&Bullet; physics 14, p35
A quantum dot can measure ultra-cold temperatures without the need for a direct electrical connection to the outside world.
Accurate temperature measurements are essential to understand the behavior of ultra-cold circuits as used in quantum computers. There are a number of “electron thermometers” for this purpose, but they all require a direct electrical connection to the circuit, which can disrupt the electronic behavior of the circuit. Now researchers have circumvented this requirement with a quantum dot thermometer that can measure temperatures down to 1 K with accuracy without a direct electrical connection  .
When the temperature of a circuit changes, the range of energy that electrons can occupy in the circuit also changes. This distribution can be observed in a component of the capacitance of a quantum dot known as the “tunnel capacitance”. Earlier quantum dot thermometers worked by measuring the current through the dot or by detecting charge directly. Joshua Chawner of Lancaster University in the UK and his colleagues are instead using an indirect approach that takes advantage of this tunnel capacity.
In their experiments, the quantum dot was located near a gate of a transistor in a circuit, but did not directly touch it. A high frequency signal was sent through the circuit to this gate. Then they measured the phase of the returned signal, which changed as the tunnel capacitance of the points changed.
When testing the thermometer in a cryogenic refrigerator – with a second thermometer as a control – the team showed that their setup works for temperatures down to about 1K. Below this, the readings of the two thermometers diverged, but remained within one standard deviation of each other. The team is now working to improve the accuracy of the quantum dot thermometer below 1K, which requires redesigning the quantum dot to change its tunneling capacity.
Christopher Crockett is a freelance writer based in Arlington, Virginia.
- J. Chawner et al., “Non-galvanic calibration and operation of a quantum dot thermometer”, Phys. Rev. Appliedfifteen, 034044 (2021).