Efficient Readout of a Single Spin State in Diamond via Spin-to-Charge Conversion

At a Glance

Metadata	Details
Publication Date	2015-03-31
Journal	Physical Review Letters
Authors	Brendan Shields, Quirin Unterreithmeier, Nathalie P. de Leon, Hongkun Park, Mikhail D. Lukin
Institutions	Harvard University
Citations	224
Analysis	Full AI Review Included

Technical Documentation and Analysis: Efficient Spin-to-Charge Conversion in MPCVD Diamond

Documentation generated by 6CCVD engineering team, specializing in MPCVD Single Crystal (SCD) and Polycrystalline (PCD) Diamond solutions for quantum technologies.

Executive Summary

This research demonstrates a highly efficient, single-shot spin readout technique for Nitrogen-Vacancy ($\text{NV}^-$) centers in diamond at room temperature, crucial for scalable quantum computing and advanced magnetometry.

Core Mechanism: Achieves spin readout via a Spin-to-Charge Conversion (SCC) process, mapping the $\text{NV}^-$ electron spin state ($m_s = 0$ vs. $m_s = \pm 1$) to a distinct charge state distribution ($\text{NV}^-$ vs. $\text{NV}^0$).
Performance Breakthrough: The SCC method drastically reduces spin readout noise to a minimum factor of 2.76 ± 0.09 times the fundamental spin projection noise level.
Nanofabrication Requirement: Efficient photon collection is enabled by highly precise, subwavelength-dimension nanobeams (300 nm triangular cross-section) fabricated from Type IIa Chemical Vapor Deposition (CVD) diamond.
Operational Environment: Readout is performed reliably at room temperature, overcoming limitations typically requiring cryogenic conditions.
Magnetometry Potential: The achieved noise reduction translates directly to enhanced sensor performance, projecting a sensitivity limit of 900 pT/Hz^1/2 for $\text{NV}^-$ centers in high-purity, isotopically engineered diamond.
SCC Pulse Sequence: SCC relies on a two-step optical sequence: 594 nm excitation for spin-dependent shelving into the singlet state manifold, followed by intense 638 nm ionization specific to the triplet manifold.

Technical Specifications

The following key parameters and results were extracted from the study:

Parameter	Value	Unit	Context
Diamond Material	Type IIa CVD	-	Element6 Source Material
Nitrogen Concentration	1	ppm	NV precursor defect concentration
Nanobeam Cross-Section	300	nm	Width of triangular waveguide
Measured $T_2$ Coherence	201 ± 7	µs	Hahn echo measurement in nanobeam
Projected $T_2$ Coherence	2	ms	Target for ¹²C isotopically pure diamond
Readout Noise ($O_R^{SCC}$) Limit	2.76 ± 0.09	-	Times the Spin Projection Noise Level
Initial Spin Polarization ($p_0(0)$)	0.92 ± 0.01	-	Fraction of population in $m_s = 0$ state
Charge Readout Fidelity ($F_c$)	0.975 ± 0.007	-	Conditioned on detection of $\ge 1$ photon
Minimum Readout Time ($t_R$)	10	µs	Achieved while maintaining $F_c \sim 0.9$
Shelving Pulse Wavelength	594	nm	Drives $\text{NV}^-$ to triplet excited state
Ionization Pulse Wavelength	638	nm	Ionizes $\text{NV}^-$ triplet to $\text{NV}^0$
Microwave Field Frequency	2.917	GHz	Used for spin sublevel manipulation
Magnetometry Sensitivity (Measured $T_2$)	4	nT/Hz^1/2	For 200 µs $T_2$ nanobeam
Magnetometry Sensitivity (Projected $T_2$)	900	pT/Hz^1/2	For 2 ms $T_2$ in ¹²C diamond

Key Methodologies

The core of the experiment relies on precision material control and a complex, two-pulse optical sequence for spin state conversion.

Material Growth and Specification:
- Utilized bulk Type IIa MPCVD diamond wafers with controlled Nitrogen concentration (1 ppm N).
Device Fabrication (Angled RIE):
- Diamond material was etched using an Angled Reactive Ion Etching (RIE) technique to form suspended nanobeams (20 µm length).
- Nanobeams featured a triangular cross-section (300 nm wide) to facilitate waveguiding and enhance photon collection efficiency, yielding a maximum CW fluorescence count rate of 0.945 Mcps (532 nm).
Charge State Initialization ($\text{NV}^-$):
- A high-power 532 nm pump pulse (150 ns @ 300 µW) followed by a 594 nm probe pulse was used to rapidly prepare the $\text{NV}^-$ charge state with high fidelity ($F_c > 0.97$).
Spin Preparation (Microwave Control):
- Electron spin sublevels ($m_s = 0$ or $m_s = \pm 1$) were coherently manipulated using a 2.917 GHz microwave field delivered via an adjacent copper wire.
Spin-to-Charge Conversion (SCC) Sequence Optimization:
- Shelving Pulse: Short, intense 594 nm pulse (145 µW) applied for $t_{shelf}$ (optimized to 60 ns). This selectively routes the $m_s = \pm 1$ population to the protected singlet manifold.
- Ionization Pulse: Short, high-power 638 nm pulse (22.5 mW) applied for $t_{ion}$ (optimized to 20 ns). This rapidly ionizes any population remaining in the exposed triplet manifold ($m_s = 0$) to $\text{NV}^0$.
Single-Shot Readout:
- The resulting charge distribution ($\text{NV}^-$ vs. $\text{NV}^0$) was measured using low-power 594 nm illumination coupled with a 655 nm longpass filter, exploiting the 20-30x contrast difference between the charge states.

6CCVD Solutions & Capabilities: Enabling Next-Generation Quantum Sensing

This research underscores the critical dependence of high-fidelity NV spin readout on two factors: material purity (controlling defect density for $T_2$ maximization) and nanoscale device engineering (for enhanced photon collection). 6CCVD is uniquely positioned to supply the advanced MPCVD diamond substrates required to replicate and extend these high-performance quantum demonstrations.

Applicable Materials

To achieve the projected 900 pT/Hz^1/2 sensitivity, researchers require diamond with maximal coherence time ($T_2$), mandating extremely low defect and impurity concentrations.

Research Requirement	6CCVD Material Solution	Optimization Feature
Enhanced Coherence ($T_2 > 2$ ms)	Optical Grade SCD (Isotopically Pure)	Ultra-low ¹³C content for nuclear spin noise reduction. Essential for advanced magnetometry projects.
Nanofabrication Substrate	MPCVD SCD Thin Film Plates	Customized thinness (0.1 µm - 500 µm) for optimal RIE etching and nanobeam creation. Superior crystalline quality (Ra < 1nm).
Current Research Replication	Standard SCD (Controlled N Doping)	Available with specific, controlled 1 ppm Nitrogen doping levels to match the material used in this experiment.

Customization Potential

The experimental success hinges on the precise geometry of the 300 nm nanobeams. 6CCVD’s advanced engineering services directly support the necessary fabrication steps:

Custom Dimensions: We supply SCD or PCD wafers up to 125 mm, providing ample substrate area for large-scale production of nanophotonic devices using RIE or similar techniques.
Precision Thickness Control: We offer growth and polishing services for substrates and thin films (SCD or PCD) ranging from 0.1 µm to 10 mm, ensuring the ideal starting material thickness required for high-yield nanodevice manufacturing.
Integrated Metalization Services: While this study used an external copper wire for MW delivery, future integrated quantum devices will require on-chip metal layers. 6CCVD offers in-house deposition of standard metals (Au, Pt, Pd, Ti, W, Cu) for complex microwave and electrical structures directly on the diamond surface.
Polishing Excellence: Our SCD surfaces are polished to Ra < 1 nm, providing a pristine, low-loss interface essential for subsequent high-resolution e-beam lithography and RIE etching used in nanobeam fabrication.

Engineering Support

The Spin-to-Charge Conversion method introduces complex dynamics involving photoionization and intersystem crossing rates ($g_0, g_1, \gamma_0, \gamma_1$). 6CCVD’s in-house PhD team provides authoritative support for projects focused on NV-based Quantum Sensing, Magnetometry, and Photon Collection Enhancement.

We assist clients in selecting the optimal MPCVD substrate specification (e.g., controlling N concentration and ¹³C isotopic purity) necessary to maximize the coherence time ($T_2$) and ultimately improve the ultimate magnetic sensitivity projected by this research (900 pT/Hz^1/2).

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Efficient readout of individual electronic spins associated with atomlike impurities in the solid state is essential for applications in quantum information processing and quantum metrology. We demonstrate a new method for efficient spin readout of nitrogen-vacancy (NV) centers in diamond. The method is based on conversion of the electronic spin state of the NV to a charge-state distribution, followed by single-shot readout of the charge state. Conversion is achieved through a spin-dependent photoionization process in diamond at room temperature. Using NVs in nanofabricated diamond beams, we demonstrate that the resulting spin readout noise is within a factor of 3 of the spin projection noise level. Applications of this technique for nanoscale magnetic sensing are discussed.

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevlett.114.136402