplified in the presence of naturally occurring random phases, which remove the need for active input-state modulation.Comment: replaced by the published version, 5 pages, 4 figure

Nonlinear entanglement witnesses

Entanglement detection typically relies on linear inequalities for mean values of certain observables (entanglement witnesses), where violation indicates entanglement. We provide a general method to improve any of these inequalities for bipartite systems via nonlinear expressions. The nonlinearities are of different orders and can be directly measured in experiments, often without any extra effort.Comment: 4 pages, 1 figure, v3: small changes, final versio

Unconditional Security of the Bennett 1992 quantum key-distribution over lossy and noisy channel

We show that the security proof of the Bennett 1992 protocol over loss-free channel in (K. Tamaki, M. Koashi, and N. Imoto, Phys. Rev. Lett. 90, 167904 (2003)) can be adapted to accommodate loss. We assumed that Bob's detectors discriminate between single photon states on one hand and vacuum state or multi-photon states on the other hand.Comment: 5 pages, 2 figures. We have presented this topic at QIPC 2003 as a poster sessio