Physicists have set a new low temperature record

The cryostat used by the Basel physicists to reach the record temperature of 220 micro-Kelvin. In the center of the image (golden rectangle) you can see the special thermometer together with a scale bar. Credit: University of Basel, Department of Physics

When materials are cooled to extremely low temperatures, their behavior often differs greatly from that at room temperature. A well-known example is superconductivity: below a critical temperature some metals and other substances conduct electric current without loss. At even lower temperatures, further quantum-physics effects can occur, which are relevant to basic research and applications in quantum technologies.

However, reaching such temperatures, less than a thousandth of a degree above zero Kelvin, or -273.15 degrees Celsius, is extremely difficult. The physicists of the research group of Prof. dott. Dominik Zumbühl of the University of Basel, together with colleagues from the VTT Technical Research Center in Finland and Lancaster University in England, have now set a new low temperature record. Their findings have just been published in Physical revision research.

Cooling with magnetic fields

“The very strong cooling of a material is not the only problem,” explains Christian Scheller, a senior scientist in Zumbühl’s laboratory. “You also need to reliably measure those extremely low temperatures.”

In their experiments, the researchers cooled a tiny electrical circuit made of copper on a silicon chip by first exposing it to a strong magnetic field, then cooling it with a special refrigerator known as a cryostat, and finally slowly decreasing the magnetic field. In this way, the nuclear spins of the copper atoms in the chip were initially aligned as small magnets and effectively cooled further when, eventually, the lowering of the magnetic field led to a decrease in their magnetic energy.

“We have been working with such techniques for a decade now, but so far the lowest temperatures that could be reached this way were limited by the vibrations of the refrigerator,” says Omid Sharifi Sedeh, who was involved in the experiments as a PhD alumnus.

Those vibrations, which result from the continuous compression and rarefaction of the helium cooling agent in a so-called “dry” cryostat, greatly heat the chip. To avoid this, the researchers developed a new sample holder that is wired so tightly that the chip can be cooled to very low temperatures despite vibrations.

Robust thermometer

To accurately measure those temperatures, Zumbühl and his collaborators improved a special thermometer integrated into the circuit. The thermometer consists of copper islands connected by so-called tunnel junctions. Electrons can move through those junctions more or less easily depending on the temperature.

Physicists have found a method to make the thermometer more robust against material defects and, at the same time, more sensitive to temperature. This finally allowed them to measure a temperature of just 220 millionths of a degree above absolute zero (220 micro Kelvin).

In the future, the Basel researchers want to use their method to lower the temperature by another factor of ten and, in the long run, also cool semiconductor materials. This will pave the way for the study of new quantum effects and different applications, such as the optimization of qubits in quantum computers.

The coldest chip in the world

More information:
Mohammad Samani et al, Microkelvin Electronics on a Pulse Tube Cryostat with a Coulomb Gate Block Thermometer, Physical Review Research (2022). DOI: 10.1103 / PhysRevResearch. 4.033225

Provided by the University of Basel

Citation: Development of ultracold circuits: Physicists set new low temperature record (2022, September 22) retrieved September 23, 2022 from temperature.html

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