Coherent spin control of a nanocavity-enhanced qubit in diamond (1409.1602v2)

Abstract: A central aim of quantum information processing is the efficient entanglement of multiple stationary quantum memories via photons. Among solid-state systems, the nitrogen-vacancy (NV) centre in diamond has emerged as an excellent optically addressable memory with second-scale electron spin coherence times. Recently, quantum entanglement and teleportation have been shown between two NV-memories, but scaling to larger networks requires more efficient spin-photon interfaces such as optical resonators. Here, we demonstrate such NV-nanocavity systems with optical quality factors approaching 10,000 and electron spin coherence times exceeding 200 $\mu$s using a silicon hard-mask fabrication process. This spin-photon interface is integrated with on-chip microwave striplines for coherent spin control, providing an efficient quantum memory for quantum networks.

Coherent Spin Control of a Nanocavity-Enhanced Qubit in Diamond

This paper presents significant advancements in the field of quantum information processing, particularly in the development of efficient spin-photon interfaces using NV-nanocavity systems. The researchers have achieved a high optical quality factor, approaching 10,000, and electron spin coherence times exceeding 200 µs, accomplished using a silicon hard-mask fabrication process. These advancements indicate strong economic potential for integrating coherent spin control with on-chip microwave striplines, thereby providing a highly efficient quantum memory suitable for scaling quantum networks.

The paper illustrates the interaction between photons and quantum states of an emitter within the strong Purcell regime. In this regime, the emitter interacts with a singular optical mode, achieving a Purcell factor greater than one ($F > 1$). The spectrally-resolved spontaneous emission rate enhancement by the Purcell factor is detailed, and it is shown that the NV zero-phonon line (ZPL) in diamond, when coupled to a cavity with optimized parameters, can enhance emission by a Purcell factor of up to 70.

NV-nanocavity structures were meticulously designed using finite-difference time-domain (FDTD) simulations, optimizing the ratio of quality factor $Q$ to the mode volume $V_\text{mode}$. The fabrication process involved high-purity single-crystal diamond, which was etched using silicon membranes as masks, resulting in uniform vertical sidewalls and low surface roughness. Notably, the fabrication process demonstrated a high yield (94%), with mean $Q$ of 6,200 and a maximum $Q$ of 9,900 ± 200.

Optical characterization revealed significant spectral properties when examining NV-nanocavity systems at ambient and cryogenic temperatures. System A demonstrated substantial photoluminescence enhancement by meticulously tuning cavity resonances to overlap with NV ZPL transitions. System B, with a higher density of NV centers and Q factor 3,300 ± 50, exhibited remarkable spontaneous emission enhancements, underscoring the potential for strong Purcell regime applications.

The coherent spin control aspect of the paper is particularly noteworthy. Using optically detected magnetic resonance (ODMR), the research demonstrated phase coherence times (T) similar to the parent CVD diamond crystal, confirming that the nanofabrication process preserves long spin coherence times. A coherence time exceeding 200 µs is more than two orders of magnitude beyond previous reports for cavity-coupled NV centers and solid-state qubits.

From a theoretical standpoint, the implications of this research extend to applications in quantum communications and computing. The strong Purcell regime and high overlap factor would facilitate an approximately 800-fold increase in entanglement generation rates compared to existing systems, greatly benefitting quantum teleportation and enabling large-scale quantum networks. Coupled NV systems can be easily integrated into circuits with microwave capabilities for controlling electron and nuclear spins, fostering advancements in quantum repeaters and quantum memories.

Conclusively, the development outlined in this paper offers promising directions for achieving coherent optical control in scalable quantum memory systems, spin-based microprocessors, and quantum networks. Future studies could explore increasing NV-nanocavity overlap probability and implementing near-surface NV strategies to further enhance coherent spin control capabilities.