Dilution Refrigerators (DRs)
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).
The most advanced DR in operation at the WMI was purchased just recently from Oxford Instruments. It is an advancement of a cryogen-free DR which was constructed at the WMI some years ago. The DR is pre-cooled by a pulse tube cryocooler which replaces the dewar with liquid helium and nitrogen of traditional DRs. The cryostat is equipped with a (cryogen-free) vector rotate superconducting magnet (6T-1T-1T) and is used for research on hybrid quantum systems.
A second cryogen-free DR is being built in a new lab for experiments on superconducting quantum circuits. The dilution refrigeration unit has already been operated in a test refrigerator; its refrigeration capacity was 350µW@100mK, and its base temperature was 8 mK. In addition to large cooling powers, the specifications for the projected experiments are mainly dictated by the dimensions of bulky microwave components such as circulators or microwave switches. Therefore the experimental space at all temperature stages including the location of the mixing chamber is big; the diameter of the mixing chamber plate is 300 mm, and the height available there is 400 mm. The DR is mounted in a trestle with air springs to keep vibrations of the building from the delicate experiments in the DR.
The sample space of one of the liquid helium precooled dilution refrigerators has been extended from a cylindrical volume with 11 cm diameter and 15 cm height to one with 55 cm height. By further increasing the number of microwave amplifiers at the 4 K-stage from two to four, the number of broadband input lines from four to seven, and the number of twisted pair DC-lines from 32 to 80, we are able to mount four experiments simultaneously and avoid idle times by interleaved measurements. The fridge has been successfully cooled to 20 mK and first measurements on superconducting quantum circuits at very low temperatures have been performed.
An additional liquid helium precooled dilution refrigerator is currently being set up. Here, the key components such as the gas handling system, pumps and vacuum installations, cryogenic insert, and the dilution unit are finished, the first test cooldown is planned soon.
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.
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.