Coupling a Surface Acoustic Wave to an Electron Spin in Diamond via a Dark State

At a Glance

Metadata	Details
Publication Date	2016-12-20
Journal	Physical Review X
Authors	D. Andrew Golter, Thein Oo, Mayra Amezcua, Ignas Lekavicius, Kevin A. Stewart
Institutions	Oregon State University, University of Oregon
Citations	140
Analysis	Full AI Review Included

Technical Documentation & Quantum Acoustics Prospectus

Executive Summary

The reported research successfully demonstrates strong coherent coupling between a Surface Acoustic Wave (SAW) and an electron spin housed within a Nitrogen-Vacancy (NV) center in diamond, leveraging the excited-state strain coupling mechanism.

Core Achievement: Realization of phonon-assisted Coherent Population Trapping (CPT) and optically-driven sideband spin transitions in a solid-state NV system using a SAW.
Mechanism: Excited-state mediation via a $\Lambda$-type three-level system enables coupling between the ground state electron spin and the mechanical degree of freedom (SAW phonons).
Material Requirement: High-purity, single-crystal diamond (SCD) is crucial for hosting NV centers with long spin decoherence times, necessary for quantum computing and spintronics applications.
Performance Metric: The resulting single-phonon Rabi frequency ($g_{ss}$) is estimated to be three orders of magnitude greater than that achievable through direct ground-state spin-phonon coupling, validating the excited-state approach.
Future Outlook: Establishes a solid-state platform analogous to successful trapped-ion quantum systems, opening new avenues for spin-based quantum acoustics and universal quantum transducers.
Fabrication Complexity: The experiment requires complex heterostructures, specifically a high-quality piezoelectric film (ZnO, 400 nm thick) deposited onto the diamond substrate, followed by high-precision patterning of Inter-Digital Transducers (IDTs) via electron beam lithography (EBL).

Technical Specifications

Parameter	Value	Unit	Context
Operating Temperature	8	K	Cold-finger optical cryostat
NV Ground State Splitting	2.88	GHz	$m_s = \pm 1$ from $m_s = 0$
Built-in DC Strain Split (Ex/Ey)	9.6	GHz	Indication of substrate strain
SAW Center Frequency ($\omega_{m}$)	Near 900	MHz	Measured frequency from IDT structure
SAW Velocity ($v_{s}$)	$\approx 5600$	m/s	Calculated from IDT parameters
Piezoelectric Layer Thickness	400	nm	Sputtered ZnO onto diamond surface
IDT Finger Width ($w$)	1.5	µm	Used in $v_{s}/(4w)$ calculation
IDT Electromechanical Coupling	$\approx 0.05$	%	Efficiency of SAW generation
SAW Amplitude (1W RF Input)	Order of a pm	pm	Generated acoustic wave displacement
CPT Rabi Frequency ($\Omega_{R}/2\pi$)	8	MHz	Effective Rabi frequency for CPT dip
Sideband Rabi Frequency ($\Omega_{ss}/2\pi$)	0.3	MHz	Effective spin transition rate
Spin Coherence Decay ($\gamma_{s}/2\pi$)	0.35	MHz	Primarily due to nuclear spin bath
Upper State Total Decay ($\Gamma/2\pi$)	14	MHz	Determined experimentally
Green Initialization Wavelength ($\lambda$)	532	nm	Used to initialize $m_s=0$ state
Zero Phonon Line Wavelength ($\lambda$)	$\approx 637$	nm	Probe laser frequency

Key Methodologies

The experiment relies on combining high-quality MPCVD diamond substrates with advanced nanofabrication techniques for integrated acoustic control.

Diamond Substrate Preparation:
- Used a diamond sample hosting NV centers situated a few µm below the surface.
- Requires low-nitrogen, high-purity Single Crystal Diamond (SCD) to ensure optimal NV spin coherence times.
Piezoelectric Layer Deposition:
- A 400 nm thick layer of piezoelectric ZnO was sputtered onto the diamond surface. This layer is critical for electro-mechanical transduction.
IDT Fabrication:
- Inter-Digital Transducers (IDTs) were patterned onto the ZnO layer using high-resolution Electron Beam Lithography (EBL).
- IDT design included 40 pairs of fingers, each $1.5 \text{ µm}$ wide, tuned to generate SAWs near $900 \text{ MHz}$.
Cryogenic and Optical Setup:
- Experiments conducted at $8 \text{ K}$ using a cold-finger optical cryostat.
- NV centers were initialized using an off-resonant $532 \text{ nm}$ green laser.
- Spin coherence and transitions were driven by two frequency-stabilized tunable dye lasers (near $637 \text{ nm}$) controlled by Acoustic Optical Modulators (AOMs).
Quantum State Control:
- A small magnetic field was applied to induce a Zeeman splitting ($24 \text{ MHz}$) between the $m_s = \pm 1$ states.
- The system was operated in the adiabatic limit (large detuning $\Delta=100 \text{ MHz}$) to minimize optically-induced decoherence while enabling strong sideband spin transitions.

6CCVD Solutions & Capabilities: Enabling Quantum Acoustics Platforms

6CCVD is uniquely positioned to supply the foundational materials and advanced processing required to replicate and scale this critical research into operational quantum acoustic devices.

Applicable Materials

The foundation of this research is the high-quality NV center, which demands superior material characteristics:

Material of Choice: Optical Grade Single Crystal Diamond (SCD):
- Required for hosting NV centers with maximum spin coherence. 6CCVD provides SCD substrates grown via MPCVD, offering exceptional purity and controlled nitrogen concentration essential for creating dense, yet isolated, NV ensembles or targeted single NVs.
- Purity: Guaranteed low substitutional nitrogen concentration ([N] < 1 ppb) to maximize $T_2$ and minimize spectral diffusion, a key factor addressed in the paper ($\gamma_{s}/2\pi = 0.35 \text{ MHz}$).
- Custom Substrates: Available as plates/wafers up to $125 \text{ mm}$ size, allowing research to move from small cryostat samples to integrated wafer-scale manufacturing.
Future Development: Boron-Doped Diamond (BDD) / Polycrystalline Diamond (PCD):
- While the paper focuses on NV centers (SCD), related quantum sensing and micro-electromechanical systems (MEMS) applications may require BDD for high conductivity contacts or PCD for robustness and large-area coverage. 6CCVD supplies both materials up to $500 \text{ µm}$ thickness.

Customization Potential

The experimental setup requires intricate material layering and high-precision patterning, services 6CCVD provides in-house:

Service	Requirement Match	6CCVD Capability
Layering & Bonding	Piezoelectric film (400 nm ZnO) deposition for IDTs.	We supply precision diamond substrates optimized for heterogeneous integration with sputtered layers like ZnO, AlN, or PZT.
Metalization & IDT Patterning	IDT fabrication requires high-resolution lithography.	Custom metalization capabilities (Au, Pt, Pd, Ti, W, Cu) for contact pads and transducer wiring. We support partners requiring subsequent EBL patterning for $\text{µm}$-scale features like the $1.5 \text{ µm}$ IDT fingers.
Surface Finish	High-fidelity optical coupling and low optical scattering.	Ultra-low roughness polishing: SCD surfaces achieved with $R_a < 1 \text{ nm}$, critical for coupling quantum emitters to external fields (optical and acoustic).
Nanomechanical Components	Outlook mentions diamond nanomechanical oscillators (bulk or diamond-on-silicon).	Custom laser machining and etching for fabricating nanomechanical structures (cantilevers, beams) cited in the references [43-47], supporting the realization of the solid-state trapped-ion analogy.
Thickness Control	SCD thickness (up to $500 \text{ µm}$) for bulk NV experiments; Substrates up to $10 \text{ mm}$.	Precise control over MPCVD growth thickness, allowing fine-tuning of material depth relative to the SAW propagation profile.

Engineering Support

The density matrix models and Hamiltonian derivation presented in this paper highlight the sophisticated theoretical demands of quantum acoustics research.

6CCVD’s in-house PhD engineering team specializes in the material science and growth parameters necessary for complex defect physics.
We offer consultation on material selection, doping control, strain management, and integration processes required for similar spin-based quantum acoustic projects.
Our global logistics network ensures DDP/DDU shipping worldwide, guaranteeing materials arrive safely and efficiently for cryogenic and sensitive fabrication environments.

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

View Original Abstract

The emerging field of quantum acoustics explores interactions between\nacoustic waves and artificial atoms and their applications in quantum\ninformation processing. In this experimental study, we demonstrate the coupling\nbetween a surface acoustic wave (SAW) and an electron spin in diamond by taking\nadvantage of the strong strain coupling of the excited states of a nitrogen\nvacancy center, while avoiding the short lifetime of these states. The SAW-spin\ncoupling takes place through a lamda-type three-level system where two ground\nspin states couple to a common excited state through a phonon-assisted as well\nas a direct dipole optical transition. Both coherent population trapping and\noptically-driven spin transitions have been realized. The coherent population\ntrapping demonstrates the coupling between a SAW and an electron spin coherence\nthrough a dark state. The optically-driven spin transitions, which resemble the\nsideband transitions in a trapped ion system, can enable the quantum control of\nboth spin and mechanical degrees of freedom and potentially a trapped-ion-like\nsolid state system for applications in quantum computing. These results\nestablish an experimental platform for spin-based quantum acoustic, bridging\nthe gap between spintronics and quantum acoustics.\n

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevx.6.041060