Differential-quadrature-phase-shift quantum digital signature.

A novel quantum digital signature (QDS) scheme, called "differential quadrature phase-shift QDS," is presented. A message sender broadcasts a weak coherent pulse train with four phases of {0, π/2, π, 3π/2} and recipients create their own authentication keys from the broadcasted signal. Unlike conventional QDS protocols, there is no post-processing of information exchange between the sender and recipients and that between the recipients. Therefore, secured channels and/or authenticated channels for information exchange are not needed, and the key creation procedure is simpler than that of conventional QDS. Security issues are also discussed, using binominal distributions instead of Hoeffding's inequality utilized in conventional QDS studies, and calculation examples for system conditions achieving the QDS function are presented.

Dependence of the amplification performance of unsaturated degenerate phase-sensitive amplification on wavelength allocation.

This paper theoretically investigates the dependence of the performance of dual-pump degenerate phase-sensitive amplification (PSA) on wavelength allocation. A fiber-based PSA under unsaturated-gain conditions is considered. Phase mismatch is formalized in terms of incident light frequencies, taking the nonlinear phase shift into account, based on which PSA performances, such as signal gain, noise figure, and phase-clamping effect, are evaluated as a function of the signal wavelength. The results quantitatively indicate that these PSA properties are degraded as the signal wavelength is detuned from the phase-matched condition.

Comparison of the efficacy of three topical antiseptic solutions for the prevention of catheter colonization: a multicenter randomized controlled study.

BACKGROUND: To compare the efficacy of three antiseptic solutions [0.5%, and 1.0% alcohol/chlorhexidine gluconate (CHG), and 10% aqueous povidone-iodine (PVI)] for the prevention of intravascular catheter colonization, we conducted a randomized controlled trial in patients from 16 intensive care units in Japan. METHODS: Adult patients undergoing central venous or arterial catheter insertions were randomized to have one of three antiseptic solutions applied during catheter insertion and dressing changes. The primary endpoint was the incidence of catheter colonization, and the secondary endpoint was the incidence of catheter-related bloodstream infections (CRBSI). RESULTS: Of 1132 catheters randomized, 796 (70%) were included in the full analysis set. Catheter-tip colonization incidence was 3.7, 3.9, and 10.5 events per 1000 catheter-days in 0.5% CHG, 1% CHG, and PVI groups, respectively (p = 0.03). Pairwise comparisons of catheter colonization between groups showed a significantly higher catheter colonization risk in the PVI group (0.5% CHG vs. PVI: hazard ratio, HR 0.33 [95% confidence interval, CI 0.12-0.95], p = 0.04; 1.0% CHG vs. PVI: HR 0.35 [95% CI 0.13-0.93], p = 0.04). Sensitivity analyses including all patients by multiple imputations showed consistent quantitative conclusions (0.5% CHG vs. PVI: HR 0.34, p = 0.03; 1.0% CHG vs. PVI: HR 0.35, p = 0.04). No significant differences were observed in the incidence of CRBSI between groups. CONCLUSIONS: Both 0.5% and 1.0% alcohol CHG are superior to 10% aqueous PVI for the prevention of intravascular catheter colonization. TRIAL REGISTRATION: Japanese Primary Registries Network; No.: UMIN000008725 Registered on 1 September 2012.

A coherent Ising machine for 2000-node optimization problems.

The analysis and optimization of complex systems can be reduced to mathematical problems collectively known as combinatorial optimization. Many such problems can be mapped onto ground-state search problems of the Ising model, and various artificial spin systems are now emerging as promising approaches. However, physical Ising machines have suffered from limited numbers of spin-spin couplings because of implementations based on localized spins, resulting in severe scalability problems. We report a 2000-spin network with all-to-all spin-spin couplings. Using a measurement and feedback scheme, we coupled time-multiplexed degenerate optical parametric oscillators to implement maximum cut problems on arbitrary graph topologies with up to 2000 nodes. Our coherent Ising machine outperformed simulated annealing in terms of accuracy and computation time for a 2000-node complete graph.

Polarization or phase insensitive fiber parametric amplifier with clamped output phase.

This paper proposes a low polarization- or phase-dependent fiber parametric amplifier system with a clamped output phase, which consists of an orthogonally pumped nonlinear fiber and a fiber loop with a polarization beam splitter. Numerical calculations show that the proposed system exhibits a constant output phase, low insensitive to the signal input phase or polarization state.

Polarization-independent, differential-phase-shift, quantum-key distribution system using upconversion detectors.

We propose and demonstrate a polarization-independent, differential-phase-shift, quantum-key distribution system with upconversion detectors. Even though the detectors have polarization dependency, use of alternative polarization modulation and a two-bit delay interferometer achieves polarization-insensitive operation. In an experiment, sifted key bits were polarization-independently generated over 50 km fiber.

Differential-phase quantum key distribution experiment using a series of quantum entangled photon pairs.

We report what we believe to be the first differential-phase quantum key distribution experiment using a series of quantum entangled photon pairs. We employed two outstanding techniques. As an entangled photon source, we used a 1.5 microm band entangled photon pair source based on spontaneous four-wave mixing in a cooled dispersion-shifted fiber. As receivers, photon pairs were actively phase modulated with LiNbO3 phase modulators followed by very stable planar light-wave circuit Mach-Zehnder interferometers, which provided two nonorthogonal measurements. As a consequence, we successfully generated sifted keys with a quantum bit error rate of 8.3% and a key generation rate of 0.3 bit/s and revealed the feasibility of this QKD scheme.

Differential-phase-shift quantum key distribution with an extended degree of freedom.

Differential-phase-shift quantum key distribution (DPS-QKD) with an extended degree of freedom of measurement is proposed. Extending the degree of freedom makes it possible to strengthen the DPS-QKD scheme against intercept-and-resend attacks. The feasibility of this idea is experimentally demonstrated.

Generation of pulsed polarization-entangled photon pairs in a 1.55-microm band with a periodically poled lithium niobate waveguide and an orthogonal polarization delay circuit.

We report a scheme for generating pulsed polarization-entangled photon pairs based on conversion from time-bin entanglement to polarization entanglement by use of an orthogonal polarization delay circuit and post-selection. We have experimentally demonstrated the scheme, using a periodically poled lithium niobate waveguide, and successfully obtained polarization entanglement in the 1.55-microm telecom wavelength band.

1.5-microm band quantum-correlated photon pair generation in dispersion-shifted fiber: suppression of noise photons by cooling fiber.

Spontaneous four-wave mixing in a dispersion-shifted fiber (DSF) is a promising approach for generating quantum-correlated photon pairs in the 1.5 microm band. However, it has been reported that noise photons generated by the spontaneous Raman scattering process degrade the quantum correlation of the generated photons. This paper describes the characteristics of quantum-correlated photon pair generation in a DSF cooled by liquid nitrogen. With this technique, the number of noise photons was sufficiently suppressed and the ratio of true coincidence to accidental coincidence was increased to ~30.

Entanglement formation and violation of Bell's inequality with a semiconductor single photon source.

We report the generation of polarization-entangled photons, using a quantum dot single photon source, linear optics, and photodetectors. Two photons created independently are observed to violate Bell's inequality. The density matrix describing the polarization state of the postselected photon pairs is reconstructed and agrees well with a simple model predicting the quality of entanglement from the known parameters of the single photon source. Our scheme provides a method to create no more than one entangled photon pair per cycle after postselection, a feature useful to enhance quantum cryptography protocols based on shared entanglement.

Differential phase shift quantum key distribution.

A novel quantum cryptography scheme is proposed, in which a single photon is prepared in a linear superposition state of three basis kets. A photon split to three pulses is sent from Alice to Bob, where the phase difference between sequential two pulses carries bit information. Bob measures the phase difference by passive differential phase detection. This scheme is suitable for fiber transmission systems and offers a key creation efficiency higher than conventional fiber-based BB84.