Teleportation is necessary for faithful quantum state transfer through noisy channels of maximal rank

Quantum teleportation enables deterministic and faithful transmission of quantum states, provided a maximally entangled state is pre-shared between sender and receiver, and a one-way classical channel is available. Here, we prove that these resources are not only sufficient, but also necessary, for deterministically and faithfully sending quantum states through any fixed noisy channel of maximal rank, when a single use of the cannel is admitted. In other words, for this family of channels, there are no other protocols, based on different (and possibly cheaper) sets of resources, capable of replacing quantum teleportation.Comment: 4 pages, comments are welcom

Simple Proof of Security of the BB84 Quantum Key Distribution Protocol

We prove the security of the 1984 protocol of Bennett and Brassard (BB84) for quantum key distribution. We first give a key distribution protocol based on entanglement purification, which can be proven secure using methods from Lo and Chau's proof of security for a similar protocol. We then show that the security of this protocol implies the security of BB84. The entanglement-purification based protocol uses Calderbank-Shor-Steane (CSS) codes, and properties of these codes are used to remove the use of quantum computation from the Lo-Chau protocol.Comment: 5 pages, Latex, minor changes to improve clarity and fix typo

Remote State Preparation

Quantum teleportation uses prior entanglement and forward classical communication to transmit one instance of an unknown quantum state. Remote state preparation (RSP) has the same goal, but the sender knows classically what state is to be transmitted. We show that the asymptotic classical communication cost of RSP is one bit per qubit - half that of teleportation - and becomes even less when transmitting part of a known entangled state. We explore the tradeoff between entanglement and classical communication required for RSP, and discuss RSP capacities of general quantum channels.Comment: 4 pages including 1 epsf figure; v3 has an additional author and discusses relation to work of Devetak and Berger (quant-ph/0102123); v4 improves low-entanglement protocols without back communication to perform as well as low-entanglement protocols with back communication; v5 (journal version) has a few small change

Entanglement properties of optical coherent states under amplitude damping

Through concurrence, we characterize the entanglement properties of optical coherent-state qubits subject to an amplitude damping channel. We investigate the distillation capabilities of known error correcting codes and obtain upper bounds on the entanglement depending on the non-orthogonality of the coherent states and the channel damping parameter. This work provides a first, full quantitative analysis of these photon-loss codes which are naturally reminiscent of the standard qubit codes against Pauli errors.Comment: 7 pages, 6 figures. Revised version with small corrections; main results remain unaltere

Nonadditivity of Bipartite Distillable Entanglement follows from Conjecture on Bound Entangled Werner States

Assuming the validity of a conjecture in quant-ph/9910026 and quant-ph/9910022 we show that the distillable entanglement for two bipartite states, each of which individually has zero distillable entanglement, can be nonzero. We show that this also implies that the distillable entanglement is not a convex function. Our example consists of the tensor product of a bound entangled state based on an unextendible product basis with a Werner state which lies in the class of conjectured undistillable states.Comment: 4 pages RevTex, 1 figure, to appear in Phys. Rev. Lett. Title changed and small paragraph adde

When should we stop mammography screening for breast cancer in elderly women?

There is insufficient evidence to recommend for or against routine screening mammography beyond the age of 69 years. The best candidates to stop screening are elderly women who have significant comorbidities, poor functional status, low bone mineral density (BMD), little interest in preventive care, or an unwillingness to accept the potential harm of screening. (Grade of Recommendation: C, based on retrospective cohort studies.

Regulation of isocitrate dehydrogenase in Escherichia coli

Abstract Not Provided