Daiji Fukuda

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.

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.

Development of a new multi-grid-type microstrip gas chamber

Abstract A new multi-grid-type MSGC (M-MSGC) has been designed and fabricated. This new MSGC has very narrow gaps between neighboring electrodes which can considerably reduce a surface charge effect. Several types of test detectors have been fabricated and the maximum gas gain was found to be beyond 10 4 for a 10 μm gap M-MSGC. The observed amplitude of the cathode signal was almost the same as the anode signal. This MSGC can be a promizing detector for the field, where both the high gain and the stable operation are required.

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.

Signal processing for CdTe detectors using a fast signal digitizing technique

Abstract To suppress the charge collection loss variation in CdTe detectors, a signal processing method has been developed using a fast signal digitizing technique. Signal pulse waveforms were described as a modeling function. Then a set of model waveform patterns has been calculated for reference patterns. The most probable waveform pattern was selected by the pattern matching method which assures a finite calculation time. Obtained residue values were used for the further selection in the correction method. An energy resolution of 8.9 keV was obtained after the correction and selection of measured pulse signals for the 137Cs 662 keV photopeak. This method can be applied to a detector having any electrode configuration if the signal pulses from that configuration are given.

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 Based Transition Edge Microcalorimeters for Optical Photon Measurements

Transition edge sensor microcalorimeters can be used in many optical quantum measurements because of its low dark counts, high quantum efficiency, and high resolving power of a photon number in weak light pulses. In order to increase count rates up to a few MHz, we have developed a titanium transition edge sensor for the optical measurements, and its performances were analysed. Titanium is one of the ideal superconductor because of its higher transition temperature and lower optical reflectance at 1.5 wavelength. Our titanium film was fabricated with electron-beam evaporation, and showed high residual resistance ratio of 3.5. The sharp superconducting transition also was found at 359 mK, which is close to the critical temperature in bulk. The fabricated device showed a fast response to pulsed laser illumination of 1.5 wavelength with the fall time constant of 300 ns. These features are very promising for high-speed single photon detection in many quantum optical measurements.

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.

A new multi-grid type MSGC with pad readout

Abstract Recently, a new multi-grid type MSGC has been proposed. Between the anode and the cathode, additional grid strips are placed in this new type of MSGC. Gaps between these strips are chosen to be around 10 μm which assure efficient removal of surface charges which even do not need the lower surface resistivity and bare glass can be used up to 10 6 cps / mm 2 . Another feature of the MSGC is its high gain capability. Owing to the existence of other strips of lower potentials, the field strength around the opposing grid to the anode strip is not so high as conventional small gap detectors. Furthermore, the contribution of the surface streamer is greatly suppressed because the electric field parallel to the surface is screened by the intermediate grid electrodes. However, the existence of additional electrodes also screens all the electric field upper than the substrate and we cannot observe induced signals from backside of the substrate. To overcome the difficulty, we propose another signal readout method using patterning approach. Floating pads are placed close to the cathode strip on the surface of the M-MSGC and induced charges are read out via the pads. If the area of pads is sufficiently large and the positive charges are moving toward the pads, backside electrodes can sense the induced charge. Collected charges on the pads are leaked through the surface resistivity. The basic experiment has been performed and the readout of the position along the cathode strips is successfully ensured.

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.