Hybrid spin-nanomechanics with single spins in diamond mechanical oscillators

Abstract

Hybrid spin-oscillator systems, formed by single spins coupled to mechanical oscillators, have attracted ever-increasing attention over the past few years, triggered largely by the prospect of employing such devices as high-performance nanoscale sensors or transducers in multi-qubit networks. Provided the spin-oscillator coupling is strong and robust, such systems can even serve as test-beds for studying macroscopic objects in the quantum regime. In this thesis we present a novel hybrid spin-oscillator system that consists of a diamond cantilever whose mechanical motion couples to the spin degree of freedom of embedded NV centers through crystal strain. This thesis starts with a characterization of the coupling strength between NV spin and resonator motion. Static cantilever bending experiments reveal spin-strain coupling constants of several GHz per unit of strain, corresponding to a single phonon coupling strength $g_0 \approx$\,Hz. Although we demonstrate that our hybrid system resides deep in the resolved sideband regime, our current experimental conditions prevent bringing the diamond resonator to its motional ground state, since spin decoherence rate and mechanical heating rate exceed $g_0$ by several orders of magnitude. However, cooling the resonator, even to its motional ground state, is possible if cantilever dimensions are reduced to the nanometer scale and corresponding experiments are performed at cryogenic temperatures. While spin-strain coupling is not favorable for such experiments in the quantum regime, it offers many other exciting features. In the second part of this thesis, we report on the implementation of a novel continuous decoupling scheme that protects the NV spin from environmental noise, increasing both Rabi oscillation decay time and inhomogeneous coherence time by two orders of magnitude. The remarkable coherence protection is explained by the robust, drift-free strain-coupling mechanism and the narrow linewidth of the high-quality diamond mechanical oscillators. A major finding of this thesis is the demonstration of coherent spin manipulation with transverse AC strain fields, which is presented in the third part of this thesis. We show that AC strain driving not only addresses a magnetic dipole forbidden transition, but also allows working in the strong driving regime, in which the induced spin rotation frequency exceeds the initial spin splitting. Few systems have reached this regime, despite the appeal of studying dynamics beyond the rotating wave approximation. Additionally, continuous strain driving enhances the NV’s spin coherence time by decoupling it from environmental magnetic noise. In the last part of this thesis, we combine coherent MW and strain spin driving to realize a three-level $\nabla$-system in the NV ground state by coherently addressing all three spin transitions. Our studies of the spin dynamics not only confirm the theoretical prediction that the global phase (i.e.\,the relative phase of the three driving fields) governs the occurring spin dynamics, but also that closed-contour driving shields the NV's spin from environmental noise without applying complicated decoupling schemes. The corresponding decoupling mechanism is well explained by the effect of noise on the $\nabla$-system Hamiltonian. Based on our findings, we believe our closed-contour interaction scheme will have future applications in sensing and quantum information processing, for example as a phase sensor or as a test-bed for state transfer protocols.