Extending the Quantum Coherence of a Near-Surface Qubit by Coherently Driving the Paramagnetic Surface Environment

At a Glance

Metadata	Details
Publication Date	2019-10-04
Journal	Physical Review Letters
Authors	Dolev Bluvstein, Zhiran Zhang, Claire A. McLellan, Nicolas R. Williams, Ania C. Bleszynski Jayich
Institutions	University of California, Santa Barbara
Citations	40
Analysis	Full AI Review Included

Technical Documentation & Analysis: Coherent Surface Spin Driving for Enhanced NV Coherence

Reference Paper: Extending the Quantum Coherence of a Near-Surface Qubit by Coherently Driving the Paramagnetic Surface Environment (arXiv:1905.06405v1)

Executive Summary

This research demonstrates a breakthrough technique—Coherent Radio-Frequency Driving of Surface Electron Spins (SSD)—to mitigate surface-induced decoherence in solid-state qubits, using the Nitrogen-Vacancy (NV) center in diamond as a model platform.

Decoherence Suppression: SSD successfully suppresses the fluctuating magnetic fields from paramagnetic surface electron spins, which are a primary source of decoherence for near-surface qubits.
Coherence Enhancement: The Hahn echo $T_{2}$ coherence time of shallow NV centers was significantly extended, showing an increase from 65 µs to 94 µs in the Single-Quantum (SQ) basis, and 41 µs to 75 µs in the Double-Quantum (DQ) basis.
Sensitivity Gain: By combining SSD with operation in the DQ basis, the researchers achieved an overall fivefold sensitivity enhancement in NV magnetometry measurements.
Material Requirements: The experiment relies on ultra-high purity, isotopically enriched (99.99% $^{12}$C) Single Crystal Diamond (SCD) grown via MPCVD, optimized for shallow ion implantation (target depth $\approx$ 7 nm).
Methodological Advantage: The SSD technique is complementary to existing materials processing methods and requires no additional manipulation of the qubit itself, offering a robust path toward achieving $T_{1}$-limited coherence times.
Broad Applicability: The demonstrated technique is broadly applicable to other qubit platforms afflicted by paramagnetic environments, including superconducting qubits and adatom qubits.

Technical Specifications

The following hard data points were extracted from the research paper detailing the material properties and experimental results:

Parameter	Value	Unit	Context
Diamond Isotopic Purity	99.99	%	$^{12}$C enrichment in CVD layer
CVD Layer Thickness	50	nm	Grown on (100) substrate
Substrate Thickness (Initial)	0.5	mm	Element Six electronic grade
Substrate Thickness (Final)	150	µm	After slicing and polishing
NV Center Implantation	4	keV	$^{14}$N ions
Implantation Dosage	5.2 x 10$^{10}$	ions/cm$^{2}$	Targeting shallow NVs
NV Center Depth Range	4 - 17	nm	Measured via proton NMR
Applied Magnetic Field (B$_{0}$)	382 / 281	G	Used for g = 2 spin resonance
Surface Spin Rabi Frequency ($\Omega_{\text{Rabi}}/2\pi$)	10	MHz	Used for continuous driving (Fig 2)
SQ T$_{2}$ Coherence (Undriven)	65 ± 2	µs	Hahn echo measurement
DQ T$_{2}$ Coherence (Driven)	75 ± 3	µs	Hahn echo measurement
Magnetometry Sensitivity Enhancement	5	fold	Achieved via DQ basis + SSD
Estimated Surface Spin Density	0.01 - 0.1	/nm$^{2}$	Bounded range estimate

Key Methodologies

The experiment relied on precise material engineering and advanced quantum control techniques:

Substrate Preparation: Commercial electronic grade (100) diamond was sliced and polished to 150 µm thickness. Polishing damage was mitigated by etching 1 µm using ArCl$_{2}$ plasma.
High-Purity CVD Growth: A 50-nm-thick layer of 99.99% $^{12}$C diamond was grown onto the substrate using Plasma-Enhanced Chemical Vapor Deposition (PECVD).
NV Center Creation: $^{14}$N ions were implanted at 4 keV with a dosage of 5.2 x 10$^{10}$ ions/cm$^{2}$ (7° tilt) to create near-surface NV centers (expected depth $\approx$ 7 nm).
Annealing: The sample was annealed in vacuum (< 10$^{-6}$ Torr) at 850 °C for 2.5 hours to activate the NV centers.
Surface Cleaning: Multiple rounds of acid cleaning (tri-acid mix at 230-240 °C) and oxygen annealing (400 °C) were performed to prepare the surface and minimize adsorbates.
RF/MW Control: Three separate radio-frequency (RF) signal generators were used to control the NV spin qutrit ($m_{s} = 0, \pm 1$) and the surface spin qubits ($| \uparrow \rangle_{ss}, | \downarrow \rangle_{ss}$) via a single patterned microwave waveguide on the diamond surface.
Measurement Sequence: Double Electron Electron Resonance (DEER) and Hahn echo sequences were employed, with continuous RF driving applied to the surface spins during the echo sequence to achieve motional narrowing and suppress decoherence.

6CCVD Solutions & Capabilities

The success of this research hinges on the availability of high-specification diamond materials and precision fabrication capabilities—areas where 6CCVD excels. We are uniquely positioned to supply the necessary components to replicate, extend, and scale this quantum technology.

Requirement from Research Paper	6CCVD Solution & Capability	Technical Advantage
Ultra-High Purity SCD Layer (50 nm, 99.99% $^{12}$C)	Optical Grade Single Crystal Diamond (SCD). We offer custom MPCVD growth of isotopically enriched $^{12}$C layers, ensuring minimal bulk spin noise (P1 centers) for optimal $T_{2}$ performance.	Provides the low-noise environment critical for achieving relaxation-limited coherence times in shallow NV centers.
Precision Thickness Control (50 nm layer on 150 µm substrate)	Custom Thickness & Substrates. SCD layers available from 0.1 µm up to 500 µm, and substrates up to 10mm thick.	Enables engineers to precisely tailor the material stack for specific ion implantation energies and optical integration requirements.
High-Quality Surface Finish (Required for shallow NV implantation)	Advanced Polishing Services. SCD wafers polished to achieve surface roughness Ra < 1nm.	Minimizes surface defects and reduces the density of paramagnetic surface spins, directly supporting the goals of this research.
Integrated RF/MW Delivery (Patterned waveguide used)	Custom Metalization Services. Internal capability for depositing Au, Pt, Pd, Ti, W, Cu layers.	Facilitates the integration of on-chip microwave structures necessary for high-fidelity coherent surface spin driving (SSD) and complex quantum control sequences.
Scalability and Custom Dimensions (2 mm x 2 mm plates used)	Large Area Wafers. We supply plates and wafers up to 125mm (PCD) and large-area SCD, supporting scaling from R&D to commercial production.	Allows for the transition of this coherence enhancement technique to wafer-scale quantum device fabrication.

Engineering Support

6CCVD’s in-house PhD team specializes in the material science of solid-state quantum systems. We can assist researchers and engineers in selecting the optimal diamond specifications (e.g., isotopic purity, layer thickness, and surface preparation) required for similar near-surface NV magnetometry and quantum sensing projects. Our expertise ensures that the base material supports the most demanding quantum control techniques, such as the coherent surface spin driving demonstrated here.

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

View Original Abstract

Surfaces enable useful functionalities for quantum systems, e.g., as interfaces to sensing targets, but often result in surface-induced decoherence where unpaired electron spins are common culprits. Here we show that the coherence time of a near-surface qubit is increased by coherent radio-frequency driving of surface electron spins, where we use a diamond nitrogen-vacancy (NV) center as a model qubit. This technique is complementary to other methods of suppressing decoherence and, importantly, requires no additional materials processing or control of the qubit. Further, by combining driving with the increased magnetic susceptibility of the double-quantum basis, we realize an overall fivefold sensitivity enhancement in NV magnetometry. Informed by our results, we discuss a path toward relaxation-limited coherence times for near-surface NV centers. The surface-spin driving technique presented here is broadly applicable to a wide variety of qubit platforms afflicted by surface-induced decoherence.

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevlett.123.146804