Kazumasa Makise

Origin of intrinsic dark count in superconducting nanowire single-photon detectors

The origin of the decoherence in superconducting nanowire single-photon detectors, the so-called dark count, was investigated. We measured the direct-current characteristics and bias-current dependencies of the dark count rate in a wide range of temperatures from 0.5u2009K to 4.0u2009K, and analyzed the results by theoretical models of thermal fluctuations of vortices. Our results indicate that the current-assisted unbinding of vortex-antivortex pairs is the dominant origin of the dark count.

Superconducting single photon detectors integrated with single flux quantum readout circuits in a cryocooler

We report the operation of superconducting single photon detectors (SSPDs) with integrated readout electronics based on a single-flux-quantum (SFQ) circuit. The SSPDs were connected to a SFQ readout circuit without bias tee in a cryocooler system, and detection efficiencies (DEs) were measured to verify their correct operation. The DE curves matched well with those measured by conventional readout electronics, and the maximum bias current for normal operation was increased. In addition, we confirmed that the SFQ circuit correctly merged the input signals from two SSPDs. SSPDs with a SFQ readout circuit demonstrated a favorable timing jitter of 50 ps.

Crosstalk-free operation of multielement superconducting nanowire single-photon detector array integrated with single-flux-quantum circuit in a 0.1 W Gifford-McMahon cryocooler.

We demonstrate the successful operation of a multielement superconducting nanowire single-photon detector (SSPD) array integrated with a single-flux-quantum (SFQ) readout circuit in a compact 0.1 W Gifford-McMahon cryocooler. A time-resolved readout technique, where output signals from each element enter the SFQ readout circuit with finite time intervals, revealed crosstalk-free operation of the four-element SSPD array connected with the SFQ readout circuit. The timing jitter and the system detection efficiency were measured to be 50 ps and 11.4%, respectively, which were comparable to the performance of practical single-pixel SSPD systems.

The First Six ALMA Band 10 Receivers

The first six Atacama Large Millimeter/submillimeter Array (ALMA) Band 10 (787-950 GHz) receivers have been developed and characterized during the receiver preproduction phase. State-of-the-art measurement systems at THz frequencies have been implemented and successfully used to measure the performance of the first six receivers. Extensive tests ranging from receiver sensitivity and stability to optical aperture efficiency on the secondary antenna have been performed. Performance of all six receivers is well within the stringent ALMA requirements. Moreover, our extensive tests have shown that there are no big performance differences between receivers. These results indicate that the ALMA Band 10 receiver is ready for the production phase, during which an additional 67 receivers will be produced and characterized.

Characterization of NbTiN Thin Films Deposited on Various Substrates

Niobium titanium nitride (NbTiN) thin films were deposited on a selection of different substrates to explore the optimum depositing condition of high-quality NbTiN thin films. The NbTiN films deposited on MgO, Al₂O₃, and fused quartz substrates showed excellent superconducting properties with a high <i>T</i>_C of 14.7 K, 14.5 K, 15.2 K, and low resistivity ρ_{20 K} of 55 μΩcm, 116 μΩcm, 78 μΩcm, respectively. The chemical composition, crystal structure, and relationship between superconducting properties and crystal structure were systematically investigated by changing the N₂/(Ar+N₂) ratio in sputtering gases. The lattice parameter systematically changed as N₂/(Ar+N₂) ratio was varied. We found that the superconducting properties depend on the lattice parameter, and the film with the best superconducting properties had a lattice parameter of 4.380 Å (on MgO), 4.345 Å (on Al₂O₃), 4.335 Å (on fused quartz).

Low-jitter single flux quantum signal readout from superconducting single photon detector

We developed a single-flux-quantum (SFQ) readout technology for superconducting single-photon detectors (SSPDs) to achieve low-jitter signal readout. By optimizing circuit parameters of the SFQ readout circuit, the input current sensitivity was improved below 10 μA, which is smaller than a typical critical current of SSPD. The experiment using a pulse-pattern generator as an input pulse source revealed that the measured jitter of the SFQ readout circuit is well below the system jitter of our measurement setup for the input current level above 15 μA. The measured jitter of the SSPD connected to the SFQ readout circuit was 37 ps full width at half maximum (FWHM) for an SSPD bias current of around 18 μA, which is a significant improvement on 67 ps FWHM jitter observed in conventional readout without an SFQ readout circuit.

Performance of the ALMA Band 10 SIS Receiver Prototype Model

We have developed a dual polarization prototype model of the Atacama Large Millimeter/submillimeter Array (ALMA) Band 10 (787-950 GHz) receivers. The front-end optics comprises a pair of ellipsoidal mirrors, a wire grid, and two corrugated feed horns. A waveguide mixer block is attached to each feed horn in which a mixer chip employing Nb/AlOx/Nb juncions and NbTiN/SiO2/Al microstrip tuning circuits is mounted to a WR-1.2 full-height waveguide. A local oscillator (LO) signal receiving horn and a waveguide 10-dB LO coupler are integrated in the block to provide the LO signal to the mixer chip. A fixed-tuned multiplier with a diagonal horn located at the 110-K stage is used to transmit the LO power. The LO signal is then quasi-optically coupled to the mixer receiving horn. A very wide intermediate frequency (IF) system with a bandwidth of 4-12 GHz is employed. The receiver demonstrated double sideband (DSB) noise temperatures of about 160 K (4hν/kB) without any correction for loss in front of the receiver at the LO frequency of 834 GHz at an operating physical temperature of 4 K.

High-quality epitaxial NbN/AlN/NbN tunnel junctions with a wide range of current density

We have developed high-quality epitaxial NbN/AlN/NbN Josephson tunnel junctions with a wide range of current density Jc. The junctions show excellent tunneling properties with a large gap voltage of 5.6u2009mV and a large IcRN product of 3.5u2009mV. The quality factor Rsg/RN is about 60 for the junctions with a Jc of 2.2u2009A/cm2, and above 10 for the junctions with a Jc of 25u2009kA/cm2. The crystal structures across the junction barrier are investigated using x-ray diffraction and cross-sectional scanning transmission electron microscopy, and demonstrate epitaxial growth of the NbN/AlN/NbN trilayers for the wide range of Jc.

Design and Fabrication of All-NbN SFQ Circuits for SSPD Signal Processing

We have developed an all-NbN integrated circuit technology for future integration of single flux quantum (SFQ) circuit with superconducting single-photon detector array. The all-NbN integrated circuits consist of epitaxial NbN/AlN/NbN tunnel junctions fabricated on single-crystal MgO substrates, Mo shunt resistors and 500-nm-thick NbN ground plane on the top. Critical current density and sheet resistance of the Mo resistor were set at 2.5 kA/cm2 and 1.2 Ω, respectively. Current uniformity of the NbN/AlN/NbN tunnel junctions was measured to be σ = 3% for 3 μm × 3 μm junction size. The NbN-based SFQ logic cells were designed for and tested at 4.2 K. We actually measured circuit inductances of the fabricated SFQ cells. Circuit simulation revealed that the measured circuit inductances in the fabricated SFQ cells should allow operating margins of more than +/-20%.

Relationship between variable range hopping transport and carrier density of amorphous In2O3–10 wt. % ZnO thin films

The electrical transport characteristics in amorphous Zn doped In2O3 films have been investigated in the range from 2u2009×u20091017u2009cm−3 to 6u2009×u20091020u2009cm−3 of the carrier concentration Ne. For films with Neu2009>u20093u2009×u20091020u2009cm−3, it is found that the Hall mobility μH is limited by ionized impurity scattering. However, for films with Neu2009 u20090 to insulating behavior with dρ/dTu2009<u20090 near Ne≈1u2009×u20091020u2009cm−3 with decreasing Ne. The transport mechanism of carriers in the high-resistivity region is discussed by considering a model based on the Ioffe-Regel criterion. For the film with highest resistivity with Neu2009≈u2009(5u2009−u20096)u2009×u20091017u2009cm−3 among the present films, the ρ(T) show a change from Mott variable-range hopping (ρ ∝u2009expu2009T−1/4) to ρu2009∝u2009expT−1/2 at approximately 10u2009K with decreasing temperature.