[Paper Review] Real-Time Charge Initialization of Diamond Nitrogen-Vacancy Centers for Enhanced Spin Readout

David A. Hopper, J. Lauigan+2 more|arXiv (Cornell University)|Jul 20, 2019

Diamond and Carbon-based Materials Research62 references42 citations

TL;DR

This paper presents a real-time feedback control method to deterministically initialize the charge state of diamond nitrogen-vacancy (NV) centers at room temperature, achieving a charge initialization fidelity of 99.4±0.1%. By integrating real-time charge initialization with spin-to-charge conversion readout, the method enables a factor-of-20 speedup in quantum sensing, resulting in an ac magnetic sensitivity of 1.3 nT/Hz¹² for a single NV center.

ABSTRACT

A common impediment to qubit performance is imperfect state initialization. In the case of the diamond nitrogen-vacancy (NV) center, the initialization fidelity is limited by fluctuations in the defect's charge state during optical pumping. Here, we use real-time control to deterministically initialize the NV center's charge state at room temperature. We demonstrate a maximum charge initialization fidelity of 99.4$\pm$0.1% and present a quantitative model of the initialization process that allows for systems-level optimization of the spin-readout signal-to-noise ratio. Even accounting for the overhead associated with the initialization sequence, increasing the charge initialization fidelity from the steady-state value of 75% near to unity allows for a factor-of-two speedup in experiments while maintaining the same signal-to-noise-ratio. In combination with high-fidelity readout based on spin-to-charge conversion, real-time initialization enables a factor-of-20 speedup over traditional methods, resulting in an ac magnetic sensitivity of 1.3 nT/Hz$^{1/2}$ for our single NV-center spin. The real-time control method is immediately beneficial for quantum sensing applications with NV centers as well as probing charge-dependent physics, and it will facilitate protocols for quantum feedback control over multi-qubit systems.

Motivation & Objective

• To overcome the limitation of probabilistic charge state initialization in NV centers, which degrades spin readout contrast and signal-to-noise ratio.
• To develop a real-time feedback control protocol that enables deterministic initialization of the NV− charge state at room temperature.
• To enhance spin readout efficiency and reduce measurement time in quantum sensing applications.
• To optimize the signal-to-noise ratio by enabling high-fidelity initialization into either NV− or NV0 states, or arbitrary intermediate distributions.
• To demonstrate a factor-of-20 speedup in quantum sensing by combining real-time initialization with spin-to-charge conversion readout.

Proposed method

• A real-time feedback loop uses an FPGA to monitor photon counts during a red laser charge probe, triggering exit from the initialization loop upon detecting a preset threshold.
• The system employs a 532 nm green laser for optical pumping and a 635 nm red laser for charge state readout, with timing controlled by an arbitrary waveform generator (AWG) and data acquisition (DAQ) system.
• A photon distribution model accounts for transitions between NV− and NV0 states, with rates γ−, γ0, ΓIon (ionization), and ΓRec (recombination), enabling quantitative optimization.
• The protocol uses a threshold-based detection scheme: higher thresholds increase fidelity by requiring more photons before exit, reducing error rates.
• The method is validated using Rabi oscillations and lifetime measurements, with spin polarization and decay rates extracted via joint fitting of transient fluorescence to a three-level spin model.
• A comprehensive model of the initialization process allows for systems-level optimization of initialization and readout parameters for arbitrary experiment durations.

Experimental results

Research questions

• RQ1 Can real-time feedback control achieve deterministic charge state initialization of NV centers at room temperature, overcoming the 75% steady-state fidelity limit?
• RQ2 How does increasing charge initialization fidelity affect the spin readout signal-to-noise ratio in both traditional photoluminescence and spin-to-charge conversion readout?
• RQ3 What is the maximum achievable charge initialization fidelity using real-time control, and how does it scale with probe power and threshold settings?
• RQ4 Can real-time charge initialization combined with spin-to-charge conversion enable a significant speedup in quantum sensing experiments?
• RQ5 What is the impact of initialization fidelity on the overall magnetic field sensitivity of a single NV center?

Key findings

• The real-time charge initialization (RTI) protocol achieves a maximum charge initialization fidelity of 99.4±0.1% by using a higher detection threshold (ν=2) and optimized probe parameters.
• The average time-to-initialize for 99.4% fidelity is 7 ms, with a corresponding initialization error rate of 0.6%, representing a 2.5-fold reduction in error compared to a threshold of 1.
• Even with overhead, increasing fidelity from 75% to near-unity enables a factor-of-two speedup in experiments while maintaining the same signal-to-noise ratio.
• Combining RTI with spin-to-charge conversion readout results in a factor-of-20 speedup over traditional methods, achieving an ac magnetic sensitivity of 1.3 nT/Hz¹².
• Lifetime measurements confirm higher spin polarization: 94.4±0.7% for RTI versus 91.5±0.7% for steady-state initialization, validating improved initialization quality.
• The signal-to-noise ratio is maximized when the initialization duration is optimized, with a 43 µs initialization time yielding a single-shot SNR of 0.4 and readout noise σR = 3.67.