SQUID-based current sensing noise thermometry for quantum resistors at dilution refrigerator temperatures

Area of Science:

• Physics
• Electrical Engineering
• Cryogenics

Background:

• Accurate low-temperature thermometry is crucial for studying quantum phenomena in resistors.
• Existing amplifiers, like High Electron Mobility Transistor (HEMT) based ones, have limitations in noise performance, particularly 1/f noise.
• Superconducting QUantum Interference Devices (SQUIDs) offer high sensitivity but require careful integration and shielding.

Purpose of the Study:

• To develop a novel dc SQUID-based current amplifier with ultra-low input-referred noise.
• To assess the amplifier's suitability for Johnson noise thermometry of kilohm-range quantum resistors at millikelvin temperatures.
• To compare the performance and practicality of the new SQUID amplifier against existing technologies.

Main Methods:

• Design and construction of a dc Superconducting QUantum Interference Device (SQUID) based current amplifier.
• Measurement of the amplifier's input-referred noise spectral density.
• Integration of the amplifier with a 3.25 kΩ quantum resistor for Johnson noise thermometry experiments.
• Testing the system's performance down to 16 mK.

Main Results:

• The developed SQUID amplifier exhibits an exceptionally low input-referred noise of 2.3 fA/√Hz.
• The circuit demonstrates negligible added noise to the Johnson noise of a 3.25 kΩ resistor at temperatures as low as 16 mK.
• The SQUID amplifier shows significantly reduced 1/f noise compared to HEMT-based amplifiers.

Conclusions:

• The novel dc SQUID-based current amplifier is a highly effective tool for sensitive Johnson noise thermometry in the millikelvin range.
• It provides a viable and advantageous alternative to HEMT amplifiers due to its superior noise performance, especially reduced 1/f noise.
• This SQUID circuit offers comparable low noise to cryogenic current comparator systems but is simpler to construct and shield.