Improved Readout Scheme for SQUID-Based Thermometry

NASA’s Jet Propulsion Laboratory, Pasadena, California

Thursday, November 01 2007 The proposed scheme is intended to eliminate these drawbacks. The scheme calls for including a secondary SQUID and its readout instrumentation that would register a small fraction of the magnetic flux passing through a primary SQUID. The scheme includes the following elements:

• Winding a secondary coil of fewer turns around the core to a second readout; or
• Forming a circuit branch parallel to the main coil with the secondary SQUID input coil in series with a large (compared to the SQUID input coil inductance) inductor; or
• Forming a circuit branch parallel to the main coil with a large inductor in series with a SQUID input coil shunted by a small inductor (see figure).

The goal is to avoid having to reset the secondary SQUID in the temperature range of interest, while maintaining the capability of determining the flux state of the primary SQUID unambiguously. If the secondary SQUID readout were monitored by a 16-bit data-acquisition board and the digitization effected by the board determined the readout accuracy, then the dynamic range afforded by this scheme could be optimized by designing the secondary SQUID readout to be about 1/32,000 as sensitive as is the primary SQUID readout. In a typical application, this level of secondary-SQUID sensitivity would correspond to a temperature range ≈3 mK. In this temperature range, there would be no need to actively track the flux state to maintain fidelity of the readout. To avoid the need for counting hardware altogether, a tertiary readout could be added.

This work was done by Konstantin Penanen of Caltech for NASA’s Jet Propulsion Laboratory.