Coherent spin control of a nanocavity-enhanced qubit in diamond

At a Glance

Metadata	Details
Publication Date	2015-01-28
Journal	Nature Communications
Authors	Luozhou Li, Tim Schröder, Edward H. Chen, Michael Walsh, Igal Bayn
Institutions	Element Six (United States), Massachusetts Institute of Technology
Citations	192
Analysis	Full AI Review Included

Coherent Spin Control in Nanocavity-Enhanced Diamond Qubits

Technical Analysis and Materials Solutions from 6CCVD

Executive Summary

This research successfully demonstrates the crucial integration of nitrogen-vacancy (NV) centers in high-purity single-crystal diamond (SCD) with photonic crystal (PhC) nanocavities to create a highly efficient, coherent quantum memory.

Strong Purcell Regime: Achieved a record high spectrally resolved Purcell enhancement factor (F_ZPL) of 62 for an NV center coupled to a PhC nanocavity.
Ultra-High Q Factor: Demonstrated optical quality factors (Q) up to 9,900 ± 200 near the NV zero-phonon line (ZPL) emission (~637 nm).
Preserved Coherence: The novel fabrication process preserves the intrinsic quality of the host diamond, yielding exceptional electron spin coherence times (T₂) exceeding 200 µs.
Efficient Spin-Photon Interface: The system operates in the strong Purcell regime (β > 0.5), meaning over 50% of the NV emission is coupled into the cavity mode.
Scalable Architecture: The on-chip integration of the PhC nanocavities with metallic microwave striplines validates an architecture suitable for future large-scale quantum networks and quantum repeaters.
Methodology: Utilized MPCVD grown, high-purity SCD membranes (~200 nm thick), ¹⁵N implantation, Si hard-mask lithography, and precision RIE to define the high-Q PhC structures.

Technical Specifications

The following hard data points were extracted, characterizing the quantum memory system and fabrication process:

Parameter	Value	Unit	Context
Cavity Quality Factor (Q)	9,900 ± 200	dimensionless	Maximum measured Q factor (System B).
Spectrally Resolved Purcell Enhancement (F_ZPL)	62	dimensionless	Strong Purcell coupling achieved (System B).
Electron Spin Coherence Time (T₂)	230	µs	Measured via Hahn echo sequence.
Rabi Oscillation Decay Time (T₂’)	> 6	µs	Coherent spin control demonstration.
Quantum Efficiency (β Factor)	0.54	dimensionless	Implies > 50% emission into cavity mode.
On-Resonance SE Lifetime (τ_on)	6.7	ns	Accelerated spontaneous emission.
Off-Resonance SE Lifetime (τ_off)	18.4	ns	Baseline NV spontaneous emission.
Diamond Membrane Thickness	~200	nm	Final RIE thickness for PhC fabrication.
Operating Temperature	~18	K	Cryogenic optical characterization.
NV ZPL Resonance Wavelength	~637	nm	Target resonance for cavity coupling.
¹⁵N Implantation Energy	80	keV	Used to create NV centers ~100 nm below surface.
Annealing Conditions	850 °C for 2h	vacuum	NV activation in MTI OTF-1500X-4 furnace.

Key Methodologies

The experiment relies on complex, precision engineering steps to produce high-quality SCD membranes, create the NV centers, and fabricate the intricate PhC structures integrated with control electronics.

Material Preparation: High-purity, single-crystal diamond (SCD) plates were grown using Microwave Plasma-Assisted Chemical Vapor Deposition (MPCVD).
Initial Thinning and Polishing: Diamond was laser-cut to 2x2 mm plates, mechanically polished to 5 µm thickness, and then subjected to chlorine- and argon-reactive ion etching (RIE) to achieve a final thickness of ~200 nm.
NV Creation: ¹⁵N ions were implanted at 80 keV (~100 nm below the surface) at dosages ranging from 5 x 10¹⁰ cm^-2 (System A) to 5 x 10¹¹ cm^-2 (System B). Samples were annealed at 850 °C in high vacuum (1.5 x 10^-6 mbar) to form NV centers.
Si Hard Mask Fabrication: Silicon (Si) masks were patterned on Si-on-insulator wafers using electron beam lithography (EBL) and cryogenic plasma etching (SF₆/O₂).
Pattern Transfer: The Si masks were transferred onto the thinned diamond membranes using a PDMS-tipped probe. The PhC pattern was etched into the diamond via Oxygen Plasma RIE (20 sccm gas flow, 50 mTorr, 100 W power).
Integration with Control Circuitry: After mask removal and isotropic RIE to suspend the cavities, the patterned diamond devices were transferred onto a separate Si chip containing integrated metallic microwave striplines for coherent spin control.
Optical Tuning: Cavity resonance alignment to the NV ZPL was achieved in situ at cryogenic temperatures (~18 K) using Xenon (Xe) gas condensation, which adjusted the effective refractive index of the diamond (tuning rate ~8 pm/s).

6CCVD Solutions & Capabilities

This research highlights the critical need for ultra-high quality MPCVD diamond materials and precision fabrication control, core competencies of 6CCVD. Our capabilities are perfectly aligned to support the replication and scaling of these quantum memory systems.

Applicable Materials

The foundation of this success is the use of high-purity, low-strain single-crystal diamond (SCD) that maintains long T₂ coherence times even after rigorous nanofabrication.

Optical Grade Single Crystal Diamond (SCD): Essential for replicating this work. We recommend electronic-grade SCD, ensuring the initial nitrogen concentration is below the detection limit (< 1 ppb). This quality is necessary to minimize background paramagnetic defects and achieve T₂ coherence times exceeding 200 µs.
Custom Doping (N-control): While the paper uses implantation, 6CCVD can offer SCD grown with highly controlled concentrations of ¹⁵N during the CVD process, or provide material optimized for external implantation processes.

Customization Potential

The PhC fabrication and integration of MW control lines require specialized material dimensions, surface preparation, and thin-film deposition that 6CCVD provides as standard or custom services.

Requirement	6CCVD Capability	Value Proposition
Precision Thinning	Custom thickness control for SCD (0.1 µm to 500 µm) and substrate thinning (up to 10 mm).	Provides highly uniform ~200 nm to 5 µm SCD membranes compatible with RIE processes.
Surface Quality	SCD polishing achieving roughness Ra < 1 nm.	Critical for minimizing optical scattering losses and enabling Q factors approaching 10,000 in PhC nanocavities.
Custom Metalization	In-house multi-layer deposition capabilities: Au, Pt, Pd, Ti, W, Cu.	Allows researchers to deposit the necessary metallic striplines (e.g., Ti/Pt/Au stack) directly onto SCD substrates or integrated components for robust MW control.
Dimension Scaling	SCD/PCD wafers available up to 125 mm.	Supports the transition from small research samples (2x2 mm) to industrial, wafer-scale fabrication necessary for scalable quantum networks.
Patterning/Etching Support	Expertise in preparing diamond for high-aspect ratio RIE processing and clean etch stops.	Ensures reliable yield during hard-mask pattern transfer and deep etching of the diamond PhC structures.

Engineering Support

Replicating and extending quantum memory research, particularly in coupling solid-state qubits to nanophotonic structures, requires deep materials expertise.

6CCVD’s in-house PhD team provides expert consultation on material selection, surface termination, and defect engineering (including specific ¹⁵N incorporation and annealing protocols) necessary to optimize the NV environment for strong Purcell enhancement and long coherence times in quantum memory networks and coherent spin control projects.

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/ncomms7173