Control of Spin Defects in Wide-Bandgap Semiconductors for Quantum Technologies

At a Glance

Metadata	Details
Publication Date	2016-05-24
Journal	Proceedings of the IEEE
Authors	F. Joseph Heremans, Christopher G. Yale, D. D. Awschalom
Institutions	Argonne National Laboratory, University of Chicago
Citations	70
Analysis	Full AI Review Included

Technical Documentation & Quantum Brief: Spin Defects in Wide Band-Gap Semiconductors

6CCVD Material Scientist Analysis: This review paper confirms the critical role of high-pquality MPCVD diamond, specifically the Nitrogen-Vacancy (NV⁻) center, as the leading solid-state platform for quantum information processing and nanoscale sensing. The requirements for extended coherence times (T₂) and integrated quantum photonics necessitate ultra-high purity, isotopically enriched, and highly polished Single Crystal Diamond (SCD).

Executive Summary

NV⁻ Diamond Dominance: The negatively charged Nitrogen-Vacancy (NV⁻) center in diamond remains the archetypal optically addressable spin defect, enabling quantum control protocols at room temperature (T > 600 K).
Extended Coherence: Spin coherence times (T₂) are maximized by using isotopically pure diamond (e.g., ¹²C growth), extending T₂ from hundreds of µs up to ~2 ms, facilitating complex quantum gates and metrology.
Nanoscale Sensing: NV⁻ centers function as exquisite nanoscale sensors, demonstrating high sensitivity for magnetic fields (~20 nT/√Hz) and temperature (~10 mK/√Hz).
Quantum Networks: Photonic control techniques, including resonant excitation and DC Stark effect tuning, are essential for achieving entanglement and quantum interference between distant NV⁻ centers (up to 3 meters separation demonstrated).
SiC as Alternative: Silicon Carbide (SiC) divacancies (VV⁰) are a promising, technologically mature alternative, exhibiting millisecond T₂ coherence times (> 1.2 ms) without isotopic engineering.
Hybrid Control: The versatility of control mechanisms—microwave, photonic, electrical, and mechanical (strain)—suggests pathways for developing robust quantum transducers and hybrid quantum systems.

Technical Specifications

The following hard data points were extracted from the analysis of spin defect performance and material properties:

Parameter	Value	Unit	Context
Diamond Bandgap (E_g)	5.5	eV	Wide band-gap host material for defect isolation
NV⁻ Zero-Phonon Line (ZPL)	1.945 (637)	eV (nm)	Key optical transition for initialization and readout
NV⁻ Ground State Splitting (D)	2.87	GHz	Crystal field splitting at zero magnetic field
NV⁻ Spin Coherence Time (T₂)	~2	ms	Achieved using isotopically purified ¹²C diamond [30]
NV⁻ Magnetic Field Sensitivity	~20	nT/√Hz	High-resolution nanoscale magnetometry [63], [64]
NV⁻ Thermal Sensitivity (Enhanced)	~10	mK/√Hz	Achieved using thermal echo pulse sequence [39]-[41]
SiC VV⁰ ZPL Range	1.09 - 1.20	eV	Range of ZPLs across various divacancy configurations
SiC VV⁰ Spin Coherence Time (T₂)	> 1.2	ms	Isolated electron spins in SiC [23]
SiC Photonic Crystal Q-Factor	~1500	N/A	Demonstrated quality factor in 3C-SiC L3 cavity [119]

Key Methodologies

The control and measurement of solid-state spin defects rely on sophisticated techniques leveraging microwave, optical, and mechanical interfaces:

Optically Detected Magnetic Resonance (ODMR):
- Mechanism: Microwave frequencies drive coherent manipulation of the spin-triplet ground state (m_s = 0 ⇔ m_s = ±1).
- Detection: Signal contrast results from spin-dependent photoluminescence (PL) intensity changes due to the intersystem crossing (ISC).
Rabi Oscillations:
- Mechanism: Applying resonant microwave pulses of varying duration to induce coherent oscillation and achieve full spin inversion (π pulse) or superposition (π/2 pulse).
- Performance: Demonstrated Rabi oscillation speeds up to ~50 ns (20 MHz) in SiC VV⁰ defects.
Spin Coherence Probing (Hahn Echo & Dynamical Decoupling):
- Hahn Echo (T₂): Uses a refocusing π pulse to reverse slow phase accumulation, characterizing the T₂ coherence time.
- Dynamical Decoupling: Increases the number of refocusing pulses (N > 1) to mitigate effects from increasingly faster local fluctuations, extending effective T₂.
Photoluminescence Excitation (PLE) & Resonant Excitation:
- Mechanism: Uses a tunable, narrow-line laser (e.g., 637 nm for NV⁻) at cryogenic temperatures (T < 20 K) to resolve sharp optical transitions.
- Application: Enables high-fidelity single-shot readout (fidelity up to 99.7% demonstrated) and optical Rabi oscillations between ground and excited states.
DC Stark Effect Tuning:
- Mechanism: Applying external electric fields to tune the orbital levels of the excited state.
- Application: Crucial for bringing separate NV⁻ centers into optical degeneracy, a prerequisite for two-photon quantum interference and entanglement protocols.
Mechanical and Electrical Control:
- Mechanism: Utilizing strain waves (via MEMS transducers) or lithographically deposited electrodes to drive magnetic-dipole forbidden transitions (Δm_s = 2) or tune the crystal field splitting (D).

6CCVD Solutions & Capabilities

6CCVD provides the foundational MPCVD diamond materials and advanced processing required to replicate and extend the cutting-edge quantum research detailed in this review.

Applicable Materials for Quantum Qubits and Sensing

To achieve the long coherence times and high-fidelity optical control demonstrated in the paper, researchers require diamond with exceptional purity and precise isotopic control.

Research Requirement	6CCVD Material Solution	Technical Advantage
Long T₂ Coherence	Isotopically Purified SCD (Single Crystal Diamond)	Ultra-low concentration of ¹³C (natural abundance 1.07%) minimizes nuclear spin bath noise, enabling T₂ times in the millisecond range.
High-Fidelity Optical Control	Optical Grade SCD	Extremely low nitrogen content (< 1 ppb) ensures minimal background defects (P1 centers) and stable NV⁻ charge state, critical for resonant excitation and PLE.
Nanoscale Sensing (Shallow NV)	Custom Thickness SCD Plates	SCD wafers available from 0.1 µm to 500 µm thick, allowing precise control over the depth of implanted or delta-doped NV⁻ centers for optimal surface sensing performance.
Quantum Hybrid Systems	Boron-Doped Diamond (BDD)	BDD films offer tunable conductivity, essential for creating integrated electrical gates and electrodes directly on the diamond surface for DC Stark effect tuning and electrically driven spin resonance (EDSR).

Customization Potential for Integrated Quantum Devices

The development of quantum networks relies heavily on integrating spin defects with photonic and electrical structures (nanowires, photonic crystals, electrodes). 6CCVD offers specialized fabrication services to meet these complex engineering demands:

Advanced Polishing: We provide ultra-smooth surfaces (Ra < 1 nm for SCD; Ra < 5 nm for inch-size PCD) necessary for high-Q photonic crystal fabrication and minimizing spectral diffusion, a key challenge noted in the paper.
Custom Dimensions: We supply SCD and PCD plates/wafers up to 125mm, supporting large-scale lithography and device integration, moving beyond the millimeter-scale samples often cited in early research.
Integrated Metalization: 6CCVD offers in-house deposition of thin-film metals (Au, Pt, Pd, Ti, W, Cu). This capability is vital for creating the lithographically deposited electrodes required for electrical control (DC Stark effect) and mechanical transducers (MEMS) discussed in Section V of the review.
Substrate Flexibility: We offer diamond substrates up to 10 mm thick, providing robust platforms for complex multi-layer device stacking and mechanical resonator integration.

Engineering Support

6CCVD’s in-house team of PhD material scientists and quantum engineers specializes in optimizing MPCVD growth parameters (pressure, temperature, gas flow) to control defect density and isotopic purity. We can assist researchers in material selection for similar NV⁻ Qubit and Nanoscale Sensing projects, ensuring the starting material meets the stringent requirements for achieving long T₂ coherence and high optical addressability.

Call to Action: For custom specifications, isotopic purification requirements, or material consultation on integrating spin defects into quantum devices, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Deep-level defects are usually considered undesirable in semiconductors as they typically interfere with the performance of present-day electronic and optoelectronic devices. However, the electronic spin states of certain atomic-scale defects have recently been shown to be promising quantum bits for quantum information processing as well as exquisite nanoscale sensors due to their local environmental sensitivity. In this review, we will discuss recent advances in quantum control protocols of several of these spin defects, the negatively charged nitrogen-vacancy (NV<sup>-</sup>) center in diamond and a variety of forms of the neutral divacancy (VV<sup>0</sup>) complex in silicon carbide (SiC). These defects exhibit a spin-triplet ground state that can be controlled through a variety of techniques, several of which allow for room temperature operation. Microwave control has enabled sophisticated decoupling schemes to extend coherence times as well as nanoscale sensing of temperature along with magnetic and electric fields. On the other hand, photonic control of these spin states has provided initial steps toward integration into quantum networks, including entanglement, quantum state teleportation, and all-optical control. Electrical and mechanical control also suggest pathways to develop quantum transducers and quantum hybrid systems. In conclusion, the versatility of the control mechanisms demonstrated should facilitate the development of quantum technologies based on these spin defects.

Tech Support

Original Source

DOI: https://doi.org/10.1109/jproc.2016.2561274