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New Sub-Kelvin Mini-Thermometer for Superconductor-Based Quantum Computers

At the National Institute of Standards and Technology (NIST), scientists have created a new tiny thermometer with huge possible applications, such as tracking the temperature of processor chips in superconductor-based quantum computers, which have to remain cool to work well.

Two of NIST’s superconducting thermometers for measuring cryogenic temperatures are glued to the lower left and upper right of this amplifier. The miniature thermometers, made of niobium on a layer of silicon dioxide, measure the temperature of the amplifier or other device based on a frequency signal. Image Credit: J. Wheeler/NIST.

NIST’s superconducting thermometer is capable of measuring temperatures below 1 K (−272.15 °C or −457.87 °F), down to 50 milliKelvin (mK) and possibly 5 mK. It is faster, smaller, and more convenient compared to traditional cryogenic thermometers used for chip-scale devices. Moreover, it can be manufactured on a large scale.

The NIST team has presented the design and working of their new thermometer in a new paper published in Applied Physics Letters.

The new thermometer measures just 2.5 by 1.15 mm in size and can be integrated into or mounted on another cryogenic microwave device to quantify its temperature when positioned on a chip. The team used the thermometer to show fast, exact measurements of the heating of a superconducting microwave amplifier.

The technology is a derivative of NIST’s custom superconducting sensors for telescope cameras, particularly microwave detectors supplied for the BLAST balloon and TolTEC.

This was a fun idea that quickly grew into something very helpful. The thermometer allows researchers to measure the temperature of a wide range of components in their test packages at very little cost and without introducing a large number of additional electrical connections. This has the potential to benefit researchers working in quantum computing or using low-temperature sensors in a wide range of fields.

Joel Ullom, Study Group Leader, NIST

The thermometer includes a superconducting niobium resonator coated with silicon dioxide. The coating interacts with the resonator to change the frequency at which it freely vibrates. Researchers speculate this is because of atoms “tunneling” between two locations, a quantum-mechanical effect.

The thermometer made by the NIST team is built around a new application of the concept that the natural frequency of the resonator relies on the temperature. The thermometer maps variations in frequency, as measured by electronics, to a temperature.

On the contrary, traditional thermometers for sub-Kelvin temperatures rely on electrical resistance. They need wiring routed to room-temperature electronics, introducing complexity and possibly causing heating and interference.

The superconducting thermometer tracks temperature within about 5 ms (one-thousandths of a second), a lot faster than the majority of conventional resistive thermometers at around one-tenth of a second. The novel thermometers are also easy to produce in just a single process stage. They can be manufactured on a large scale, with over 1,200 fitting on a 3-inch (about 75-mm) silicon wafer.

Journal Reference:

Wheeler, J., et al. (2020) Sub-Kelvin thermometer for on-chip measurements of microwave devices utilizing two-level systems in superconducting microresonators. Applied Physics Letters.

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