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Ultra-Cold Nano-Mechanics: from Classical to Quantum Complexity

Periodic Reporting for period 3 - ULT-NEMS (Ultra-Cold Nano-Mechanics: from Classical to Quantum Complexity)

Reporting period: 2018-11-01 to 2020-04-30

The ULT-NEMS project aims at studying ultra-low temperature (ULT) physics by means of dedicated NEMS devices (Nano Electro Mechanical Systems). The project has four WP (work-packages) that distinguish different axes of research, focused on microscopic or macroscopic aspects of quantum physics:

WP1 : NEMS materials properties down to ULT, amorphous matter, supraconductivity at small scales. So-called Quantum Solids axis (QS)
WP2 : developing NEMS probes for Quantum Fluids (QF)
WP3 : ULT cooling of NEMS device: reaching by “brute force” cryogenics the mechanical ground state (QM)
WP4 : quantum operation on a quantum NEMS beam (final aim of QM axis; studying basic aspects of Quantum Mechanics)

This project tackles fundamental aspects of quantum mechanics in a unique way. On the microscopic side, the aim is to study elementary excitations of condensed matter within quantum solids and fluids. The actual existence of some of these has not been demonstrated so far in some specific areas of physics (two-level systems in amorphous matter, and Majorana Fermions in topological states). On the macroscopic level, the question addressed is how a quantum mechanical object becomes classical when it moves on macroscopic distances? This is linked to topical theoretical questions on wave packet reduction and decoherence, addressed in specific theories of collapse and quantum gravity.
These questions are extremely important for scientists, because they are at the heart of our description of Nature with quantum theory. More generally, these questions are important because they tackle the way we understand the world we live in.
"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 to purchase a new dry dilution cryostat (called BF1), replacing our own home-made dry prototype. Indeed, these machines, which were state-of-art ten 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 are now ready, fully equipped with microwave optomechanics (HEMT, circulators, etc.) and base temperature below 1 mK for DN1, and below 10 mK for BF1 (see pictures). Some thermometry developments are still ongoing on the DN1 platform. Two microwave chips have been realized and are being measured now (see pictures below: actual device, PCB board for sample cell and numerical simulation of the NEMS beam).
A first dry demag. prototype has been designed and is almost ready for tests (the various constitutive pieces are ready: coil, heat switch, PrNi5 stage).
New NEMS probes for quantum fluids and solids have been realised (see picture below). They are being tested, and mounted in benchmark experiments.

During this reporting period we have been performing some preliminary experiments touching essentially WP2 and WP3. Two main issues for these have been addressed: how does a NEMS device behave in presence of a confined (quantum) fluid, and second how cold can we “brute force” cool a NEMS? The last question is particularly demanding, and indeed all other laboratories rely on active cooling schemes, at best performed from typically 10 mK starting temperatures. In our project (especially for WP4), we need to cool down all degrees of freedom in equilibrium, thus active schemes are prohibited.
A related important question is how good can our NEMS devices be, regarding intrinsic sources of noise and mechanical damping? This has both practical implications (good devices are mandatory for good physics), but also comes in with fundamental studies on amorphous materials (the WP1) which are the constitutive materials of our NEMS objects.

Some preliminary works on (classical) fluids, frequency noise (and related aspects of Brownian motion), and amorphous thin films have been published. An experiment studying the mechanical damping in a silicon nitride amorphous NEMS has been conducted down to 25 mK, and is being analyzed. It is part of a PhD thesis defended 26 march 2018, but it is still unpublished. This work is an extremely interesting test for the Two Level System model describing glasses, and shall be published soon. A collaboration with University of Lancaster is ongoing on the interaction of a NEMS with superfluid 4He.
We are presently measuring the optomechanical NEMS setups on both DN1 and BF1 cryostats. The plan is to study their properties and design the best measurement schemes. And see the impact of the measurements on the actual temperature.
Another quantum-limited detection scheme for ultra-cold NEMS is being considered: a SQUID amplifier. This work is performed in collaboration with Royal Holloway University of London. Instead of a microwave cavity, the NEMS device (mounted also on a demag. cryostat) is embedded in the pickup loop of a SQUID amplifier. The idea is to compare the capabilities of both schemes."
"The lowest temperatures achievable are one of the frontiers of physics. Knowing if we can ""brute force"" cool a NEMS device to sub-milliKelvin temperatures is a great challenge, which would open a new field for fundamental science.
Thus, the first demanding step was to mount an optomechanics experiment on the demagnetization cryostat including an almost quantum-limited detection scheme, based on a microwave cavity, and to be able to measure locally the temperature of the phonons inside the NEMS. This is the experiment we are conducting right now, based on the setup and device which pictures are attached. Up to now the results are encouraging, even if we experienced some technical problems.

Such an experiment has never been tried up to now (demag. cryostat + microwave optomechanics). It is a crucial step of the project, which will tell us how far we can cool down these objects.
We expect to be able to demonstrate this cooling before the end of the project, and to use it for some demonstration experiment on ""brute-force"" ground-state cooled macroscopic NEMS devices.

Another key aspect of the project is the dissemination of the technology. At the moment, sub-milliKelvin temperatures are reached only in a few laboratories. The development of ""dry"" Nuclear Demagnetization could widen the field to many communities outside ultra-low temperature physics.
Moreover, the new NEMS probes we are developing for quantum fluids could also be disseminated in all laboratories studying superfluids (4He and 3He). This would open new capabilities with the study of quantum fluids down to smaller scales, in confined geometries.
One of the output would be the study of exotic elementary excitations, like the Majorana states present in superfluid 3He.

The dissemination aspect, and the comparison of different technologies (e.g. microwave optomechanics versus SQUIDs, or NEMS in various fluids) cannot be performed in a single laboratory, and to do this we rely on a broad collaboration that exists in Europe: the EMP consortium (European Microkelvin Platform). This is a unique strength that could be built in Europe thanks to European funds. We rely on this networking in order to magnify the final impact on the community of the ULT-NEMS project.

Platform NEMS for elementary exitations in superfluid 3He
BF1 dry dilution unit with microwave cell
DN1 demag. cryostat with microwave wiring
Optomechanical NEMS for cooling experiment
3He ultra-low temperature thermometer cell
Comsol simulation of opto-mechanics NEMS
PCB in sample cell with optomechanics chip