Hybrid Quantum Systems With Nitrogen Vacancy Centers and Mechanical Resonators

Abstract

Hybrid quantum systems involving coupled mechanical and spin degrees of freedom are promising candidates for applications in quantum metrology and quantum information processing. Specific examples range from sensitive magnetic field measurements to preparation of non-classical states of macroscopic objects and quantum transducers for mediating long range interactions between quantum bits. Nitrogen-vacancy (NV) centers in diamond represent a particularly promising spin system for these application. They feature long coherence times, well developed control methods, and a possibility of magnetic coupling to mechanical systems. However, achieving strong coupling between spin and mechanical degrees of freedom is challenging, as it requires a combination of large resonator zero point motion, magnetic field gradient, and mechanical quality factor within the same setting. This thesis presents two approaches for magnetic coupling between individual NV centers and mechanical oscillators. In the first approach, we demonstrate progress towards a high-cooperativity system with magnetically functionalized, doubly clamped silicon nitride resonators. We engineer high quality factor (Q > 10^5) resonators with large magnetic field gradients, and show how NVs can be integrated with this platform. Prospects for ground state cooling and quantum gate operations mediated by a mechanical bus are discussed. In the second approach, single micromagnets are trapped using a type-II superconductor nearby to spin qubits, enabling direct magnetic coupling between the two systems. Controlling the distance between the magnet and the superconductor during cooldown, we demonstrate three-dimensional trapping with quality factors above 1 x 10^6 and kHz trapping frequencies. The large magnetic moment to mass ratio of this mechanical oscillator is further exploited to couple its motion to the spin degrees of freedom of an individual NV center, and the resulting coupling is measured. This represents a new platform for ultrasensitive metrology, testing quantum mechanics with mesoscopic objects, and realizing quantum networks.