An improved readout scheme has been proposed for high-resolution thermometers, (HRTs) based on the use of superconducting quantum interference devices (SQUIDs) to measure temperature- dependent magnetic susceptibilities. The proposed scheme would eliminate counting ambiguities that arise in the conventional scheme, while maintaining the superior magnetic-flux sensitivity of the conventional scheme. The proposed scheme is expected to be especially beneficial for HRT-based temperature control of multiplexed SQUID-based bolometer sensor arrays.
SQUID-based HRTs have become standard for measuring and controlling temperatures in the sub-nano-Kelvin temperature range in a broad range of low-temperature scientific and engineering applications. A typical SQUID-based HRT that utilizes the conventional scheme includes a coil wound on a core made of a material that has temperature- dependent magnetic susceptibility in the temperature range of interest. The core and the coil are placed in a DC magnetic field provided either by a permanent magnet or as magnetic flux inside a superconducting outer wall. The aforementioned coil is connected to an input coil of a SQUID. Changes in temperature lead to changes in the susceptibility of the core and to changes in the magnetic flux detected by the SQUID.
The SQUID readout instrumentation is capable of measuring magneticflux changes that correspond to temperature changes down to a noise limit ≈0.1 nK/Hz1/2. When the flux exceeds a few fundamental flux units, which typically corresponds to a temperature of ≈100 nK, the SQUID is reset. The temperature range can be greatly expanded if the reset events are carefully tracked and counted, either by a computer running appropriate software or by a dedicated piece of hardware.
While adequate for many applications, the conventional scheme has drawbacks: If the temperature is changed rapidly or the temperature noise is high, the counting hardware and/or software loses flux count. In the case of a software counter, the temperature reading is lost entirely if the software is reset or restarted. In the case of a multiplexed SQUID controller, these drawbacks become more severe because flux readings are taken less frequently.
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.
This Brief includes a Technical Support Package (TSP).
Improved Readout Scheme for SQUID- Based Thermometry
(reference NPO-41757) is currently available for download from the TSP library.
Don't have an account?
Overview
The document titled "Improved Readout Scheme for SQUID-Based Thermometry" (NPO-41757) from NASA's Jet Propulsion Laboratory outlines advancements in the readout technology for SQUID (Superconducting Quantum Interference Device) thermometers, which are essential for high-resolution temperature measurements in the sub-nano-Kelvin range. These devices are widely used in low-temperature science and engineering applications due to their superior sensitivity to magnetic flux changes.
The primary focus of the document is to address the limitations of current SQUID readout systems, particularly their dynamic range and the challenges associated with tracking magnetic flux changes. The existing systems can detect minute changes in magnetic flux corresponding to approximately 0.1 nK/(Hz)–1/2, but they face issues when the flux exceeds a few fundamental units, typically around 100 nK, necessitating a reset of the SQUID. This reset can lead to loss of data, especially during rapid temperature changes or high temperature noise, which can disrupt the counting of flux changes.
To overcome these challenges, the document proposes an alternative measuring scheme that incorporates a secondary SQUID readout. This secondary readout is designed to register a small fraction of the magnetic flux passing through the primary SQUID, thereby avoiding the need for frequent resets. The proposed methods for implementing this secondary readout include winding a second coil around the salt material with fewer windings, and creating circuits that allow for the secondary SQUID to operate in parallel with the primary one.
The document emphasizes that by optimizing the sensitivity of the secondary SQUID to be approximately 32,000 times less than that of the primary, the dynamic range can be significantly expanded, allowing for accurate readings in a temperature range of about 3 mK without the need for active flux state tracking. This innovation not only enhances the fidelity of the readout but also simplifies the overall measurement process.
In summary, the proposed improvements in SQUID-based thermometry aim to provide a more robust and reliable method for measuring temperature at extremely low levels, which is crucial for various scientific and engineering applications. The document serves as a technical support package, detailing the innovative approaches and potential benefits of the new readout scheme.
