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

Periodic Reporting for period 4 - ULT-NEMS (Ultra-Cold Nano-Mechanics: from Classical to Quantum Complexity)

Reporting period: 2020-05-01 to 2021-04-30

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. 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 (final aim of QM axis; studying basic aspects of Quantum Mechanics in true thermal equilibrium)

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 fully demonstrated so far, in 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.
Besides, quantum transport of heat is underlying ultra-cold experiments with phononic modes, adressing in a unique way quantum thermodynamics issues.
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.

ULT-NEMS reported on materials properties in NEMS devices down to 500 µK. The physics of two-level-systems has been discussed in this framework. Ground-state cooling has been achieved for a macroscopic device with a first flexural mode at 15 MHz using only passive cryogenics. This breakthrough opens the path for future experiments in a completely new area of experimental research, with a quantum-mechanical object in-equilibrium with its environment. Unique results on the fundamental mode population fluctuations have been reported, down to 500 µK.
New promising probes for quantum fluids have been developed, with cross-dimensions of the order of the 3He superfluid coherence length: about 50 nm. Their first application to superfluid 4He has been reported during the project.
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).
The lowest temperature achievable is one of the frontiers of physics. Cooling down passively to its quantum ground state a whole mechanical macroscopic object had never been achieved before; the technique in use was active cooling, which cools a single mode, leaving it very damped and out-of-equilibrium (the environment remains hot).

Knowing if we can "brute force" cool a NEMS device to sub-milliKelvin temperatures was in itself a great challenge, which opens a new field for fundamental science: quantum thermodynamics with phonons, tests of collapse models and quantum gravity.
The setup we have built during ULT-NEMS is unique up to now, and demonstrated the capabilities of the combination microkelvin platform/microwave detection.

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 will also be disseminated in laboratories studying superfluids (4He and 3He). This will open new capabilities with the study of quantum fluids down to smaller scales.
One of the output would be the study of exotic elementary excitations, like the Majorana states present 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
PCB in sample cell with optomechanics chip