J. T. Skidmore

Single superconducting quantum interference device multiplexer for arrays of low-temperature sensors

We present the design and experimental evaluation of a superconducting quantum interference device (SQUID) multiplexer for an array of low-temperature sensors. Each sensor is inductively coupled to a superconducting summing loop which, in turn, is inductively coupled to the readout SQUID. The flux-locked loop of the SQUID is used to null the current in the summing loop and thus cancel crosstalk. The sensors are biased with an alternating current, each with a separate frequency, and the individual sensor signals are separated by lock-in detection at the SQUID output. We have fabricated a prototype 8 channel multiplexer and discuss the application to a larger array.

Superconducting bolometer for far-infrared Fourier transform spectroscopy

A voltage-biased superconducting bolometer with a heat sink temperature of 4.2 K has been developed for Fourier transform spectroscopy in the far infrared. This device is based on a Nb transition edge sensor with Tc=8.1 K. It will operate for absorbed infrared power up to 3×10−6 W and has an absorber area of 7 mm2. The response is inherently linear and the noise equivalent power (NEP)=1.2×10−13 W Hz−1/2 is dominated by thermal fluctuation noise. This NEP is at least a factor 10 better than that expected for a conventional 4.2 K semiconductor bolometer which is optimized for 1% saturation at the same infrared power. The optical response time τ=1.2 ms is dominated by the internal thermalization time. A smaller version of this bolometer could be useful for diffraction-limited spectroscopy of small samples throughout the infrared. Estimates suggest that values of detectivity D*>1011 cm Hz+1/2 W−1 and time constants approaching 270 μs could be achieved.

Single SQUID multiplexer for arrays of Voltage-biased Superconducting Bolometers

We describe a frequency domain superconducting quantum interference device (SQUID) multiplexer which monitors a row of low-temperature sensors simultaneously with a single SQUID. Each sensor is ac biased with a unique frequency and all the sensor currents are added in a superconducting summing loop. A single SQUID measures the current in the summing loop, and the individual signals are lock-in detected after the room temperature SQUID electronics. The current in the summing loop is nulled by feedback to eliminate direct crosstalk. In order to avoid the accumulation of Johnson noise in the summing loop, a tuned bandpass filter is inserted in series with each sensor. To multiplex 32 Voltage-biased Superconducting Bolometers (VSB) with 1 msec time constants a total bandwidth of 100 kHz is required. The minimum bias frequency is limited to ≳500 kHz by practical limitations in the size of photolithographed inductors and capacitors. The major limitation of our multiplexing scheme is in the slew rate of the read...

New technologies for the detection of millimeter and submillimeter waves

Voltage-biased superconducting bolometers have many operational advantages over conventional bolometer technology including sensitivity, linearity, speed, and immunity from environmental disturbance. A review is given of the Berkeley program for developing this new technology. Developments include fully lithographed individual bolometers in the spiderweb configuration, arrays of 1024 close-packed absorber-coupled bolometers, antenna-coupled bolometers, and a frequency-domain SQUID readout multiplexer.