Coupling NVs to mechanical resonators

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.

Scanning electron microscope image of silicon nitride nanobeam with iron magnet evaporated in the middle. Typical dimensions: 145 um x 1 um x 150 nm.

  • 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 LettersPhys. Rev. Lett. 124, 163604 (2020).

Top-down view of micromagnet in gold-coated pocket (~ 200 um x 200 um). Magnet is initially stuck to pocket but "pops off" to levitate as superconductor is raised.