Controlling the coherence of a diamond spin qubit through its strain environment

At a Glance

Metadata	Details
Publication Date	2018-05-16
Journal	Nature Communications
Authors	Young-Ik Sohn, Srujan Meesala, Benjamin Pingault, Haig A Atikian, Jeffrey Holzgrafe
Institutions	Harvard University, Sandia National Laboratories
Citations	150
Analysis	Full AI Review Included

Technical Analysis and Documentation: Strain-Controlled Diamond Qubits

This technical documentation analyzes the research “Controlling the coherence of a diamond spin qubit through its strain environment,” which demonstrates a novel method using strain engineering to significantly enhance the coherence time ($T_{2}^{*}$) of silicon-vacancy ($\text{SiV}^{-}$) spin qubits in MPCVD diamond. The findings are highly relevant to quantum computing, sensing, and scalable photonic networks, fields where 6CCVD provides critical, ultra-high purity diamond substrates.

Executive Summary

This study successfully leverages mechanical strain, applied via a Nano-Electro-Mechanical System (NEMS), to mitigate thermal decoherence in solid-state quantum bits (qubits). This method drastically improves qubit performance without requiring complex sub-Kelvin cryogenic systems.

Core Achievement: Demonstrated a 6-fold prolongation of the spin coherence time ($T_{2}^{*}$) for the $\text{SiV}^{-}$ center in diamond by suppressing phonon-mediated decoherence.
Performance Metric: Spin coherence time $T_{2}^{*}$ reached $0.25 \pm 0.02$ µs, saturating at high strain conditions ($\Delta_{gs} \approx 467$ GHz).
Operating Advantage: Achieved robust spin coherence at a standard helium-4 temperature ($T \approx 4$ K), circumventing the need for resource-intensive dilution refrigerator temperatures (mK).
Mechanism: Static strain increased the Ground State (GS) orbital splitting energy scale (up to 1.2 THz), pushing it far beyond the thermal energy ($k_{B}T$) and rapidly quenching phonon absorption.
Material Requirement: Requires high-quality, ultra-high purity (nitrogen < 5 ppb) Single Crystal Diamond (SCD) as the foundational material for the NEMS device structure.
Future Applications: The work lays the foundation for phonon-mediated quantum gates, non-linear quantum phononics, and scalable strain-tunable photonic quantum networks.

Technical Specifications

The following hard data points were extracted, detailing the experimental conditions and performance metrics achieved through strain tuning of the $\text{SiV}^{-}$ center.

Parameter	Value	Unit	Context
Diamond Type	Type IIa SCD	N/A	(100)-cut, MPCVD synthesized
Impurity Concentration (N)	< 5	ppb	Required for ultra-high purity
SiV Generation Method	FIB Implantation	$^{28}\text{Si}^{+}$	Targeted ion implantation followed by annealing
Annealing Temperature	1100	°C	Required for SiV center formation
Operating Temperature	3.8 - 4	K	Closed-cycle liquid helium cryostat
Max Applied DC Voltage	200	V	Used for NEMS cantilever deflection
Initial GS Splitting ($\Delta_{gs}$)	46	GHz	Zero strain condition
Max Tunable GS Splitting	Up to 1.2	THz	Achieved under highest strain
Max Spin Coherence ($T_{2}^{*}$)	$0.25 \pm 0.02$	µs	Measured via CPT at 467 GHz GS splitting
CPT Linewidth (Minimum)	$\approx 1$	MHz	Linewidth saturation point
Electrode Metalization	Ta (10 nm) / Au (200 nm)	nm	Used for electrical actuation of NEMS

Key Methodologies

The experiment relies heavily on precise material synthesis, nanoscale fabrication, and sophisticated optical measurements.

CVD Substrate Preparation: Sourcing of ultra-high purity, (100)-cut, Type IIa Single Crystal Diamond (SCD) with Nitrogen concentration less than 5 ppb.
NEMS Fabrication: Patterning of cantilever arrays using Electron-Beam Lithography (EBL), followed by vertical etching (oxygen plasma) and angled etching (ion-milling) to create free-standing monolithic structures.
SiV Incorporation: Precise, targeted implantation of $^{28}\text{Si}^{+}$ ions using Focused Ion Beam (FIB) into the cantilever structure.
Defect Activation: High-vacuum annealing at 1100 °C to activate the $\text{SiV}^{-}$ color centers, followed by a tri-acid surface clean.
Electrode Deposition: Bi-layer PMMA process used for electrode patterning, followed by evaporation of a 10 nm Tantalum (Ta) adhesion layer and a 200 nm Gold (Au) contact layer.
Strain Actuation: Static strain is controllably applied via electrostatic attraction by applying a DC voltage (up to 200 V) across the Ta/Au electrodes, deflecting the cantilever.
Qubit Characterization: Measurements include strain-dependent Photoluminescence Excitation (PLE) and Coherent Population Trapping (CPT) to probe orbital splittings and measure spin coherence ($T_{2}^{*}$) at 4 K.

6CCVD Solutions & Capabilities

This research highlights the absolute necessity of ultra-high quality, customizable MPCVD diamond substrates for breakthroughs in solid-state quantum technology. 6CCVD is uniquely positioned to supply and enhance the critical components required for replicating and extending this work, particularly in NEMS-based quantum control.

Applicable Materials

To achieve the exceptional $T_{2}^{*}$ coherence times demonstrated, the research requires the highest quality diamond material, free from competing defects.

Requirement	6CCVD Solution	Technical Detail
Ultra-High Purity Substrate	Optical Grade Single Crystal Diamond (SCD)	Ultra-low concentration of residual nitrogen (N $\ll$ 5 ppb) essential for minimizing parasitic spin baths and decoherence mechanisms.
NEMS Structural Base	Custom Thin-Film SCD	Thicknesses are available from 0.1 µm up to 500 µm, allowing fine control over cantilever dimensions and resonant properties required for optimal strain transfer.
Alternative Qubit Substrates	Boron-Doped Diamond (BDD)	For future work exploring high-bandwidth sensor applications or thermal management, BDD is available in both SCD and PCD formats.

Customization Potential

The NEMS architecture demands precision engineering in substrate geometry and metal contact preparation, capabilities that are central to 6CCVD’s expertise.

Custom Dimensions and Crystal Orientation: The experiment utilizes (100)-cut diamond wafers. 6CCVD provides custom crystal orientations and wafer sizes up to 125 mm (PCD) and large-area SCD, ready for subsequent processing (FIB implantation, EBL, and etching).
Precision Surface Finish: Achieving reliable lithography and nanoscale etching (crucial for NEMS cantilevers) requires flawless surface quality. 6CCVD guarantees $R_{a} < 1$ nm polishing on SCD, minimizing surface defects that can interfere with fabrication and qubit performance.
Integrated Metalization Services: The device relied on a Ta/Au bilayer for stable, high-voltage actuation. 6CCVD offers in-house metal deposition, including Ti, W, Ta, Pt, Pd, Au, and Cu. We can supply ready-to-use substrates with specified metal stacks tailored for NEMS or superconducting qubit integration.

Engineering Support

The complexity of strain-mediated control requires deep collaboration between material scientists and quantum engineers.

Material Design Consultation: 6CCVD’s in-house $\text{PhD}$ team specializes in diamond synthesis for quantum applications. We offer consultation on optimizing material parameters (e.g., surface termination, thickness, crystal orientation) to maximize strain coupling efficiency for similar $\text{SiV}^{-}$ or $\text{NV}^{-}$ projects.
Process Optimization: We assist researchers planning post-processing steps like the 1100 °C high-temperature annealing or the targeted FIB implantation, ensuring the diamond substrate maintains its integrity and purity.

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

Tech Support

Original Source

DOI: https://doi.org/10.1038/s41467-018-04340-3