Continuous-variable random-number generation from an amplified spontaneous emission light source.

An amplified spontaneous emission (ASE) light source using an Er-doped fiber is an ideal random-number source in principle because ASE originates from vacuum fluctuations. Interferometrically measured ASE light directly reflects vacuum fluctuations in phase space; the interferometer does not need to be stabilized because the phase is completely random, and measurable random numbers are a continuous variable because vacuum fluctuations are continuous. These characteristics make the random-number source practical and ideal. Evaluated randomness was sufficiently ideal at the accuracy level of measurements and evaluations.

Surface code for low-density qubit array.

Surface code is a promising candidate for the quantum error corrections needed for fault-tolerant quantum computations because it can operate on a two-dimensional grid of qubits. However, the gates and control lines become dense as more and more qubits are integrated, making their design and control difficult. This problem can be alleviated if the surface code can operate on sparse qubit arrays. Here, we give an solution for an array in which qubits are placed on edges as well as on nodes of a two-dimensional grid. The qubits on the edges are divided into two groups: those in one group act as the deputies of data qubits; the others act as deputies of the syndrome qubits. Syndrome outputs are obtained by multiplying the measured values of the syndrome and edge qubits. The procedure for the quantum part is the same as that of the ordinary surface code, making the surface code applicable to sparse qubit arrays.

LD light antisqueezing through fiber propagation in reflection-type interferometer.

Antisqueezed light is a possible resource to apply quantum information technologies to the real world. When antisqueezed light is used in secure optical communications, an LD is a preferable light source from an engineering point of view. Although LD output power is low, LD light can be antisqueezed with the help of an EDFA in a reflection-type interferometer consisting of a standard single-mode fiber of typically 5 km. The ellipticity of the obtained antisqueezed light was 9 at maximum in a balanced interferometer case, and the angle that was subtended by antisqueezed fluctuations at the origin of phase space was 23 degrees at maximum. The feasibility of secure optical communications using antisqueezed light is demonstrated.