Robust All-Optical Single-Shot Readout of Nitrogen-Vacancy Centers in Diamond

This work describes a new single-shot readout scheme for nitrogen-vacancy (NV) centers in diamond, which is even applicable when photon collection is poor (<1000 clicks per second), and for shallow implanted NV centers.
NV centers are a prosperous quantum system because of their potential for applications and the simplicity of using them. NV centers allow for groundbreaking applications in the near future, foremost for quantum sensing. A prominent example is nano-scale magnetic resonance imaging (MRI) with chemical contrast and single-atom resolution. Other innovative applications exist already such as nano-scale magnetic imaging.
The simplicity of using NV centers derives from the fact that their spin state can be initialized and read out optically—the NV center’s fluorescence intensity correlates with its spin state. While this readout is technically easy to implement, it suffers from low signal-to-noise ratio (SNR). To achieve an SNR > 1 typically hundreds to thousands of experimental repetitions are necessary.
However, for many applications in quantum sensing, readout within a single experimental repetition (“single-shot readout”) is beneficial, or even required for quantum computing. So far, there are two single-shot methods: readout by resonant excitation and nuclear assisted readout. Yet these methods require highly engineered systems like nano-fabricated photonic structures around the NV, aligned magnetic fields and/or deep natural NV centers in low-strain environments, a category of NV centers which is not suitable for most quantum sensing applications. These drawbacks render existing single-shot readout methods impractical for some promising sensing applications like nano-MRI.
Spin-to-charge conversion (SCC) is another approach to improve the readout fidelity. The idea is to spin-selectively ionize the NV– center and subsequent read out the charge state. This approach can enhance the SNR, although not beyond a value of 1.
Here we present an all-optical robust single-shot readout method, which delivers a single-shot SNR above 1 even in less demanding experimental conditions and if photon collection is poor (<1000 clicks/second).
We demonstrate the protocol on a deep natural NV center below a planar surface without the need for aligned magnetic field. In addition, we demonstrate a single-shot SNR of 1.29 for a shallow implanted NV center in a high-strain environment (>10 GHz), with misaligned magnetic field and without any photonic structures.
The readout method is implemented by combining resonant excitation at low temperature and spin-to-charge conversation. Resonant excitation allows to excite an NV– center highly spin-selectively. By applying a second high-power laser, the NV– center will be ionized out of the excited state. Together, these two lasers implement a deterministic two-photon ionization protocol, which converts the fragile spin states into stable charge states.
The charge state is stable on a timescale of seconds under illumination and is read out by applying the resonant laser plus continuous wave (cw) microwave (MW). This readout method provides a higher SNR than the common optical charge readout with an orange laser.
For high-purity spin preparation (>90 %), we use resonant optical pumping on the same transition alternated with population inversion by a microwave pulse. Spin-purity and spin life-time under optical illumination are characterized and fitted with a transfer matrix model.
The low-temperature SCC readout method is estimated to work for almost any magnetic field and strain value as indicated by group theoretical calculations of the optical transitions according to Doherty et al..
A side-project examines the influence of high-power laser illumination onto the spin-splitting within the NV– center optical ground state (light shift).