Optimal photon energies for initialization of hybrid spin quantum registers of nitrogen-vacancy centers in diamond

At a Glance

Metadata	Details
Publication Date	2020-01-30
Journal	Physical review. A/Physical review, A
Authors	‎K‎. ‎R‎. ‎K‎. ‎Rao, Yihua Wang, Jingfu Zhang, Dieter Suter
Institutions	Bennett University, TU Dortmund University
Citations	6
Analysis	Full AI Review Included

Optimal Photon Energies for NV Center Quantum Registers: A 6CCVD Technical Analysis

This document analyzes the research detailing the optimization of NV center initialization using specific laser wavelengths, focusing on the implications for material requirements and how 6CCVD’s specialized MPCVD diamond products can enable and advance this critical quantum technology.

Executive Summary

The research demonstrates a significant advancement in initializing hybrid spin quantum registers based on Nitrogen-Vacancy (NV) centers in diamond by optimizing the optical repolarization step.

Problem Solved: Standard green laser excitation (532 nm) causes unwanted ¹⁴N nuclear spin depolarization during the electron spin repolarization cycle, limiting overall quantum register fidelity.
Optimal Solution: Utilizing orange laser light (594 nm) for the repolarization pulse significantly inhibits the photo-induced ionization of the NV center (NV^- <—> NV⁰), which is a primary source of noise.
Performance Gain: The optimized protocol achieved a high ¹⁴N nuclear spin polarization of 89.0 (±2.7) % using 594 nm illumination, a substantial improvement over the 76.3 (±1.9) % achieved with conventional 532 nm illumination.
Material Requirement: Achieving these high polarization levels relies fundamentally on ultra-high purity, isotopically enriched diamond (99.99% ¹²C, Nitrogen concentration < 5 ppb).
6CCVD Value Proposition: 6CCVD specializes in providing the necessary high-purity Single Crystal Diamond (SCD) substrates and thin films, customized for isotopic enrichment and low nitrogen content, essential for high-fidelity quantum applications.

Technical Specifications

The following hard data points were extracted from the experimental results, highlighting the performance difference between the two illumination wavelengths.

Parameter	Value	Unit	Context
Maximum ¹⁴N Nuclear Spin Polarization (594 nm)	89.0 (±2.7)	%	Achieved after 4 cycles of transfer/repolarization.
Maximum ¹⁴N Nuclear Spin Polarization (532 nm)	76.3 (±1.9)	%	Achieved after 3 cycles of transfer/repolarization.
Optimal Repolarization Wavelength	594	nm	Orange light excitation, minimizes NV ionization noise.
Standard Excitation Wavelengths	532, 520	nm	Green light excitation, used for initial polarization.
Nuclear Spin Depolarization Time Constant (594 nm)	16.6 (±4.8)	µs	Significantly slower depolarization rate.
Nuclear Spin Depolarization Time Constant (532 nm)	8.4 (±2.5)	µs	Faster depolarization rate.
Electron Spin Polarization Time Constant (594 nm)	110 (±22)	ns	Similar rate to 532 nm (101 ± 16 ns).
Static Magnetic Field (B)	2.8	mT	Applied along the NV axis.
Material Purity (¹²C Enrichment)	99.99	%	Isotopic purity required for long coherence times.
Nitrogen Concentration	< 5	ppb	Ultra-low concentration required for isolated NV centers.

Key Methodologies

The experiment utilized a hybrid optical-microwave-radiofrequency (MW/RF) pulse sequence on a single NV center to achieve high nuclear spin polarization.

Material Selection: Used a bulk diamond sample characterized by ultra-high isotopic purity (99.99% ¹²C) and extremely low native nitrogen concentration (< 5 ppb) to ensure long spin coherence times.
Initial Electron Spin Polarization: A 4 µs laser pulse (520 nm or 532 nm) was applied to initialize the NV center into the NV^-, m_s = 0 state.
Polarization Transfer: Electron spin polarization was transferred to the ¹⁴N nuclear spin using a sequence of transition-selective MW π pulses (duration 1 µs) followed by RF π pulses (duration 62 µs).
Electron Spin Repolarization (Critical Step): A second laser pulse was applied to repolarize the electron spin, which was left in a mixed state after transfer.
- Optimization: The repolarizing pulse wavelength was varied (520 nm, 532 nm, or 594 nm). The 594 nm pulse duration was 700 ns (532 nm was 500 ns).
- Mechanism: 594 nm light excites NV^- but significantly inhibits the unwanted ionization into NV⁰, thereby suppressing nuclear spin depolarization.
Measurement: Electron spin Free-Induction Decay (FID) was measured using a Ramsey sequence (π/2 - τ - π/2) followed by a 400 ns readout laser pulse. The resulting spectral line amplitudes were used to calculate the nuclear spin polarization (p).
Iteration: The transfer-repolarization cycle was repeated (N cycles) to maximize the final nuclear spin polarization.

6CCVD Solutions & Capabilities

This research confirms the critical role of high-quality diamond material in achieving high-fidelity quantum initialization. 6CCVD is uniquely positioned to supply the necessary custom diamond substrates and films required to replicate, scale, and integrate this optimized protocol into functional quantum devices.

Applicable Materials

To replicate or extend this research, engineers require diamond with exceptional purity and controlled defect density. 6CCVD recommends the following materials:

Optical Grade SCD (Single Crystal Diamond): Required for high-fidelity quantum registers.
- Purity: Ultra-low native nitrogen concentration (< 5 ppb) to minimize background noise and ensure isolated NV centers.
- Isotopic Control: Available with high ¹²C enrichment (e.g., 99.99%) to maximize spin coherence times (T₂* and T₂).
Thin Film SCD: For integrated quantum photonics and sensing applications, 6CCVD provides SCD films from 0.1 µm up to 500 µm thick, allowing for precise NV layer placement and integration into waveguides or resonators.

Customization Potential

The implementation of this optimized protocol in a practical device requires precise material engineering and integration capabilities, all available in-house at 6CCVD:

Requirement from Research/Application	6CCVD Custom Capability	Technical Specification
High-Purity Substrates	SCD Growth & Isotopic Control	¹²C enrichment up to 99.99% available.
Integrated Devices	Custom Dimensions & Thickness	Plates/wafers up to 125mm (PCD); SCD thickness up to 500 µm.
MW/RF Delivery	Custom Metalization Services	Deposition of Ti/Pt/Au, Pd, W, or Cu electrodes for on-chip microwave delivery.
Optical Interface Quality	Ultra-Precision Polishing	Surface roughness (Ra) of < 1 nm for SCD, ensuring minimal scattering loss for 594 nm and 532 nm excitation.
Device Integration	Laser Cutting & Shaping	Custom geometries and precise dicing for integration into cryogenic or vacuum systems.

Engineering Support

The successful implementation of hybrid spin quantum registers depends on balancing material purity, defect creation, and surface preparation. 6CCVD’s in-house PhD team offers specialized consultation for projects involving:

NV Center Creation: Advising on optimal implantation energy and annealing protocols to achieve desired NV depth and density in low-N SCD.
Material Selection: Guiding the choice between bulk SCD, thin film SCD, or Boron-Doped Diamond (BDD) for specific applications like quantum sensing or integrated photonics.
Interface Optimization: Assisting with metalization stack design to ensure low-loss MW/RF transmission and robust adhesion on diamond surfaces.

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

View Original Abstract

Initializing quantum registers with high fidelity is a fundamental precondition for many applications like quantum information processing and sensing. The electronic and nuclear spins of a Nitrogen-Vacancy (NV) center in diamond form an interesting hybrid quantum register that can be initialized by a combination of laser, microwave, and radio-frequency pulses. However, the laser illumination, which is necessary for achieving electron spin polarization, also has the unwanted side-effect of depolarizing the nuclear spin. Here, we study how the depolarization dynamics of the $^{14}$N nuclear spin depends on the laser wavelength. We show experimentally that excitation with an orange laser (594 nm) causes significantly less nuclear spin depolarization compared to the green laser (532 nm) typically used for excitation and hence leads to higher nuclear spin polarization. This could be because orange light excitation inhibits ionization of NV$^{0}$ into NV$^{-}$ and therefore suppresses one source of noise acting on the nuclear spin.

Tech Support

Original Source

DOI: https://doi.org/10.1103/physreva.101.013835