Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - ULT-NEMS (Ultra-Cold Nano-Mechanics: from Classical to Quantum Complexity)

Teaser

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...

Summary

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.

Work performed

The starting date of the project has been delayed for technical reasons; first the purchase of a new cryostat, which was not foreseen at the date of the writing of the proposal, took some time on the administrative side. Second, the laboratory needed some refreshing (redoing electricity, etc…) that implied to move equipment out of the rooms, and took time.
As we eventually started the activity, the first thing to do was to set up new experiments dedicated to this new physics, and to proceed with preliminary tests.
The ULT group possesses the last functional Nuclear Demagnetization cryostat in France (called DN1). Even though the design of the machine is 20 years old, it is an extremely good cryostat, able to reach 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. 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, 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.

Final results

During this reporting period we have been performing some preliminary experiments touching essentially WP2 and WP3. Two main issues for these have to be addressed: how does a NEMS device behave in presence of a quantum fluid, and second how cold can we “brute force” cool a NEMS?
For the first question, and some related aspects of Brownian noise in NEMS, papers are published or being submitted/written. 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. We do plan to cool down much lower with the demagnetization scheme, keeping the whole object (all its degrees of freedom, plus the outside world coupled to it) in thermal equilibrium. This is essential for the fundamental studies we plan to do.
Thus, the first demanding step was to mount an 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.
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.