Dilution Refrigerators

Dilution refrigeration is most important for low temperature research as it is the only method to continuously produce temperatures in the millikelvin regime. At the WMI dilution refrigerators have been used in many experiments of solid state physics and quantum fluid and solid research.

The WMI has a long history of dilution refrigerator construction. The first fridge went into operation in 1969; it was the first one of its kind in Germany at that time. Many fridges followed over the years. Of special importance for the further development of dilution refrigerators was a fridge which had no separate cooling stage to liquefy the backstreaming ³He gas, but utilized an integrated Joule-Thomson stage for condensation (1976).

Today the oldest fridge in operation is the one which is part of our nuclear demagnetization cryostat (BM II). It has a cooling capacity of 100 µW@100mK and a very low base temperature of 4 mK.

In another cryostat installed just recently a dilution refrigerator and a superconducting vector magnet are combined where precise 2-axes rotations of the magnetic field (B ≤ 3.5 kG) at temperatures down to ~ 20 mK are feasible. The cryostat is currently used for studying the superconducting properties of low-dimensional organic metals.

For heat capacity measurements a dilution refrigerator is available where samples of a volume of up to 1 cm3 can be cooled to 17 mK; their specific heat can be probed in a temperature regime of 17 mK and 6 K. The measurement method is the so-called quasi-adiabatic heat pulse method. The addenda heat capacity is 10-8 J/K@0.1K.

In recent years miniature dilution inserts were developed which fit in superconducting magnets with an inner diameter of 2 inches; measuring times of several months are common. Alternatively, they can conveniently be operated in helium transport dewars. Temperatures as low as 15 mK are available. What makes miniature fridges especially efficient for scientists is their short cooldown time of only four hours between installation and full operation. Several mini-fridges are in use at the WMI.

For time domain measurements of the coherent quantum dynamics of superconducting qubits a newly installed dilution refrigerator is at our disposal. The cryostat is placed in a shielded room on a trestle with anti-vibration air springs; it is equipped with several coaxial lines for frequencies up to 40 GHz. The cooling capacity of the fridge is 100 µW@100mK with a base temperature of 13 mK.

A development which is still ongoing is the construction of a “dry” fridge; no cryogens are needed for this type of refrigerator. At present, this fridge has a cooling capacity of 350 µW@100mK and a base temperature of well below 10 mK. Recent work aims to increase the refrigeration capacity of our “dry” fridge.