Masahide Sasaki

Superconducting Nanowire Single-Photon Detectors for Quantum Communication Applications

Single-photon detectors are a key enabling technology for optical quantum information processing applications such as quantum key distribution. A new class of single-photon detectors have emerged based on superconducting nanowires. These detectors offer sensitivity at telecommunication wavelengths (1310nm and 1550nm) with low dark counts and excellent timing resolution at an operating temperature of ~4 K. We have integrated four independent fibre-coupled detectors into a practical closed-cycle refrigerator and plan to employ this multichannel detector system in advanced quantum information processing experiments.

Generation of entanglement between frequency bands via a nonlinear fiber propagation and a spectral pulse shaping

A novel fiber-based scheme for generating quadrature entanglement between a desired pair of frequency bands is proposed. The scheme is based on a nonlinear fiber, a spectral pulse shaper, and an adaptive feedback loop.

Photonic quantum digital signatures operating over kilometer ranges in installed optical fiber

The security of electronic communications is a topic that has gained noteworthy public interest in recent years. As a result, there is an increasing public recognition of the existence and importance of mathematically based approaches to digital security. Many of these implement digital signatures to ensure that a malicious party has not tampered with the message in transit, that a legitimate receiver can validate the identity of the signer and that messages are transferable. The security of most digital signature schemes relies on the assumed computational difficulty of solving certain mathematical problems. However, reports in the media have shown that certain implementations of such signature schemes are vulnerable to algorithmic breakthroughs and emerging quantum processing technologies. Indeed, even without quantum processors, the possibility remains that classical algorithmic breakthroughs will render these schemes insecure. There is ongoing research into information-theoretically secure signature schemes, where the security is guaranteed against an attacker with arbitrary computational resources. One such approach is quantum digital signatures. Quantum signature schemes can be made information-theoretically secure based on the laws of quantum mechanics while comparable classical protocols require additional resources such as anonymous broadcast and/or a trusted authority. Previously, most early demonstrations of quantum digital signatures required dedicated single-purpose hardware and operated over restricted ranges in a laboratory environment. Here, for the first time, we present a demonstration of quantum digital signatures conducted over several kilometers of installed optical fiber. The system reported here operates at a higher signature generation rate than previous fiber systems.

Quantum Information Networks with Superconducting Nanowire Single-Photon Detectors

The advent of high performance practical superconducting nanowire single photon detectors (SNSPDs) has enabled rapid progress in a range of quantum information technologies, including quantum key distribution, characterization of single photon sources and quantum interface technologies. This chapter gives an overview of these recent advances.

Quantum displacement receiver for M-ary phase-shift-keyed coherent states

We propose quantum receivers for 3- and 4-ary phase-shift-keyed (PSK) coherent state signals to overcome the standard quantum limit (SQL). Our receiver, consisting of a displacement operation and on-off detectors with or without feedforward, provides an error probability performance beyond the SQL. We show feedforward operations can tolerate the requirement for the detector specifications.