Go Fujii

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.

Bright narrowband source of photon pairs at optical telecommunication wavelengths using a type-II periodically poled lithium niobate waveguide

We report on the generation of narrowband photon pairs at telecommunication wavelengths using a periodically poled lithium niobate waveguide that utilizes the nonlinear tensor element d(24) for type-II quasi phase matching. The FWHM bandwidth of the spontaneous parametric downconversion was 1 nm. The brightness of the photon pair source was ~6x10(5)/s/GHz when the pump power was 1 mW. The indistinguishability of the signal and idler photons generated by the degenerate spontaneous parametric downconversion process was studied in a Hong-Ou-Mandel type interference experiment.

Differential phase shift quantum key distribution using single-photon detectors based on a sinusoidally gated InGaAs/InP avalanche photodiode

The authors report a quantum key distribution experiment, in which they implemented a differential phase shift quantum key distribution protocol, using single-photon detectors based on InGaAs∕InP avalanche photodiodes operated with a sinusoidal gating. The single-photon detectors were operated at a repetition frequency of 500MHz with low after pulsing probabilities and low dark counts. A sifted key generation rate of 1.5Mbit∕s was achieved over a communication distance of 15km. Taking account of the security of the protocol against general individual attacks, secure keys can be generated with a rate of 0.33Mbit∕s.

Quantum receiver beyond the standard quantum limit of coherent optical communication.

The most efficient modern optical communication is known as coherent communication, and its standard quantum limit is almost reachable with current technology. Though it has been predicted for a long time that this standard quantum limit could be overcome via quantum mechanically optimized receivers, such a performance has not been experimentally realized so far. Here we demonstrate the first unconditional evidence surpassing the standard quantum limit of coherent optical communication. We implement a quantum receiver with a simple linear optics configuration and achieve more than 90% of the total detection efficiency of the system. Such an efficient quantum receiver will provide a new way of extending the distance of amplification-free channels, as well as of realizing quantum information protocols based on coherent states and the loophole-free test of quantum mechanics.

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.

Sub-shot-noise-limit discrimination of on-off keyed coherent signals via a quantum receiver with a superconducting transition edge sensor.

We demonstrate a sub-shot-noise-limit discrimination of on-off keyed coherent signals by an optimal displacement quantum receiver in which a superconducting transition edge sensor is installed. Use of a transition edge sensor and a fiber beam splitter realizes high total detection efficiency and high interference visibility of the receiver and the observed average error surpasses the shot-noise-limit in a wider range of the signal power. Our technique opens up a new technology for the sub-shot-noise-limit detection of coherent signals in optical communication channels.

Titanium Superconducting Photon-Number-Resolving Detector

High-efficiency photon-number-resolving detectors have been developed using titanium-based transition-edge sensors. Device performances have been evaluated with respect to the response photon distribution, dark count, and timing jitter. In the analysis of the photon statistics, the observed detection probabilities were consistent with Poisson distributions. From a comparison of the measured distribution with theory, 98% detection efficiency was deduced at 850 nm. The dark count probability of the device is less than 10-6, corresponding to a dark count rate of 0.6 Hz. The timing jitter is 25 ns, the best value ever reported (to our knowledge) in transition-edge sensors used for optical photon measurements.

Preservation of photon indistinguishability after transmission through surface-plasmon-polariton waveguide.

We experimentally demonstrated preservation of indistinguishability between two photons via mode conversions, namely, photon-to-plasmon and plasmon-to-photon conversions. A two-photon interference experiment was carried out using a broadband photon pair generated through a spontaneous parametric downconversion process. We observed the so-called Hong-Ou-Mandel dip with an interferometer including a 1-mm-long surface-plasmon-polariton (SPP) waveguide. The photon indistinguishability of 92.4% was retained after propagation in the SPP waveguide.

Superconducting Tunnel Junction Detectors for Analytical Sciences

Superconducting tunnel junction (STJ) detectors exhibit superior detection performance for photons and particles at a high spectroscopic resolution of ~ 10 eV, a short dead-time (decay time) of ~ μs, a high quantum efficiency of ~ 100%, and a low detection threshold energy of less than 1 eV, which cannot be achieved by conventional detectors. The outstanding detection performance originates from a small superconducting energy gap of ~ meV, which is three orders of magnitude smaller than ~ eV in semiconductors. This paper reports our recent progress in two applications of STJ detectors to fluorescence-yield X-ray absorption fine structure (XAFS) spectrometry for trace light elements in matrices and mass spectrometry (MS) for ions with the same mass/charge-number ratio (m/z) but different charge states and neutral fragments.

Titanium TES based photon number resolving detectors with 1 MHz counting rate and 65% quantum efficiency

A transition edge sensor (TES) is one of superconducting photon detectors, which has a photon number resolving ability in light pulses. The TES device is a kind of calorimeters operated at an extremely low temperature, and the energy of the photons is measured as a resistance change in a superconducting transition region of the TES. The advantages of the TESs are an excellent energy resolution and a high quantum efficiency. However a response speed is limited due to slow thermal recovery time. To overcome this, we fabricated new TES devices which are based on a titanium superconductor. The critical temperature of our titanium films is around 410 mK, which greatly improves the thermal recovery time. The observed decay time constant of response signals to the light pulses is around several hundreds of ns, that make it possible to operate the devices at a counting rate over 1 MHz. The photon number resolving power is 0.35 eV(FWHM) for a 5 μm size device even at the high operating temperature. The system quantum efficiency is 65 % by embedding the TES films in an optical structure with a high reflection dielectric mirror and an anti-reflection coatings fabricated by an ion beam assisted sputtering method. These features are very promising for high speed photon number resolving applications in the quantum information field.