Akio Yoshizawa

Titanium-based transition-edge photon number resolving detector with 98% detection efficiency with index-matched small-gap fiber coupling

We have realized a high-detection-efficiency photon number resolving detector at an operating wavelength of about 850 nm. The detector consists of a titanium superconducting transition edge sensor in an optical cavity, which is directly coupled to an optical fiber using an approximately 300-nm gap. The gap reduces the sensitive area and heat capacity of the device, leading to high photon number resolution of 0.42 eV without sacrificing detection efficiency or signal response speed. Wavelength dependent efficiency in fiber-coupled devices, which is due to optical interference between the fiber and the device, is also decreased to less than 1% in this configuration. The overall system detection efficiency is 98%±1% at wavelengths of around 850 nm, which is the highest value ever reported in this wavelength range.

Gated-mode single-photon detection at 1550 nm by discharge pulse counting

Gated-mode single-photon detection using an avalanche photodiode is characterized by charge and discharge pulses, which are attributable to capacitive behavior. In this letter, the discharge pulse rather than the photon-induced avalanche pulse is counted in single-photon detection, in order to reduce after pulses. A demonstration adopts an indium–gallium–arsenide avalanche photodiode operating at 1550 nm. After-pulse probability per gate is evaluated at a repetition frequency of 5 MHz.

Stable source of high quality telecom-band polarization-entangled photon-pairs based on a single, pulse-pumped, short PPLN waveguide

We demonstrate a stable source of high quality telecom-band polarization-entangled photon-pairs based on a single, pulse-pumped, short periodically-poled lithium niobate (PPLN) waveguide. Full quantum state tomographic measurement performed on the photon-pairs has revealed a very high state purity of 0.94, and an entanglement fidelity exceeding 0.96 at the low-rate-regime. At higher rates, entanglement quality degrades due to emission of multiple-pairs. Using a new model, we have confirmed that the observed degradation is largely due to double- and triple-pair emissions.

A 1550 nm Single-Photon Detector Using a Thermoelectrically Cooled InGaAs Avalanche Photodiode

We report a gated-mode single-photon detector sensitive to 1550 nm radiation using a thermoelectrically cooled InGaAs avalanche photodiode. At the operation temperature of 238 K, we have obtained a quantum efficiency of 24.3% with a dark-count probability per gate of 9.4×10-5.

Broadband source of telecom-band polarization-entangled photon-pairs for wavelength-multiplexed entanglement distribution

Studies on telecom-band entangled photon-pair sources for entanglement distribution have so far focused on their narrowband operations. Fiber-based sources are seriously limited by spontaneous Raman scattering while sources based on quasi-phase-matched crystals or waveguides are usually narrowband because of long device lengths and/or operations far from degeneracy. An entanglement distributor would have to multiplex many such narrowband sources before entanglement distribution to fully utilize the available fiber transmission bandwidth. In this work, we demonstrate a broadband source of polarization-entangled photon-pairs suitable for wavelength-multiplexed entanglement distribution over optical fiber. We show that our source is potentially capable of simultaneously supporting up to forty-four independent wavelength channels.

Distribution of polarization-entangled photonpairs produced via spontaneous parametric down-conversion within a local-area fiber network: theoretical model and experiment.

We present a theoretical model for the distribution of polarization-entangled photon-pairs produced via spontaneous parametric down-conversion within a local-area fiber network. This model allows an entanglement distributor who plays the role of a service provider to determine the photon-pair generation rate giving highest two-photon interference fringe visibility for any pair of users, when given user-specific parameters. Usefulness of this model is illustrated in an example and confirmed in an experiment, where polarization-entangled photon-pairs are distributed over 82 km and 132 km of dispersion-managed optical fiber. Experimentally observed visibilities and entanglement fidelities are in good agreement with theoretically predicted values.

10.5 km Fiber-Optic Quantum Key Distribution at 1550 nm with a Key Rate of 45 kHz

High-speed single-photon detectors are crucial for efficiently performing quantum key distribution. We demonstrate fiber-optic quantum key distribution at 1550 nm using single-photon detectors operating at 10 MHz. Discharge pulse counting is adopted for single-photon detection in order to suppress after-pulse generation. Furthermore, by setting a dead time in single-photon detection, we discard after-pulses to reduce the number of bit errors in the quantum key distribution. Using discharge pulse counting and after-pulse discarding in the quantum key distribution, we can achieve a key rate of 45 kHz with a bit-error rate of 2%. The fiber length is 10.5 km.

Wavelength-multiplexed distribution of highly entangled photon-pairs over optical fiber

We report the first experimental demonstration of wavelength-multiplexed entanglement distribution over optical fiber. Forty-four channels of polarization-entangled photon-pairs were produced from a single pulse-pumped, short periodically-poled lithium niobate waveguide and distributed over 10 km of dispersion-shifted optical fiber. Entanglement fidelities of the distributed photon-pairs exceeded 0.86 for all selected channels.

Photon number resolving detection with high speed and high quantum efficiency

Photon number resolving detectors based on titanium-transition edge sensors with high speed and high quantum efficiency have been developed for quantum sensors in the fields of quantum information and quantum radiometry. The two devices optimized at wavelengths of interest showed 81% and 64% system detection efficiencies at 850 nm and 1550 nm, respectively. The response speed of the device optimized for a high counting operation is 190 ns, which corresponds to a counting rate over 1 MHz.

Chirped-comb generation in frequency-shifted feedback laser diodes with a large frequency shift

Chirped-comb generation has been observed in an external-cavity laser diode with a grazing-incidence grating and an intracavity acousto-optic frequency shifter. In contrast to the previous works in which the ratio of the intracavity frequency shift to the cavity free spectral range is much smaller than unity, our experiment was performed for a wide range of the ratio from 0.21 to 1.4.