Broadband radio-frequency transmitter for fast nuclear spin control
At a Glance
Section titled âAt a Glanceâ| 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 |
High-Bandwidth Microcoil for Fast Nuclear Spin Control: Engineering Analysis
Section titled âHigh-Bandwidth Microcoil for Fast Nuclear Spin Control: Engineering AnalysisâExecutive Summary
Section titled âExecutive Summaryâ- Core Achievement: Demonstration of a planar multilayer microcoil system optimized for generating strong, broadband radio-frequency (RF) fields necessary for fast nuclear spin manipulation in nanoscale NMR and quantum devices.
- Performance Metrics (13C): Achieved a maximum 13C Rabi frequency of 74 kHz, enabling nuclear pi/2 rotations in 3.4 ”s. This is critical for efficient quantum gate operations.
- Extrapolated Performance (1H): The system is projected to achieve a 1H Rabi frequency of approximately 240 kHz, suggesting highly efficient homonuclear decoupling for dense proton networks.
- Broadband Capability: The untuned circuit (Design â1) maintains a -3dB bandwidth of 19.3 MHz and a Voltage Standing Wave Ratio (VSWR) below 2:1 up to 8.8 MHz, allowing simultaneous excitation of multiple nuclear species.
- Thermal Management: Optimized heat extraction using a high-thermal-conductivity CVD diamond substrate (2,300 W/mK) and non-conductive epoxy limits the temperature rise to less than 2 K, even when applying peak powers up to 280 W (at 5% duty cycle).
- Field Generation Efficiency: The coil provides a magnetic field strength of 3.6 mT/A (measured) to 4.5 mT/A (designed) at 1 mm separation, calibrated in situ using Nitrogen-Vacancy (NV) center magnetometry.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Coil Design (â1) | Planar Multilayer | N/A | 2 layers, 10 windings, Archimedean spiral |
| Inner/Outer Diameter (ID/OD) | 1.02 / 3.13 | mm | Design â1 geometry |
| Wire Thickness | 100 | ”m | Copper magnet wire |
| Design Inductance (L) | 0.77 | ”H | Design â1 (Calculated) |
| Measured -3dB Bandwidth (f3dB) | 19.3 | MHz | Design â1 (Measured S21) |
| Magnetic Field per Current (B/I) | 3.6 | mT/A | Measured in situ (Extrapolated 1 A) |
| Max 13C Rabi Frequency | 74 | kHz | Achieved at amplifier saturation |
| 13C pi/2 Rotation Time | 3.4 | ”s | Minimum achieved rotation time |
| Extrapolated 1H Rabi Frequency | 240 | kHz | Projected for 8 MHz NMR frequency |
| Coil Magnetic Field Rise Time | 8 | ns | Measured via time-resolved ODMR |
| Max Applied Peak Power | 280 | W | At 5% duty cycle |
| Maximum Dissipated Power (P) | 0.27 | W | Limit to maintain temperature rise < 2 K |
| Substrate Material | CVD Diamond | N/A | Thermal conductivity: 2,300 W/mK |
| Max Tolerated Temperature | > 370 | K | Before structural damage |
Key Methodologies
Section titled âKey MethodologiesâThe microcoil system implementation and calibration relied on specialized fabrication and advanced quantum sensing techniques:
- Planar Coil Fabrication: Microcoils were produced as Archimedean spirals using 100-”m-thick copper magnet wire, designed as a multilayer solenoid (Design â1: 2 layers, 10 windings) to balance inductance (L) and magnetic field strength (B/I).
- Optimized Thermal Anchoring: The coil was mounted onto a high-purity CVD diamond substrate (2,300 W/mK) using a non-conductive epoxy (Masterbond Supreme 18TC). This configuration was critical; simulations showed diamond minimized temperature rise, and electrical tests confirmed non-conductive mounting was necessary to prevent magnetic shorting and bandwidth degradation.
- RF Drive Circuit: An untuned, broadband 50 Ω transmission circuit was used, driven by an Arbitrary Waveform Generator (AWG) and amplified by a broadband amplifier (Rohde & Schwarz BBA150). A 50 Ω termination was used in series with the coil to match impedance and reduce the quality factor (Q).
- In-Situ AC Field Calibration (NV Magnetometry): The magnitude and transient response of the coilâs magnetic field were calibrated using optically-detected magnetic resonance (ODMR) spectroscopy of a nearby NV center in diamond.
- Field Magnitude (Bâ„): Determined by measuring the Bloch-Siegert shift of the NV electronic spin resonance under applied AC current.
- Field Magnitude (B||): Determined by measuring the amplitude of the first ODMR side band.
- Transient Response: Measured by applying a linear-ramp input signal and recording time-resolved ODMR spectra to determine the 8 ns rise time.
- Nuclear Spin Manipulation: Fast Rabi nutation experiments were performed on a single 13C nuclear spin coupled to the NV center. The sequence involved nuclear spin initialization via an amplitude-ramped NOVEL scheme, followed by the variable-duration Rabi pulse, and final detection via nuclear state tomography.
Commercial Applications
Section titled âCommercial ApplicationsâThis high-bandwidth, high-power microcoil technology is essential for advancing several fields requiring precise, fast control of nuclear spins, particularly in low-field environments:
- Nanoscale NMR Spectroscopy: Enables microfluidic analysis of small sample quantities and potential direct imaging of three-dimensional molecular structures at the single-molecule level, particularly when integrated with NV-NMR detectors.
- Quantum Computing and Spin Registers: Provides the strong, fast RF pulses required for efficient quantum gate operations and control of multi-qubit spin registers and quantum memories based on solid-state defects (like NV centers).
- Broadband NMR Systems: The untuned, broadband nature (up to 20 MHz) simplifies heteronuclear NMR schemes, allowing simultaneous excitation of multiple nuclear species (e.g., 1H, 13C, 15N) using software-defined pulse patterns uploaded to the AWG.
- High-Power RF Circuitry: The optimized thermal management structure (diamond substrate) provides a blueprint for applying high RF powers (up to 280 W peak) to micro-scale circuits while maintaining thermal stability (temperature rise < 2 K).
- Homogeneous Decoupling: The high extrapolated 1H Rabi frequency (240 kHz) is well above typical dipolar coupling frequencies, making the technology highly effective for homonuclear decoupling in solid samples.
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 âŒ20 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 Ï/2 rotations within 3.4 ÎŒs. The extrapolated values for 1H spins are about 240 kHz and 1 ÎŒs, 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.