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 electricity without loss. At even lower temperatures, additional quantum physical 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 Prof. Dr. Dominik Zumbühl from 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 results have just been published in Physical Review Research.
Cooling with magnetic fields
“Very strong cooling of a material is not the only problem,” explains Christian Scheller, a scientist in Zumbühl’s laboratory. “It also needs 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 reducing magnetic field slowly. In this way, the nuclear spins of the copper atoms in the chip were initially aligned like small magnets and effectively cooled further when, at the end, the decrease in the magnetic field led to a decrease in their magnetic energy.
“We have been working with such techniques for a decade, but until now the lowest temperatures that could be reached in this way were limited by the vibrations of the refrigerator,” says Omid Sharifi Sedeh, who was involved in the experiments as a Ph.D. student.
Those vibrations, which arise from the continuous compression and rarefaction of the helium coolant in a so-called “dry” cryostat, heat the chip significantly. To avoid this, the researchers developed a new sample holder that is so strongly connected that the chip can be cooled to very low temperatures despite the vibrations.
To accurately measure those temperatures, Zumbühl and his collaborators have developed a special thermometer that is integrated into the circuit. The thermometer consists of copper islands that are 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 eventually 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 a further ten and, in the long term, to cool semiconductor materials as well. This will open the way to studies of new quantum effects and various applications, such as optimizing qubits in quantum computers.
The coolest chip in the world
Mohammad Samani et al., Microkelvin Electronics on a Pulse Tube Cryostat with a Gate Coulomb Blocking 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 https://phys.org/news/2022-09-ultracold-circuits-physicists-low-temperature .html
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