Ultracold circuits

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

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 managed to cool an electric circuit on a chip down to 220 microkelvin – close to absolute zero.

When materials are cooled down to extremely low temperatures, their behaviour often differs strongly from that at room temperature. A well-known example is superconductivity: below a critical temperature some metals and other substances conduct electric current without any losses. 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, to reach such temperatures – less than a thousandth of a degree above the absolute zero of 0 Kelvin, or -273.15 degrees Celsius – is exceedingly difficult. Physicists in the research group of Prof. Dr. Dominik Zumbühl at the University of Basel, together with colleagues at the VTT Technical Research Centre in Finland and at Lancaster University in England, have now set a new low-temperature record. Their results have just been published in the scientific journal Physical Review Research.

Cooling with magnetic fields

“Cooling a material down very strongly is not the only problem”, explains Christian Scheller, a senior scientist in Zumbühl’s laboratory: “One also has to reliably measure those extremely low temperatures.” In their experiments, the researchers cooled a tiny electric circuit made of copper on a silicon chip by first exposing it to a strong magnetic field, then cooling it with a special fridge known as a cryostat and eventually ramping down the magnetic field slowly. In this way, the nuclear spins of the cooper atoms in the chip were initially aligned like small magnets and effectively cooled down even further when, at the end, ramping down the magnetic field led to a decrease in their magnetic energy.