Zhen Wang

0.5-THz waveguide SIS mixers with NbN/AlN/NbN technology

Heterodyne mixers based on superconducting SIS (superconductor-insulator-superconductor) tunnel junctions have been demonstrated to be the most sensitive coherent detectors at millimeter and submillimeter wavelengths. In fact, conventional superconducting SIS mixers with Nb/AlOx/Nb junction and Nb/SiO2/Nb tuning circuit have shown good performances with the noise temperature reaching as low as three times the quantum limit below 0.7THz, which is the gap frequency of Nb-based SIS junctions. However, due to the large loss in Nb thin-film superconducting microstrip lines, the noise performance of Nb SIS mixers deteriorates significantly above 0.7THz. With a gap frequency double that of Nb-based SIS junctions, NbN-based SIS junctions are of particular interest for the development of heterodyne mixers in the terahertz region. Considering the bandwidth and output power of local-oscillator (LO) signal sources are quite limited around 1THz, we firstly develop a waveguide NbN-based SIS mixer at 0.5THz. Three types of SIS junctions, i.e., long junction, parallel-connected tunnel junction (PCTJ) and distributed junction array (DJ) are investigated. They are all comprised of NbN-AlN-NbN tri-layer fabricated on an MgO substrate and have the same current density (Jc) of 10kA/cm2. In this paper, we describe their design, fabrication and preliminary experimental results.

Operation of single-flux-quantum logic cells based on all-NbN integrated circuit technology

We developed NbN-based SFQ circuits consisting of high-quality epitaxial NbN/AlN/NbN trilayer grown on MgO (100) substrate. The SFQ logic cells such as JTL, DC/SFQ converter, voltage driver, pulse splitter, TFF, and CB were designed by setting the critical current density JC at 2.5 kA/cm2. We succeeded in demonstrating correct operations of all the designed SFQ cells, but the JC range allowing the operation was quite narrow as around 70% for the designed value. The circuit simulation revealed that the TFF cell requires the JC range from 65% to 115% for the correct operation, while the DC/SFQ converter was not operational for the JC of more than 70%.

Superconducting Nanowire Single-Photon Detectors for Quantum Communication Applications

Single-photon detectors are a key enabling technology for optical quantum information processing applications such as quantum key distribution. A new class of single-photon detectors have emerged based on superconducting nanowires. These detectors offer sensitivity at telecommunication wavelengths (1310nm and 1550nm) with low dark counts and excellent timing resolution at an operating temperature of ~4 K. We have integrated four independent fibre-coupled detectors into a practical closed-cycle refrigerator and plan to employ this multichannel detector system in advanced quantum information processing experiments.

Preparation of YBa2Cu3O7-x Thin Films by Nd:YAG Laser Ablation

We have studied the preparation of YBa2Cu3O7-x (YBCO) thin films grown on SrTiO3 (100) and MgO (100) substrates using the 4th harmonics of an neodymium-doped yttrium aluminum garnet (Nd:YAG) pulse laser beam (wavelength 266 nm). The Nd:YAG laser is a solid-state laser and is safer than the excimer laser. The as-deposited YBCO thin films have been shown c-axis orientation and 90 K phase with varying the target-substrate distance and the oxygen flow rate during the ablation. We have found that the target-substrate distance is closely related to the laser-generated plume size and that the oxygen flow rate is the key preparation parameter in addition to the substrate temperature and oxygen pressure.

Performances of NbTiN superconducting nanowire single photon detector

We present high performance fiber-coupled niobium titanium nitride superconducting nanowire single photon detectors fabricated on thermally oxidized silicon substrates. By applying a double sided cavity structure, simulated absorptance in the nanowire meander reached 97% at 1550 nm wavelength. The fabricated device showed a system detection efficiency (DE) of 74% at 1550 nm, dark count rate of 100 c/s, and a full width at half maximum timing jitter of 68 ps under a bias current of 18.0 μA with a practical Gifford-McMahon cryocooler system.

Characterization of optical absorptance in superconducting nanowire single-photon detectors

We present a numerical simulation of the optical absorptance for the superconducting nanowire single-photon detectors (SNSPDs or SSPDs) designed with various layouts by using finite-element analysis software. The simulation results reveal that the electric field concentrated in the superconducting nanowire is enhanced by reducing the filling factor, and the degree of the enhancement becomes larger for thicker nanowires. Due to the large enhancement of the electric field in the nanowire with a double-side cavity structure, the high optical absorptance can be achieved even for the low-filling-factor layout by tuning the device design, which leads to the realization of SSPDs with high system detection efficiency and high counting rate.