A quantum spectrum analyzer enhanced by a nuclear spin memory

At a Glance

Metadata	Details
Publication Date	2017-08-22
Journal	npj Quantum Information
Authors	T. Rosskopf, Jonathan Zopes, J. M. Boss, Christian L. Degen
Institutions	ETH Zurich
Citations	68
Analysis	Full AI Review Included

Technical Analysis & Documentation: Memory-Enhanced Quantum Spectrum Analyzer

Executive Summary

This research successfully demonstrates a significant advancement in quantum sensing resolution using a two-qubit Nitrogen-Vacancy (NV) center sensor in single-crystal diamond (SCD). By utilizing the ¹⁵N nuclear spin as a long-lived memory qubit, the sensor overcomes the primary Fourier-limit imposed by the electronic spin’s relaxation time ($T_{1}$).

Record Resolution Achieved: Spectral resolution of 19 Hz, corresponding to an ultra-precise 2.9 ppm for external AC magnetic fields.
Performance Gain: Achieved an improvement in spectral resolution up to 100-fold compared to conventional dynamical decoupling and correlation spectroscopy methods without nuclear memory.
Memory Functionality: Extended the maximum correlation waiting time ($t$) to 45 ms by storing the electronic qubit state in the nuclear spin, significantly exceeding the electronic $T_{1}$ time (~0.5 ms).
Material Requirement: Requires high-quality, electronic-grade SCD wafers, typically isotopically pure (<0.01% ¹³C), with ultra-shallow NV center implantation (3-10 nm depth).
Application Demonstrated: Successfully applied the memory-assisted protocol to high-resolution nanoscale ¹³C Nuclear Magnetic Resonance (NMR) spectroscopy, achieving line widths of 190 Hz (74 ppm).

Technical Specifications

Data extracted from the realization of the memory-enhanced quantum spectrum analyzer.

Parameter	Value	Unit	Context
Best Spectral Resolution	19	Hz	For external AC test signals (~2.9 ppm)
Resolution Improvement Factor	Up to 100	Fold	Compared to non-memory protocols
Maximum Correlation Time ($t$)	45	ms	Limited by nuclear $T_{1,n}$
Nuclear Memory Lifetime ($T_{1,n}$)	52	ms	Measured without laser illumination
Electronic Relaxation Time ($T_{1}$)	~0.5	ms	Limits resolution without nuclear memory
¹³C NMR Line Width	190	Hz	Corresponding to ~74 ppm
NV Implantation Depth (Shallow)	3-10	nm	Created via 5 keV ¹⁵N⁺ ion implantation
DC Bias Magnetic Field	320	mT	Used for $T_{1,n}$ measurements
Electronic Spin Rabi Frequency	20-30	MHz	Achieved using selective MW pulses
¹⁵N Nuclear Spin Rabi Frequency	10-30	kHz	Enhanced by hyperfine interaction
NV Center Density Control	Two types used: 1.1% ¹³C (natural abundance) and <0.01% ¹³C (isotopically pure)	%	Material choice for noise control

Key Methodologies

The two-qubit sensor protocol hinges on advanced material engineering and precise microwave (MW) and radio frequency (RF) control sequences.

Diamond Material Preparation:
- Utilized electronic-grade SCD, including isotopically pure material (<0.01% ¹³C) to minimize environmental spin noise (¹³C bath).
- Wafers were subjected to ion implantation (5 keV ¹⁵N⁺) to create a shallow NV layer, followed by 800 °C annealing.
Surface Engineering:
- Achieved ultra-shallow NV depths (3-5 nm) via an oxygen etch at ~550 °C, crucial for nanoscale sensing applications.
- Final surface cleaning was performed via baking at 465 °C in air.
Qubit Control and Initialization:
- The electronic spin was initialized into the |0_e> state using 532 nm laser excitation.
- MW pulses (700 ns, ~0.7 MHz Rabi frequency) and RF pulses (20-50 µs) were applied via a coplanar waveguide structure to selectively address the resolved hyperfine transitions.
Memory Gate Implementation (Store/Retrieve):
- Store and retrieve operations were implemented using sequences of controlled-NOT (c-NOT) gates (based on selective spin rotations).
- The state information was transferred from the short-lived electronic spin to the long-lived ¹⁵N nuclear spin ($T_{1,n} >> T_{1}$).
Enhanced Readout:
- The nuclear memory allowed for repeated, non-destructive readouts (up to $n \approx 1000$), significantly enhancing the signal readout efficiency and reducing overall acquisition time.
Spectral Drift Correction:
- To enable long-term, high-resolution measurements (up to 48 hours total acquisition time), a tracking system was implemented to monitor and post-correct frequency drifts caused by temperature-induced changes in the external bias magnetic field (EPR tracking).

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the advanced MPCVD diamond materials and engineering services required to replicate and extend this pioneering research in high-resolution quantum spectroscopy and nanoscale NMR. Our materials ensure the ultra-low defect concentration and tailored isotopic purity critical for maximizing qubit coherence times.

Applicable Materials for NV Center Qubit Research

To achieve the long $T_{1}$ and $T_{2}$ times necessary for sub-100 Hz spectral resolution, material quality and isotopic purity are paramount.

Requirement from Paper	6CCVD Material Specification	Engineering Benefit
Isotopically Pure Host Material	Ultra-Low ¹³C SCD Wafers	We offer isotopically purified single-crystal diamond (<100 ppm ¹³C), essential for minimizing the intrinsic nuclear magnetic noise bath and enabling long coherence times ($T_{2}$) for high-resolution spectroscopy.
Electronic-Grade Quality	Optical Grade SCD (Ra < 1 nm)	Our SCD material is guaranteed to be electronic-grade, minimizing bulk defects and achieving surface polishing roughness $R_{a}$ < 1 nm, which is critical for preserving qubit integrity in shallow NV centers (3-10 nm).
Specialized NV Creation	Custom Ion Implantation Support (¹⁵N)	We partner with specialized facilities to provide SCD wafers pre-treated with custom, high-dose, shallow ¹⁵N implantation, ensuring a robust density of target NV centers for sensor array development.
Non-Planar Sensing Devices	Polycrystalline Diamond (PCD) Plates	For applications requiring larger sensing areas, 6CCVD supplies inch-size PCD plates up to 125 mm, which can be polished to $R_{a}$ < 5 nm, suitable for less coherence-sensitive, yet high-throughput, bulk spectroscopy.

Customization Potential

The experimental setup utilized precise control structures integrated onto the diamond surface. 6CCVD offers in-house services to expedite device fabrication:

Custom Metalization: We provide internal capability for depositing thin-film metals (Au, Pt, Pd, Ti, W, Cu) required for fabricating the coplanar waveguide transmission lines and control electrodes necessary for delivering selective MW and RF pulses to the NV centers.
Precision Machining: 6CCVD provides custom dimensions and laser cutting/dicing services for plates/wafers up to 125 mm, ensuring the diamond sensor chip fits precisely into existing confocal microscope and magnet setups (e.g., matching the 320 mT high-field setup used here).
Substrate Thickness: We supply robust substrates up to 10 mm thick, providing the mechanical stability required for experiments operating under high magnetic bias fields and cryogenic conditions.

Engineering Support

This research paves the way for advanced quantum register development and nanoscale detection of single molecules. 6CCVD’s in-house PhD team can assist researchers and technical engineers with material selection, surface preparation protocols (including oxygen etching), and isotopic optimization for similar nanoscale NMR and quantum sensing projects. We ensure that the diamond substrate is optimized for the intended NV creation methodology and final device performance metrics (e.g., $T_{1,n}$ optimization).

Call to Action: For custom specifications or material consultation regarding high-resolution quantum sensing or ¹⁵N implantation projects, visit 6ccvd.com or contact our engineering team directly. We offer global shipping (DDU default, DDP available) for expedited delivery of critical materials.

View Original Abstract

Abstract We realize a two-qubit sensor designed for achieving high-spectral resolution in quantum sensing experiments. Our sensor consists of an active “sensing qubit” and a long-lived “memory qubit”, implemented by the electronic and the nitrogen-15 nuclear spins of a nitrogen-vacancy center in diamond, respectively. Using state storage times of up to 45 ms, we demonstrate spectroscopy of external ac signals with a line width of 19 Hz (∼2.9 ppm) and of carbon-13 nuclear magnetic resonance signals with a line width of 190 Hz (∼74 ppm). This represents an up to 100-fold improvement in spectral resolution compared to measurements without nuclear memory.

Tech Support

Original Source

DOI: https://doi.org/10.1038/s41534-017-0030-6