The ULT group possesses the last functional Nuclear Demagnetization cryostat in France (demag. cryostat, called DN1). Even though the design of the machine is 20 years old, it is an extremely good cryostat, able to reach sub-milliKelvin (sub-mK) temperatures.
However, the ULT-NEMS project is a drastic change in the scientific research of the group, and the machine needed to be adapted. This was a major work, which took a big fraction of the first reporting period. A microwave wiring has been installed, plus a liquid 3He thermometer for temperatures below 10 mK (see pictures below). Besides, some of the equipment used on this cryostat was obsolete (current sources, control PCs, temperature regulators) which also represented a decent amount of work in upgrading.
The ULT team possessed in addition to the DN1 Nuclear Demagnetization cryostat some dilution units. But these very good machines were also quite old, with two major problems: their design did not really match the requirements of the proposal, and more problematically they were showing some signs of aging with technical problems appearing (leaks, etc…).
For these reasons we decided during RP1 to purchase a new dry dilution cryostat (called BF1), replacing our own home-made dry prototype. Indeed, these machines, which were state-of-art 15 years ago, are now commercially available. The aim of this dry dilution cryostat setup is twofold: first, make experiments for debugging at a somewhat higher temperature than ULT (namely, 10 mK instead of sub-mK on DN1), and second to develop a unique new facility for "dry" nuclear demagnetization cooling relying on home-made nuclear stages. These types of machines have been demonstrated by colleagues in recent years, but no commercially available design exists yet. We want to develop a new type of setup that could be disseminated by a company selling cryostats.
The two cryogenic platforms have been created, fully equipped with microwave optomechanics (HEMT, circulators, etc.) and base temperature at 500 µK for DN1, and about 10 mK for BF1 (see pictures).
A first dry demag. prototype (PrNi5 stage) has been designed and is under tests for further improvements; this should enable in its final realisation temperatures of order 0.5 mK to be maintained in a "dry" machine". This work is under progress.
New NEMS probes for quantum fluids and solids have been realised. They are being measured in benchmark experiments. First results in 4He gas and in superfluid 4He have been obtained, with very promising results.
In the final RP, we demonstrated the passive ground-state cooling of a macroscopic mechanical device. This breakthrough opens the way for a new type of experiments on quantum NEMS, were all modes of the mechanical element are in equilibrium with their surrounding bath. We reported, in these conditions, the fluctuations of the mechanical mode in contact with its environment; a unique result that shows the potentialities of this setup for quantum thermodynamics. These results have been advertised at (online) conferences, and an article has been sublitted (manuscript on ArXiv: 2104.09541).
