The thermometer works a

Two of the new temperature sensors (top right, bottom left) are connected to a chip under test. Also included is a conventional wired ruthenium oxide thermometer (right, top)

All this, and does not require additional cables or lasers, as it borrows the conductors present in the device under test.

“This was a fun idea that quickly became something very useful,” said NIST scientist Joel Ullom. “The thermometer allows researchers to measure the temperature of a wide range of components in their test packages at a very low 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 across a wide range of fields. “

In the technology demonstrator, the team used three of the sensors to monitor temperatures on a kinetic inductance traveling wave parametric amplifier (look at the photo). They see it as a useful technique for monitoring the temperature of processor chips in superconducting quantum computers.

In principle it is simple: a temperature sensitive LC tank circuit is coupled to any microwave signal path that passes through the device under test and its resonance peak can be seen from the outside of the cryogenic system using an analyzer of vector network.

In this case, there were two types of sensors, both about 2.4 x 1.2 mm and designed to resonate between 1.5 and 6 GHz with a ~ 4μH meander inductor and a ~ 2pF planar interdigitated capacitor. – made with a single lithographic pass on a silicon wafer (1,200 can be made on a 75 mm wafer).

The detection ranges from 50mK to 1K and the range can be shifted up to 5mK with slightly different designs. Sensitivity is 50 – 75μK / √Hz.

• Type 1 has a plate capacitor to couple in the boundary field of a host device power line (photo top right, via SMA connection)
• Type 2 has a connection plate for connecting the wire to a power line of the host devicephoto bottom left)

The two types of thermometer were optimized using 2D electromagnetic simulations for strong coupling.

As they do not need additional wiring (unlike the conventional ruthenium oxide thermometer pictured (gray on the right)), the sensors cannot introduce electromagnetic interference or conductive heating into the device under test.

But why are they temperature sensitive?

The shift in resonant frequency of LC circuits is not due to something as simple as thermal contraction.

Instead it is due to a quantum interaction between the LC and a dielectric layer of silicon dioxide coated on the inductor and capacitor tracks, which are attacked by superconducting niobium.

“The coating interacts with the resonator to shift the frequency at which it naturally vibrates,” according to NIST. “Scientists suspect this is due to the tunneling of atoms between two sites, a quantum mechanical effect.”

Tunneling is between two quantum superpositions. Although the science is not fully understood, the phenomenon – known as a two-tier system or two-state system – is well known and well characterized.

According to the paper “Sub-Kelvin Thermometer for On-Chip Measurements of Microwave Devices Using Two-Level Systems in Superconducting Microresonators,” it is these two-level temperature-dependent systems that interact with microwave photons to shift resonance.

The document, published in Applied Physics Letters, is available in full at no cost. He clearly explains the quantum interaction and goes on to put the sensors into perspective: “While [these] thermometers will not replace resistive thermometers or conventional diodes in sub-kelvin cryogenics, in certain situations they are more convenient. Specifically, thermometers provide a tool below 1K for measuring the on-chip temperature of cryogenic microwave devices, such as microwave kinetic inductance detectors, microwave SQUID multiplexers, and qubits. Thermometers can be frequency multiplexed together with these devices using the existing power line and read with the same infrastructure. “