Quantum Optomechanics
Interfacing spins and mechanical degrees of freedom allows for a variety of applications and experimental observations. For example, one can deterministically entangle pairs of spins through their coherent coupling with the dynamics of a resonator, even for large spin-spin distance separations and thermal resonator states. Additionally, the resonator could be cooled close to the quantum ground state by bringing a strongly coupled bath of spins into resonance, introducing the possibility of single phonon experiments and quantum state preparation of a mesoscopic object. In our lab, we are pursuing strong, coherent coupling of Nitrogen Vacancy (NV) center spin qubits in diamond, to mechanical resonators via a magnetic field gradient. We approach this goal with two different mechanical resonator setups:
- One of our setups consists of silicon nitride nanobeams with a magnet placed at the center. These resonators can be fabricated in a scalable way with tens of resonators per chip. Frequencies are typically ~1 MHz with quality factors of ~10^6. We are currently attempting to measure its AC magnetic field using a diamond nanopillar. These nanopillars have a geometry that allows us to bring an NV arbitrarily close to the magnet. This gives us the potential for strong coupling.
- Our other mechanical resonator setup consists of a ~10 um ferromagnetic microsphere in a microfabricated pocket levitated over a type-II superconductor (YBCO). The flux-pinning from the YBCO traps the magnet in three dimensions (center-of-mass modes) as well as its dipole orientation (librational modes). These center-of-mass modes have been shown to reach the kHz regime. Additionally, quality factors of more than 10^6 have been demonstrated. We couple these center-of-mass modes to an NV by placing a bulk diamond overhead. Our first measurement of the coupling yielded 0.048(2) Hz. We are currently working on the next generation where we remove the need for a pocket to allow for higher couplings. Read more about our work in Physical Review Letters: Phys. Rev. Lett. 124, 163604 (2020).