Development of ultracold circuits: physicists set a new low temperature record

Development of ultracold circuits: physicists set a new low temperature record

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

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

However, reaching such temperatures – less than a thousandth of a degree above absolute zero of 0 Kelvin, or -273.15 degrees Celsius – is extremely difficult. Physicists from the research group of Professor Dominik Zumbühl at the University of Basel, together with colleagues from the VTT Technical Research Center in Finland and the University of Lancaster in England, have now set a new low temperature record. Their results have just been published in Physical examination research.

Cooling by magnetic fields

“The very strong cooling of a material is not the only problem”, explains Christian Scheller, principal researcher at the Zumbühl laboratory. “You also have to reliably measure these extremely low temperatures.”

In their experiments, the researchers cooled a tiny copper electrical circuit on a silicon chip by first exposing it to a strong magnetic field, then cooling it with a special refrigerator called a cryostat and slowly reducing the magnetic field. In this way, the nuclear spins of the copper atoms in the chip were initially aligned like small magnets and effectively cooled even more when, in the end, the deceleration of the magnetic field caused their magnetic energy to decrease.

“We have been working with such techniques for a decade now, but until now the lowest temperatures that could be reached in this way were limited by the vibrations of the refrigerator,” explains Omid Sharifi Sedeh, who participated in the experiments in as a Ph.D. student.

These vibrations, which result from the continuous compression and rarefaction of the helium refrigerant in a so-called “dry” cryostat, heat the chip considerably. 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 vibration.

Robust thermometer

To accurately measure these temperatures, Zumbühl and his collaborators improved a special thermometer built into the circuit. The thermometer is made up of copper islands connected by so-called tunnel junctions. Electrons can cross these 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 allowed them, finally, to measure a temperature of only 220 millionths of a degree above absolute zero (220 micro Kelvin).

In future, the Basel researchers want to use their method to further lower the temperature by a factor of ten and, in the long term, also cool the semiconductor materials. This will pave the way for studies of new quantum effects and various applications, such as qubit optimization in quantum computers.

The coldest chip in the world

More information:
Mohammad Samani et al, Microkelvin Electronics on a Pulsed Tube Cryostat with a Coulomb Gate Blockade Thermometer, Physical examination research (2022). DOI: 10.1103/PhysRevResearch.4.033225

Provided by the University of Basel

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