Improved Electron-Nuclear Quantum Gates for Spin Sensing and Control
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2025-04-11 |
| Journal | PRX Quantum |
| Authors | Hendrik Benjamin van Ommen, G. L. van de Stolpe, N. Demetriou, Hans K. C. Beukers, J. Yun |
| Institutions | Delft University of Technology |
| Citations | 3 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This analysis summarizes the advancements in electron-nuclear quantum gates using the Dynamically Decoupled radio-frequency (DDrf) framework, primarily utilizing Nitrogen-Vacancy (NV) centers in diamond.
- Generalized Framework: The DDrf sequence was generalized to include off-resonant driving, providing a complete analytical model for predicting gate selectivity and effective Rabi frequencies (Ω).
- Sensitivity Enhancement: The optimized detuned sensing protocol achieved a 60x sensitivity enhancement for detecting weakly coupled nuclear spins, extending detection capability to hyperfine couplings as low as 115 Hz.
- Trade-off Mitigation: The research identified and provided solutions for the inherent trade-off between maintaining electron coherence (requiring short interpulse delays, τ) and maximizing the effective interaction strength (Ω).
- Detuned Gate Design: Demonstrated the ability to construct high-contrast quantum gates even when the RF driving frequency is significantly detuned, achieved by precisely updating the pulse phases.
- High-Fidelity Qubit Control: Simulations confirmed the feasibility of high-fidelity control in multiqubit registers, achieving maximum average gate fidelities of 99.7% for a six-qubit system.
- Selectivity Mechanisms: Two primary mechanisms for gate selectivity were quantified: the limited bandwidth of individual RF pulses and the constructive phase build-up enabled by the phase-increment condition.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Operating Temperature | 4 | K | Cryogenic environment for experiments |
| External Magnetic Field (Bz) | 189.1 | mT | Applied along the NV symmetry axis |
| Electron Spin Transition Frequency | 2.425 | GHz | ms=0 ↔ ms=-1 transition |
| 13C Natural Abundance | 1.1 | % | Diamond sample composition |
| Average Readout Fidelity (Favg) | 0.963(3) | - | NV electronic spin state readout |
| Maximum Experimental RF Rabi Frequency (Ω) | 1.6 | kHz | Limit due to sample heating effects |
| Maximum Simulated RF Rabi Frequency (Ω) | 10 | kHz | Upper bound used for gate optimization |
| Sensitivity Enhancement Factor | 60 | x | Achieved for weakly coupled spins (115 Hz coupling) |
| Minimum Detectable Hyperfine Coupling | 115 | Hz | Achieved using the optimized detuned sensing protocol |
| Maximum 6-Qubit Gate Fidelity (C1) | 99.7 | % | Simulated average gate fidelity |
| Nuclear Spin Coherence Time (T2*) | 10 | ms | Simulated quasistatic noise limit |
Key Methodologies
Section titled “Key Methodologies”- Sample Preparation: Experiments were conducted on a single NV center in a natural-abundance (1.1% 13C) diamond sample, grown via homoepitaxial Chemical Vapor Deposition (CVD). A Solid Immersion Lens (SIL) was milled to maximize photon collection efficiency.
- Field and Environment: A custom-built cryogenic confocal microscopy setup (4K) was used. An external magnetic field (189.1 mT) was applied along the NV symmetry axis using a permanent neodymium magnet.
- Pulse Generation and Shaping: Microwave (MW) and radio-frequency (rf) pulses were generated using a ZI HDAWG Arbitrary Waveform Generator. RF pulses were Hermite-shaped with a sin2(t) roll-on/roll-off (duration of two rf periods) to minimize signal ringing.
- Decoupling Sequence: The XY-8 sequence type was used for Dynamical Decoupling (DD) on the electron spin to mitigate low-frequency noise and pulse errors.
- DDrf Gate Implementation: The DDrf sequence interleaved electron DD pulses with nuclear RF pulses. The phase of each RF pulse was updated by a calculated increment (δφ) to ensure constructive build-up of rotations and achieve resonance.
- Gate Optimization: The optimal RF detuning (Δ1) was calculated to maximize the effective Rabi frequency (Ω) based on the interpulse delay (τ), particularly in the weak-coupling regime (Δτ less than or equal to π).
- Spectroscopy: DDrf spectroscopy was performed by sweeping both the RF frequency and the single-pulse phase increment (δφ) to map out the resonance conditions of surrounding 13C nuclear spins.
Commercial Applications
Section titled “Commercial Applications”- Quantum Computing and Qubit Registers:
- Enabling the realization of high-fidelity, selective two-qubit and multiqubit gates (up to six qubits demonstrated) in solid-state platforms (NV centers, silicon carbide, silicon defects).
- Providing a robust method for controlling nuclear spin memory qubits, essential for scalable quantum processors and quantum error correction.
- Nanoscale Nuclear Magnetic Resonance (Nano-NMR):
- The 60x sensitivity enhancement makes DDrf a promising technique for high-resolution nano-NMR, allowing for chemical structure determination at the single-molecule level.
- Applicable in biological and material science research requiring magnetic resonance imaging (MRI) or spectroscopy at ultra-small scales.
- Quantum Sensing and Metrology:
- Offers a generalized framework for designing application-specific sensing sequences, particularly for detecting weakly coupled spin ensembles or single spins outside the host crystal.
- Solid-State Defect Engineering:
- The analytical model and optimization toolbox are transferable to a wide variety of electron-nuclear spin systems, including tin-vacancy centers, silicon carbide defects, quantum dots, and rare-earth ions, accelerating the development of new quantum hardware.
View Original Abstract
The ability to sense and control nuclear spins near solid-state defects might enable a range of quantum technologies. Dynamically decoupled radio-frequency (DDrf) control offers a high degree of design flexibility and long electron-spin coherence times. However, previous studies have considered simplified models and little is known about optimal gate design and fundamental limits. Here, we develop a generalized DDrf framework that has important implications for spin sensing and control. Our analytical model, which we corroborate by experiments on a single NV center in diamond, reveals the mechanisms that govern the selectivity of gates and their effective Rabi frequencies, and enables flexible detuned gate designs. We apply these insights to numerically show a <a:math xmlns:a=“http://www.w3.org/1998/Math/MathML” display=“inline” overflow=“scroll”><a:mn>60</a:mn><a:mo>×</a:mo></a:math> sensitivity enhancement for detecting weakly coupled spins and study the optimization of quantum gates in multiqubit registers. These results advance the understanding for a broad class of gates and provide a toolbox for application-specific design, enabling improved quantum control and sensing.