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.