Periodic Reporting for period 1 - cQMM (circuit Quantum Magneto-Mechanics : interfacing single molecular spins with nanomechanical resonators in the quantum regime.)
Reporting period: 2019-04-01 to 2021-03-31
The overall objective of this project has been to develop a nano-mechanical platform enabling the control and detection of molecular spin qubits. The approach is appealing because of the potentially strong spin-phonon interactions for this class of spin, in particular if using very light mechanical oscillators such as graphene membranes. The conclusion of the action is that high-frequency oscillators are a better choice for coupling to spins. In the case of graphene oscillators, it then becomes essential to control the mechanical stress.
During the project, the spin-phonon coupling mechanisms in molecular magnets was understood better theoretically. This lead us to develop a nano-mechanical platform working at GHz frequencies instead of 10-100 MHz. An additional advantage of this strategy is that, at low temperature, working with high frequency mechanical modes reduces thermal excitations and therefore decoherence (or the need for active cooling techniques). In order to interface such high frequency graphene modes, we decided to use acoustic waves instead of the originally envisioned cavity electromechanics setup. We successfully designed, fabricated and characterized several different acoustic transmission lines working around 5 GHz and 20 mK (often referred to as acoustic delay lines). The limiting factor to our progress has been the lack of control over the mechanical stress in our nano-mechanical membranes. Aside from this graphene platform, we have developed a protocol for mechanical oscillators coupled to qubits, allowing to stabilize sub-Poissonian states in the oscillator. This protocol was shown in several oral presentations (2 conferences and 2 invited seminars).
Even though we have not been able to demonstrate the coupling of molecular spin qubits with the phonons of our membranes, we have made significant progress on qubit-phonon interactions within a different platform. In this work, the qubit is carried by a superconducting circuit which behaves as a true two-level system (like a spin-1/2) and the phonons are modes of membrane made out of aluminium. Using microwave drives, we could prepare, stabilize and detect and non-classical state of motion. This state is characterized by an energy distribution that is squeezed (i.e. fluctuations around the mean value are reduced) below the classical bound given by the Poisson distribution. In a broader context, the demonstrated protocol could have implications in quantum technologies, as it could be utilized for instance to enhance the sensitivity of force detectors.