Ultracold Circuits – ScienceDaily

Cooling materials to extremely low temperatures is important for basic physics research as well as for technological applications. By improving a special refrigerator and a low-temperature thermometer, Basel scientists have now been able to cool an electrical circuit on a chip to 220 microKelvin – close to absolute zero.

When materials are cooled to very low temperatures, their behavior often differs greatly from that at room temperature. One well-known example is superconductivity: below a critical temperature, some metals and other materials conduct electric current without any losses. Even at lower temperatures, additional quantum physical effects can occur, and are relevant for basic research as well as for applications in quantum technologies.

However, reaching these temperatures — less than a thousandth of a degree above absolute zero Kelvin, or -273.15 degrees Celsius — is very difficult. Physicists in the research group of Professor Dominic Zumbul at the University of Basel, together with colleagues at the VTT Technical Research Center in Finland and Lancaster University in England, have set a new low-temperature record. Their results have just been published in the scientific journal Physical Review Research.

Magnetic field cooling

“Cooling a substance too strongly is not the only problem,” explains Christian Schiller, a senior scientist at the Zomball Laboratory: “One also has to measure very low temperatures reliably.” In their experiments, the researchers cooled a small copper circuit 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 eventually raising the magnetic field slowly. In this way, the nuclear spins of the copper atoms in the chip were initially aligned like tiny magnets and cooled more effectively when, eventually, the reduced magnetic field resulted in a decrease in their magnetic energy.

says Omid Sharifi Sadeh, who participated in the experiments as a PhD student assistant. These vibrations, which arise from constant pressure and dislocation of the helium refrigerant in a so-called “dry” refrigerant, heat the wafer dramatically. To avoid this, the researchers developed a new sample holder connected with extremely strong wires so that the chip could be cooled to extremely low temperatures despite vibrations.

powerful thermometer

To accurately measure these temperatures, Zumbühl and his collaborators have improved a special thermometer built into the circuit. The thermometer consists of copper islands connected by what are called tunnel junctions. Electrons can move through those junctions more or less easily depending on the temperature. Physicists have found a way 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 220 millionths of a degree above absolute zero (220 microK).

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 to cool the semiconductor materials. This will pave the way towards studies of new quantum effects and diverse applications, such as improving qubits in quantum computers.

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Materials Introduction of University of Basel. Note: Content can be modified according to style and length.