Coherence of a charge stabilised tin-vacancy spin in diamond

At a Glance

Metadata	Details
Publication Date	2022-04-28
Journal	npj Quantum Information
Authors	Johannes Görlitz, Dennis Herrmann, Philipp Fuchs, Takayuki Iwasaki, Takashi Taniguchi
Institutions	National Institute for Materials Science, Saarland University
Citations	56
Analysis	Full AI Review Included

Technical Documentation & Analysis: Charge Stabilized SnV Centers in Diamond

Executive Summary

This research demonstrates a robust protocol for stabilizing the critical negative charge state (SnV^-) of tin-vacancy centers in diamond, overcoming a major limitation for their use as solid-state spin qubits in Quantum Information Processing (QIP).

Charge Stabilization Breakthrough: A single-photon charge cycle model is proposed and exploited using a 445 nm (blue) laser to efficiently and rapidly stabilize the desired SnV^- charge state.
High Efficiency & Speed: The protocol achieves a charge initialisation efficiency of 91(1)% (up to 97% on secondary emitters) with rapid initialisation times of approximately 10 µs.
Superior Coherence: The charge-stabilized SnV^- center exhibits a ground state spin dephasing time (T₂) of 5(1) µs at 1.7 K, confirming its suitability for coherent QIP protocols.
Long-Term Stability: The optical transitions maintain exceptional long-term stability, showing spectral shifts of less than 4(2) MHz over one hour, significantly below the lifetime-limited linewidth.
Qubit Readout Demonstrated: Proof-of-principle single-shot spin state readout achieved a fidelity of 74%, even when the magnetic field was misaligned relative to the defect axis.
Material Requirement: The success relies on high-purity, electronic-grade Single Crystal Diamond (SCD) substrates, which 6CCVD specializes in manufacturing via MPCVD.

Technical Specifications

The following critical performance metrics and experimental parameters were achieved using charge-stabilized SnV^- centers in diamond:

Parameter	Value	Unit	Context
Target Defect	SnV^-	N/A	Negatively charged Tin-Vacancy center
Spin Dephasing Time (T₂)	5(1)	µs	Ground state coherence at 1.7 K
Spin Lifetime (T₁)	> 20	ms	Ground state lifetime at 1.7 K, 200 mT
Charge Initialisation Efficiency	91(1)	%	Using 445 nm laser pulse
Charge Initialisation Time	~10	µs	Rapid initialization demonstrated
Single Shot Readout Fidelity	74	%	Achieved without aligned magnetic field
Spectral Stability (Low Power)	< 4(2)	MHz	Standard deviation of line center over 1 hour
Operating Temperature	1.7	K	Closed-cycle helium cryostat
Charge Repump Wavelength	445	nm	Optimized for charge stabilization
Sn Ion Implantation Energy	700	keV	Defect creation process
HPHT Annealing Temperature	2100	°C	Post-implantation damage reduction

Key Methodologies

The successful creation and stabilization of the SnV^- centers relied on precise material preparation and advanced optical techniques:

Substrate Selection: Electronic-grade bulk diamond (001) orientation was used, specified with ultra-low impurity levels (< 5 ppb N, < 1 ppb B).
Ion Implantation: Tin (Sn) ions were implanted at an energy of 700 keV with a fluence of 8x10¹³ ions/cm² to introduce the necessary vacancies and Sn atoms.
High-Temperature Annealing: Samples underwent High-Pressure-High-Temperature (HPHT) annealing at 2100 °C and 7.7 GPa to reduce implantation damage and facilitate SnV formation.
Cryogenic Setup: Experiments were conducted in a closed-cycle helium cryostat operating at a base temperature of ~1.7 K.
Charge Stabilization: A continuous wave (CW) or pulsed 445 nm diode laser was used as a charge repump to maintain the SnV^- state, exploiting the charge cycle mechanism.
Resonant Excitation: A frequency-stabilized Dye CW laser was used for resonant excitation of the SnV^- C-transition (ZPL at ~2.0 eV).
Coherent Population Trapping (CPT): An Electro-Optical Phase Modulator (EOM) and microwave signals were used to generate optical sidebands for all-optical probing of the ground state spin coherence.

6CCVD Solutions & Capabilities

6CCVD provides the foundational MPCVD diamond materials and custom engineering services required to replicate, scale, and advance this critical research into charge-stabilized Group IV-Vacancy (G4V) centers.

Applicable Materials

To achieve the high spectral stability and long coherence times demonstrated in this paper, researchers require the highest quality starting material.

Material Grade	6CCVD Offering	Application Relevance
Electronic Grade SCD	Ultra-high purity, low-strain Single Crystal Diamond (SCD) wafers.	Essential for minimizing spectral diffusion caused by environmental impurities (N, B) and lattice defects.
Custom Orientation	Standard (001) orientation, or custom orientations available upon request.	Required for alignment with the SnV defect symmetry axis and subsequent magnetic field application.
Custom Thickness	SCD plates available from 0.1 µm up to 500 µm thickness.	Allows optimization for specific QIP device architectures (e.g., thin membranes for integration with photonic structures).

Customization Potential

The creation of SnV centers involves precise post-growth processing (implantation and annealing). 6CCVD supports this workflow by providing engineered substrates and specialized services:

Substrate Engineering: We supply high-quality SCD substrates (up to 500 µm thick) ready for external ion implantation. Our material purity ensures that the resulting SnV centers are protected from fluctuating charges caused by background impurities, which is crucial for the demonstrated long-term stability (< 4 MHz shifts).
Precision Polishing: The research relies on high-fidelity optical measurements. 6CCVD offers Atomic-Scale Polishing (Ra < 1 nm for SCD) to ensure minimal surface scattering losses and optimal coupling efficiency for integrated photonic devices (e.g., optical antennas, micropillars, and waveguides mentioned in the paper’s outlook).
Custom Metalization Services: While the core experiment focused on optical control, future device integration often requires electrical contacts. 6CCVD offers in-house deposition of standard QIP metal stacks, including Ti, Pt, Au, Pd, W, and Cu, tailored to specific lithography requirements.
Large Format Availability: For scaling up QIP experiments or moving toward commercial fabrication, 6CCVD can provide large-area Polycrystalline Diamond (PCD) wafers up to 125 mm in diameter, or SCD substrates up to 10 mm thick.

Engineering Support

6CCVD’s in-house PhD team specializes in the material science of MPCVD diamond and its application in quantum technologies. We offer authoritative consultation on:

Material Selection for G4V Centers: Assistance in selecting the optimal SCD purity and orientation necessary to replicate or extend this research to other Group IV-Vacancy centers (SiV, GeV, PbV), ensuring minimal spectral diffusion and maximum T₂ coherence times.
Post-Processing Optimization: Guidance on substrate preparation (e.g., surface termination, polishing requirements) to maximize the yield and quality of SnV centers following ion implantation and HPHT annealing.
Integration Challenges: Support for engineers designing integrated quantum photonic interfaces, where precise diamond thickness and surface quality are paramount for efficient photon collection and readout fidelity.

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

View Original Abstract

Abstract Quantum information processing (QIP) with solid state spin qubits strongly depends on the efficient initialisation of the qubit’s desired charge state. While the negatively charged tin-vacancy (SnV − ) centre in diamond has emerged as an excellent platform for realising QIP protocols due to long spin coherence times at liquid helium temperature and lifetime limited optical transitions, its usefulness is severely limited by termination of the fluorescence under resonant excitation. Here, we unveil the underlying charge cycle, potentially applicable to all group IV-vacancy (G4V) centres, and exploit it to demonstrate highly efficient and rapid initialisation of the desired negative charge state of single SnV centres while preserving long term stable optical resonances. In addition to investigating the optical coherence, we all-optically probe the coherence of the ground state spins by means of coherent population trapping and find a spin dephasing time of 5(1) μ s. Furthermore, we demonstrate proof-of-principle single shot spin state readout without the necessity of a magnetic field aligned to the symmetry axis of the defect.

Tech Support

Original Source

DOI: https://doi.org/10.1038/s41534-022-00552-0