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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 5.6 mV and a large IcRN product of 3.5 mV. The quality factor Rsg/RN is about 60 for the junctions with a Jc of 2.2 A/cm2, and above 10 for the junctions with a Jc of 25 kA/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. [ABSTRACT FROM AUTHOR]

We demonstrate superconducting qubits using epitaxially grown Josephson junctions. A fully epitaxial NbN/AlN/NbN trilayer on MgO (100) substrate is processed by photolithography and dry-etching into transmon qubits with a large Josephson energy. The tunnel barrier made of cubic-phase AlN, rather than the ordinary hexagonal phase, is the key to avoid piezoelectric coupling to the phonon bath. The energy-relaxation time and the spin-echo decay time of ∼500 ns are observed in the qubits that are coupled to a monolithically made coplanar waveguide resonator. [ABSTRACT FROM AUTHOR]

Proposes and demonstrates a high-temperature superconductor multilayer structure. Details on the isolation resistance between the ground plane and other layers of the YBCO; Effects of the high temperature process during ground plane formation; Information on the measured voltage modulations of the super-conducting quantum interference device.

We demonstrate the operation of a coincidence photon detector with a short timing jitter consisting of two superconducting nanowire single photon detectors (SSPDs) and a single flux quantum (SFQ) circuit. By utilizing the timing discrimination capability of the SFQ coincidence circuit, the full width at half maximum timing jitter of the entire coincidence photon detector was evaluated as 32.3 ps, which is 36 ps less than that of the standard commercial time correlated single photon counting module, and the timing jitter of the SSPD was estimated as ∼15 ps. Owing to the short timing jitter characteristics, our coincidence photon detector could correctly capture the effect of pulse-width broadening by insertion of an optical bandpass filter. We have also demonstrated that our coincidence photon detection clearly shows Hong-Ou-Mandel interference with a weak coherent pulse. These results are a crucial step to realizing high timing resolution coincidence measurements, ushering in a technology for timing measurement based multi-photon quantum interference. [ABSTRACT FROM AUTHOR]

We have demonstrated highly pure single-photon emissions from an InAs/InP quantum dot at the wavelength of 1.5 μm. By applying quasi-resonant excitation, one exciton is deterministically generated in an excited state, which then relaxes to the exciton ground state before recombining to emit a single photon. The photon-correlation function of the emission from the exciton ground state exhibits a record g(2)(0) value of (4.4±0.2) ×10-4 measured using high-performance super-conducting single-photon detectors, without any background subtraction. This single-photon source with extremely low multi-photon emission probability paves the way to realize long distance quantum key distribution and low error-rate quantum computation. [ABSTRACT FROM AUTHOR]

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 0.5 K to 4.0 K, 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. [ABSTRACT FROM AUTHOR]