Controlling the coherence of a diamond spin qubit through strain engineering (1706.03881v2)

Abstract: The uncontrolled interaction of a quantum system with its environment is detrimental for quantum coherence. In the context of solid-state qubits, techniques to mitigate the impact of fluctuating electric and magnetic fields from the environment are well-developed. In contrast, suppression of decoherence from thermal lattice vibrations is typically achieved only by lowering the temperature of operation. Here, we use a nano-electro-mechanical system (NEMS) to mitigate the effect of thermal phonons on a solid-state quantum emitter without changing the system temperature. We study the silicon-vacancy (SiV) colour centre in diamond which has optical and spin transitions that are highly sensitive to phonons. First, we show that its electronic orbitals are highly susceptible to local strain, leading to its high sensitivity to phonons. By controlling the strain environment, we manipulate the electronic levels of the emitter to probe, control, and eventually, suppress its interaction with the thermal phonon bath. Strain control allows for both an impressive range of optical tunability and significantly improved spin coherence. Finally, our findings indicate that it may be possible to achieve strong coupling between the SiV spin and single phonons, which can lead to the realisation of phonon-mediated quantum gates and nonlinear quantum phononics.

Strain Engineering to Enhance Spin Coherence in Silicon-Vacancy Centres

This paper presents a significant advancement in the field of quantum coherence control by demonstrating a novel approach to mitigate the detrimental effects of phonon interactions on the coherence of a single electronic spin qubit in silicon-vacancy (SiV) centers in diamond. The paper effectively utilizes strain engineering through a nano-electro-mechanical system (NEMS) to manipulate the local strain environment of the SiV centers without altering the system temperature.

Key Findings and Methodology

The authors highlight the susceptibility of the electronic orbitals of SiV centers to local strain, which inherently causes high sensitivity to phonons, leading to decoherence. This susceptibility has been leveraged by using a NEMS device, which comprises a single-crystal diamond cantilever with metal electrodes, to create and control static strain on the SiV centres. This method allows significant optical tunability and enhanced spin coherence by influencing the electronic levels of the emitter to suppress its thermal phonon interactions.

The research experimentally shows that adjusting the strain in the SiV centers can alter their ground state (GS) and excited state (ES) orbital splittings, enabling researchers to probe the phonon density of states and their effect on phonon-mediated spin relaxation. Through controlled strain application, the authors achieve an orbital splitting increase up to 1.2 THz, markedly improving the SiV's spin coherence time from 40 nanoseconds to 0.25 microseconds.

Numerical Insights

Quantitative measures indicate that by controlling the orbital splitting, the number of acoustic phonon modes coupled with the GS is influenced, resulting in accelerated phonon processes at low frequencies but suppressed phonon absorption at high orbital splittings (>120 GHz at 4K). This phenomenon correlates with reduced phonon occupation at elevated splittings, contributing significantly to enhanced spin coherence.

The research also translates into practical applications, such as optical access to the SiV system via resonance laser excitation. This excitation forms coherent superpositions in spin qubits with significantly narrowed linewidths as strain increases, showcasing potential utility for quantum memory applications akin to NV centers. The longest reported T2 was on par with NV centers without dynamical decoupling strategies, emphasizing the method's effectiveness.

Implications and Future Directions

This work not only demonstrates a robust approach to enhance quantum coherence in solid-state systems but proposes a pathway towards scalable quantum technologies using diamond as a substrate. The authors suggest that similar strain engineering methods can be applied to other emitters or spin-orbit coupled systems. Moreover, this method could eventually support photonic quantum networks by enabling indistinguishable photon generation.

Future research avenues include leveraging dynamical decoupling techniques to further mitigate environmental noise, utilizing phonons as quantum resources via coherent coupling with mechanical modes, and realizing phonon-mediated two-qubit gates. These approaches may significantly influence the development of multifunctional quantum systems entailing phonon-based quantum states as intermediaries or interfaces with existing quantum platforms like superconducting qubits.

Overall, the research presents a compelling case for employing strain engineering in optimizing quantum coherence of solid-state emitters, highlighting its potential to bring forth realizable advances in constructing scalable quantum devices.