Broadband radio-frequency transmitter for fast nuclear spin control

At a Glance

Metadata	Details
Publication Date	2020-11-01
Journal	Review of Scientific Instruments
Authors	K. Herb, J. Zopes, K. S. Cujia, C. L. Degen, K. Herb
Institutions	ETH Zurich
Citations	15
Analysis	Full AI Review Included

Technical Documentation & Analysis: High-Bandwidth Microcoil for Fast Nuclear Spin Control

Executive Summary

This research successfully demonstrates a high-performance microcoil system utilizing CVD diamond substrates to achieve rapid nuclear spin manipulation, critical for nanoscale Nitrogen-Vacancy (NV) center magnetometry and quantum computing applications.

Core Achievement: Demonstrated ¹³C nuclear spin π/2 rotations in a record 3.4 µs, corresponding to Rabi frequencies exceeding 70 kHz.
Thermal Management: CVD diamond was proven to be the optimal mounting substrate, exhibiting exceptional thermal conductivity (2,300 Wm^-1K^-1) that limits temperature rise to 10 K under high heat load (1.35 W).
High Power Handling: The optimized thermal anchoring allows the application of high peak RF power (up to 280 W at 5% duty cycle) necessary for strong magnetic fields.
Broadband Performance: The untuned, impedance-matched circuit (Design №1) achieved a broad excitation bandwidth of 19.3 MHz, enabling the simultaneous actuation of multiple nuclear species.
Application Potential: The system is ideally suited for low-field nanoscale NMR detectors, multi-qubit spin registers, and complex heteronuclear decoupling schemes (e.g., extrapolated ¹H Rabi frequency of 240 kHz).
Material Requirement: The success hinges on the use of high-purity, high-thermal conductivity MPCVD diamond plates, a core offering of 6CCVD.

Technical Specifications

The following hard data points were extracted from the microcoil design (№1) and experimental results:

Parameter	Value	Unit	Context
Coil Design (№1) Bandwidth (f_3dB)	19.3	MHz	Measured cut-off frequency
Coil Design (№1) B/I Field Strength	4.5	mT/A	Calculated on-axis field (d = 1 mm)
Measured Field Magnitude (Extrapolated)	3.6	mT/A	At 1 A current (In-situ NV calibration)
¹³C Nuclear Rabi Frequency (Max)	74	kHz	Achieved at amplifier saturation (280 W peak)
¹³C Nuclear π/2 Rotation Time	3.4	µs	Minimum rotation time achieved
Extrapolated ¹H Rabi Frequency	240	kHz	For ¹H NMR frequency of 8 MHz
Diamond Thermal Conductivity	2,300	Wm^-1K^-1	Substrate material (Superior to Cu, AlN, Sapphire)
Temperature Rise (Diamond Substrate)	10	K	Simulated rise at 1.35 W heat load
Maximum Applied Peak RF Power	280	W	Used at 5% duty cycle
Coil Magnetic Field Rise Time	8	ns	Measured exponential response time

Key Methodologies

The successful implementation of the high-bandwidth microcoil relied on optimized design, material selection, and precise calibration techniques:

Microcoil Fabrication: Planar multilayer solenoids were fabricated using 100-µm-thick copper magnet wire wound in an Archimedean spiral shape, isolated by a 20-µm-thick varnish layer.
Thermal Simulation and Substrate Selection: Finite element software (Solidworks) was used to simulate temperature distribution across four potential mounting substrates (diamond, copper, AlN, sapphire). CVD diamond was selected due to its superior thermal conductivity (2,300 Wm^-1K^-1).
Thermal Anchoring: The coil was glued to the diamond mounting plate using a high-thermal-conductivity, non-conductive epoxy (Masterbond Supreme 18TC) to ensure efficient heat extraction while preserving the high electrical bandwidth.
Electrical Characterization: Vector Network Analysis (VNA) was performed to measure the reflection parameter (S₁₁) and transmission parameter (S₂₁), confirming the 50 Ω impedance match and the 19.3 MHz bandwidth.
In-situ Magnetic Field Calibration: Pulsed Optically-Detected Magnetic Resonance (ODMR) spectroscopy on a nearby NV center in the diamond was used to calibrate the magnitude (via Bloch-Siegert shift) and the time response (via time-resolved ODMR) of the AC magnetic field.
Nuclear Spin Manipulation: Rabi nutation experiments were performed on a single ¹³C nuclear spin, demonstrating fast π/2 rotations and confirming the high magnetic field amplitude generated by the microcoil.

6CCVD Solutions & Capabilities

The performance demonstrated in this research is directly dependent on the quality and thermal properties of the CVD diamond substrate. 6CCVD is uniquely positioned to supply and enhance the materials required for replicating and advancing this work.

Applicable Materials

To replicate the high-efficiency thermal management and maintain the optical quality necessary for NV center experiments, 6CCVD recommends the following materials:

Optical Grade Single Crystal Diamond (SCD):
- Purity: Essential for minimizing defects that interfere with NV center coherence and spin initialization.
- Thermal Performance: Provides the highest thermal conductivity (up to 2,300 Wm^-1K^-1), matching the requirements for high-power RF applications.
- Surface Quality: Available with ultra-low roughness polishing (Ra < 1 nm), critical for minimizing surface strain and maximizing optical readout fidelity.
High-Purity Polycrystalline Diamond (PCD) Substrates:
- Cost-Effective Thermal Solution: Suitable for larger area applications where the highest optical transparency is not strictly required, while still offering excellent thermal properties.
- Large Dimensions: Available in plates/wafers up to 125 mm in diameter.

Customization Potential

6CCVD’s advanced fabrication capabilities allow researchers to move beyond standard geometries, enabling next-generation device integration:

Requirement from Paper	6CCVD Solution & Capability	Technical Advantage
Custom Substrate Dimensions (e.g., 10 mm x 15 mm plate, up to 10 mm thick)	Precision Laser Cutting & Grinding. We provide custom-sized SCD and PCD substrates up to 10 mm thickness.	Ensures perfect mechanical fit into complex microcoil holder assemblies and translation stages.
Integrated RF/MW Circuitry (CPW for NV drive)	Advanced Metalization Services. Internal capability for depositing Au, Pt, Pd, Ti, W, and Cu layers.	Allows for the integration of coplanar waveguides (CPW) and contact pads directly onto the diamond surface, simplifying the ODMR setup (Fig. 1a).
Optimized Coil-Sample Separation (d = 1 mm used)	Custom Thickness Control. SCD and PCD wafers available from 0.1 µm to 500 µm, and substrates up to 10 mm.	Enables precise control over the vertical distance (d) between the microcoil and the NV layer, allowing researchers to maximize the magnetic field amplitude (B) at the spin location.

Engineering Support

The successful implementation of nanoscale NMR relies heavily on balancing thermal management, electrical performance, and material purity. 6CCVD’s in-house PhD team specializes in material selection and optimization for similar NV Center Magnetometry and Solid-State Quantum Device projects. We offer consultation on:

Selecting the optimal diamond grade (SCD vs. PCD) based on required optical transmission and thermal load.
Designing custom metalization stacks for low-loss RF/MW transmission lines.
Achieving ultra-low surface roughness necessary for near-surface NV applications.

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

View Original Abstract

The active manipulation of nuclear spins with radio-frequency (RF) coils is at the heart of nuclear magnetic resonance (NMR) spectroscopy and spin-based quantum devices. Here, we present a miniature RF transmitter designed to generate strong RF pulses over a broad bandwidth, allowing for fast spin rotations on arbitrary nuclear species. Our design incorporates (i) a planar multilayer geometry that generates a large field of 4.35 mT per unit current, (ii) a 50 Ω transmission circuit with a broad excitation bandwidth of ∼20 MHz, and (iii) an optimized thermal management leading to minimal heating at the sample location. Using individual 13C nuclear spins in the vicinity of a diamond nitrogen-vacancy center as a test system, we demonstrate Rabi frequencies exceeding 70 kHz and nuclear π/2 rotations within 3.4 μs. The extrapolated values for 1H spins are about 240 kHz and 1 μs, respectively. Beyond enabling fast nuclear spin manipulations, our transmitter system is ideally suited for the incorporation of advanced pulse sequences into micro- and nanoscale NMR detectors operating at a low (&lt;1 T) magnetic field.

Tech Support

Original Source

DOI: https://doi.org/10.1063/5.0013776