Transition edge sensors (TES) have achieved a low noise level and become an attractive tool for detecting photons and measuring the power of millimeter waves. While large TES arrays are awaited in many fields, multiplexing schemes limit the multiplexing factor and the number of detectors. In the case of frequency-domain multiplexing, extending the multiplexing factor requires an increase in the bandwidth of the SQUID electronics. To overcome this limitation, we use digital active nulling (DAN) on a digital frequency multiplexing platform. We present development of the large array's readout for the POLARBEAR-2 cosmic microwave background (CMB) experiment. It aims to observe B-mode polarization generated by the gravitational lensing of CMB and inflationary gravitational waves, with high sensitivity. We will build a receiver that has 7,588 TES bolometers coupled to polarization-sensitive antennas. We present implementation of DAN and show that our system is able to bias a bolometer in its superconducting transition at approximately 3 MHz.
We present our recent progress in development of a superconducting nanowire single-photon detector (SSPD) system. Our practical SSPD system with a compactGifford-McMahon cryocooler successfully achieved the high system detection efficiency of 74%, low dark count of 100 cps, and small timing jitter of 68 ps in one device simultaneously, which is useful for actual applications. Furthermore, we propose and demonstrate a new strategy to resolve a trade-off problem between the system detection efficiency and counting rate (speed) in the SSPD.
Hot electron bolometer mixers (HEBM) are expected to be used as low-noise heterodyne mixers in the terahertz frequency range. We are developing quasi-optical HEBMs for applications such as atmospheric remote sensing and radio astronomy in the terahertz frequency range. Therefore, we developed a fabrication method using niobium nitride (NbN) ultrathin films deposited on a silicone (Si) substrate for terahertz applications. First, a magnesium oxide buffer layer 50 nm thick was deposited on a Si substrate at approximately 525 °C. After deposition of this layer, NbN ultrathin films were deposited at ambient temperature. Using the buffer layers, we found the transition temperature of the 3.5-nm-thick NbN thin films to be approximately 10 K. The same method was applied to fabricate NbN-HEBMs on a Si substrate. The fabricated quasi-optical mixers were designed to operate at 3.1 THz. The receiver noise temperature was found to be approximately 1930 K (DSB) at 3.1 THz.
Here, we developed a robust HTS-SQUID magnetometer based on a bicrystal SrTiO3 substrate and single-layer YBa2Cu3O7-x film, aiming for a mobile magnetometer in an unshielded environment. The robustness in the unshielded environment was achieved by coverage of the SQUID magnetometer with another HTS cross-shaped film in a flip-chip configuration, and introduction of a mesh structure in both the magnetometer and the HTS film. The SQUID parameters in DC and AC fields, such as critical current 2Ic, resistance Rn/2, modulation depth Vpp and flux noise S∅1/2, were studied. The SQUID was rotated and moved in the environment using a robot-arm, resulting in stable operation during the motion at 20 mm/s with active magnetic shielding.