The mm-wavelength sky reveals the initial phase of structure formation, at all spatial scales, over the entire observable history of the Universe. Over the past 20 years, advances in mm-wavelength detectors and camera systems have allowed the field to take enormous strides forward – particularly in the study of the Cosmic Microwave Background – but limitations in mapping speeds, sensitivity and resolution have plagued studies of astrophysical phenomena. In fact, limitations due to inherent biases in the ground-based mm-wavelength surveys conducted over the last 2 decades continue to motivate the need for deeper and wider-area maps made with increased angular resolution. TolTEC is a new camera that will fill the focal plane of the 50m diameter Large Millimeter Telescope (LMT) and provide simultaneous, polarization-sensitive imaging at 2.0, 1.4, and 1.1mm wavelengths. The instrument, now under construction, is a cryogenically cooled receiver housing three separate kilo-pixel arrays of Kinetic Inductance Detectors (KIDs) that are coupled to the telescope through a series of silicon lenses and dichroic splitters. TolTEC will be installed and commissioned on the LMT in early 2019 where it will become both a facility instrument and also perform a series of 100 hour “Legacy Surveys” whose data will be publicly available. The initial four surveys in this series: the Clouds to Cores Legacy Survey, the Fields in Filaments Legacy Survey, the Ultra-Deep Legacy Survey and the Large Scale Structure Survey are currently being defined in public working groups of astronomers coordinated by TolTEC Science Team members. Data collection for these surveys will begin in late 2019 with data releases planned for late 2020 and 2021. Herein we describe the instrument concept, provide performance data for key subsystems, and provide an overview of the science, schedule and plans for the initial four Legacy Survey concepts.
Superconducting nanowire single photon detectors (SNSPDs) have emerged as a leading choice for high performance single photon detectors due to their low timing jitter, high detection efficiency, and low dark count rates. SNSPDs have typically been biased using a passive quenching scheme in which the bias current of the device is shunted through a resistive load to allow for recovery after a detection event. To prevent latching, the shunting resistor must be approximately an order of magnitude smaller than the peak normal domain resistance of the SNSPD. Consequentially, the pulse amplitude (∝IBRL) and recovery time (∝LK/RL) are both negatively impacted. In this talk, we will describe a novel approach to the bias and read-out of SNSPDs based upon active quenching. We will present detailed design considerations for an active quenching architecture and will show that such an approach has the potential to improve count rates while increasing signal swings to the point where external amplification is no longer required. A silicon germanium (SiGe) active-reset chip design has been designed, implemented, and integrated with a NbTiN SNSPD. The procedure for the SiGe chip design will be described and simulation results will be presented. Finally, detailed measurement results of the complete system will be shown and compared to measurements of the same detector when biased and read-out using a standard passive quenching scheme. It will be shown that the active quenching configuration enables a considerable enhancement to the system performance.
There is a growing interest in developing systems employing large arrays of SNSPDs. To make such instruments practical, it is desirable to perform signal processing before transporting the detector outputs to room temperature. We present a cryogenic eight-channel pixel combiner circuit designed to amplify, digitize, edge detect, and combine the output signals of an array of eight SNSPDs. The circuit has been fabricated and measurement results agree well with expectation. The paper will conclude with a summary of ongoing work and future directions.