Cryogenic temperatures are key aspect of experimental condensed matter physics. The 4He temperature range, down to 4K or even 1K, is routinely available for all spectroscopic techniques at PI1, but in particular for microwave spectroscopy, where photon energies at GHz frequencies correspond to thermal energies below 1K, even lower temperatures are desired. The most common approach to cool experiments down to temperatures well below 100mK is 3He-4He dilution refrigerators. One such system has been in use at PI1 for microwave spectroscopy studies on various material classes for more than a decade and relies on the traditional “wet” precooling with liquid 4He. In recent years, so-called “dry” or “cryogen-free” dilution refrigerators became popular, which precool the cryogenic dilution unit via pulse-tube cryocoolers and thus do not require supply of liquefied 4He. Furthermore, common “dry fridges” are designed for applications in the field of quantum computation and related research, where cryogenic circuits require control and readout using high-frequency signals, and thus such cryostats are optimized with implementation of numerous microwave coaxial cables in mind – in contrast to typical “wet” systems, like the one at PI1 where coaxial cables have to be installed within rather limited space of the cryogenic inserts.
PI1 now obtained a new, “dry” dilution refrigerator to widen microwave spectroscopy research with the specific focus on materials for quantum computation. The model LD400 by Finnish company Bluefors Oy is a cryogen-free dilution refrigerator type that is commonly employed by researchers worldwide in the field of solid-state quantum computation. The planned experiments at PI1 will use superconducting microwave resonators in the single-photon regime that correspond to typical tests performed in the context of superconducting quantum circuitry, e.g. for quantum computation. This will allow PI1 to apply their expertise in the fundamental physics of non-conventional superconducting materials, e.g. strongly disordered or granular superconductors, directly towards application such as high-kinetic-inductance circuitry for novel superconducting qubits.
On December 9th, 2022, the newly installed Bluefors dilution refrigerator achieved a base temperature of 7.4mK during its first cooldown at PI1. The system is now ready for implementation of PI1-designed experiments using various microwave resonator designs and superconducting materials.
Funding for this new 3He-4He dilution refrigerator was kindly provided by the European Union via EFRE (Europäischer Fonds für regionale Entwicklung als Teil der Reaktion der Union auf die COVID-19-Pandemie) and the state of Baden-Württemberg.
