| Metadata | Details |
|---|
| Publication Date | 2020-07-02 |
| Journal | Physical Review X |
| Authors | Hengyun Zhou, Joon-Hee Choi, Soonwon Choi, Renate Landig, Alexander M. Douglas |
| Institutions | Universität Ulm, University of California, Berkeley |
| Citations | 99 |
| Analysis | Full AI Review Included |
- Core Achievement: Demonstrated a novel quantum metrology approach using a dense ensemble of interacting Nitrogen-Vacancy (NV) electronic spins in diamond, successfully surpassing the sensitivity limit imposed by spin-spin interactions.
- Performance Enhancement: Achieved a five-fold enhancement in spin coherence time (T2 = 7.9 µs) compared to conventional dynamical decoupling sequences (XY-8, T2 = 1.6 µs).
- Methodology: Utilized periodic pulsed manipulation (Floquet engineering) and a robust pulse sequence (Seq. B) designed via a generalized average Hamiltonian formalism.
- Fault Tolerance: The Seq. B design systematically suppresses leading-order imperfections arising from on-site disorder, spin-spin interactions, finite pulse durations, and rotation angle errors.
- Sensing Breakthrough: Implemented a new vectorial AC field sensing picture, achieving a best volume-normalized AC magnetic field sensitivity (ην) of 8.3(8) nT·µm3/2/Hz1/2.
- Future Potential: The robust sequence design is projected to enable single-digit picotesla sensitivity levels in µm3 volumes, opening new regimes for solid-state ensemble magnetometers.
| Parameter | Value | Unit | Context |
|---|
| NV Center Density (ρ) | ~15 | ppm | Total negatively-charged NV- concentration |
| On-site Disorder (σω) | 4.0 ± 0.1 | MHz | Gaussian standard deviation of linewidth |
| Dipolar Interaction Strength (J) | 35 | kHz | Typical separation of 11 nm |
| Coherence Time (T2, Seq. B) | 7.9(2) | µs | Robust interaction-decoupled performance |
| Coherence Time (T2, XY-8) | 1.6(1) | µs | Conventional dynamical decoupling limit |
| Depolarization Time (T1) | ~100 | µs | Current limit in the dense spin ensemble |
| Pulse Spacing (τ) | 25 | ns | Fixed free evolution interval |
| π-Pulse Width (tπ) | 20 | ns | Determined by Rabi frequency Ω = 25 MHz |
| Probing Volume (V) | 8.1 ± 0.9 x 10-3 | µm3 | Nanobeam structure, effective volume |
| AC Field Sensitivity (η) | 92(2) | nT/Hz1/2 | Measured minimum sensitivity (Seq. B) |
| Volume-Normalized Sensitivity (ην) | 8.3(8) | nT·µm3/2/Hz1/2 | Best achieved performance |
| Static Magnetic Field (B0) | 260 | gauss | Used to isolate a single NV group |
| Green Laser Wavelength | 532 | nm | Excitation source for initialization/readout |
| Readout Efficiency (C) | ~2.8 x 10-3 | N/A | Overall readout efficiency per NV |
| Rotation Angle Robustness | Up to ±12 | % | Range where sensitivity surpasses interaction limit |
- Diamond Synthesis and Doping: Used Type-Ib high-pressure high-temperature (HPHT) diamond. The sample was irradiated with 2 MeV electron beams (fluence 1.4 x 1019 cm-2) and simultaneously annealed at 700 - 800 °C to achieve a high NV- concentration (~15 ppm).
- Nanobeam Fabrication: A 20 µm-long nanobeam structure with a triangular cross-section was fabricated from bulk diamond via Faraday cage angled etching to improve microwave control homogeneity and confine the probing volume.
- Spin System Isolation: A static magnetic field (260 gauss) was applied parallel to a single NV crystallographic axis to isolate an effective two-level system (|ms = 0> and |ms = -1>).
- Robust Pulse Sequence Design (Seq. B): Developed using a generalized average Hamiltonian theory and a set of algebraic rules (Rules 1-4) imposed on the toggling frame matrix (F) to achieve leading-order fault-tolerant dynamical decoupling.
- Microwave Control: π/2 and π pulses were synthesized using an arbitrary waveform generator (AWG) with 83 ps temporal resolution, achieving a Rabi frequency of Ω = 25 MHz. Pulses were delivered via a coplanar waveguide.
- Vectorial AC Sensing Protocol: Implemented a phase-synchronized sensing scheme where the effective magnetic field (Beff) points along the [1, 1, 1]-direction in the toggling frame, utilizing phase accumulation along all three axes.
- Optimal Initialization and Readout: To maximize contrast (sine magnetometry), spins were initialized perpendicular to the effective field direction (e.g., [1, -1, 0] direction) and read out using an unconventional rotation axis (around the [-1, 1, 0] axis by arccos(√2/3)).
- Quantum Sensing and Metrology:
- High-sensitivity AC magnetic field sensing for applications requiring high spatial resolution (e.g., detecting weak signals in microelectronics or biological systems).
- Development of robust, high-density solid-state ensemble magnetometers that operate effectively at room temperature.
- Biochemical Analysis and Imaging:
- Enabling nanoscale Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) of small samples (µm3 volumes) with high sensitivity, crucial for analyzing complex molecules.
- Materials Science and Characterization:
- Investigation of magnetic properties and dynamics in novel condensed matter systems, including strongly correlated materials, under ambient conditions.
- Quantum Computing and Simulation:
- The robust dynamical decoupling formalism provides a systematic tool for engineering and controlling many-body Hamiltonians, applicable to various quantum hardware platforms for simulation and information processing.
- Diamond Device Engineering:
- The techniques developed for high-density NV ensembles and nanobeam structures inform the design and fabrication of next-generation diamond quantum devices with optimized photon collection and control homogeneity.
View Original Abstract
Quantum metrology makes use of coherent superpositions to detect weak\nsignals. While in principle the sensitivity can be improved by increasing the\ndensity of sensing particles, in practice this improvement is severely hindered\nby interactions between them. Using a dense ensemble of interacting electronic\nspins in diamond, we demonstrate a novel approach to quantum metrology. It is\nbased on a new method of robust quantum control, which allows us to\nsimultaneously eliminate the undesired effects associated with spin-spin\ninteractions, disorder and control imperfections, enabling a five-fold\nenhancement in coherence time compared to conventional control sequences.\nCombined with optimal initialization and readout protocols, this allows us to\nbreak the limit for AC magnetic field sensing imposed by interactions, opening\na promising avenue for the development of solid-state ensemble magnetometers\nwith unprecedented sensitivity.\n