Optimal frequency measurements with quantum probes
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2021-04-01 |
| Journal | npj Quantum Information |
| Authors | Simon Schmitt, Tuvia Gefen, Daniel Louzon, Christian Osterkamp, Nicolas Staudenmaier |
| Institutions | Element Six (United Kingdom), Center for Integrated Quantum Science and Technology |
| Citations | 18 |
| Analysis | Full AI Review Included |
Technical Documentation & Analysis: Optimal Quantum Frequency Measurements
Section titled âTechnical Documentation & Analysis: Optimal Quantum Frequency Measurementsâ6CCVD Material Science Analysis of npj Quantum Information (2021) 7:55
Executive Summary
Section titled âExecutive SummaryâThis research demonstrates the implementation of optimal quantum strategies for frequency discrimination and estimation, leveraging the superior properties of the Nitrogen-Vacancy (NV) center in high-purity diamond. The results establish new benchmarks for quantum spectroscopy, directly impacting the development of advanced nanoscale Nuclear Magnetic Resonance (NMR) sensors.
- Optimal Sensing Achieved: The study successfully implemented the optimal frequency discrimination protocol using a single NV center in ultrapure Single Crystal Diamond (SCD).
- Record Speed: Achieved discrimination between two frequencies separated by 2 kHz in a single 44 ”s measurement, a factor of ten below the standard Fourier limit (23 kHz).
- Quadratic Scaling: Optimal control (using Ï-pulses) was employed to maximize the angle between quantum states, resulting in a quadratic (t2) accumulation of sensor phase difference, crucial for minimizing discrimination time.
- High Sensitivity: Demonstrated a frequency sensitivity of 1.6 ”Hz/Hz2 for frequency estimation, approaching the theoretical quantum limit (0.9 ”Hz/Hz2) within a factor of 2.
- Readout Optimization: Compared Ensemble Averaging (EA) and Single-Shot Readout (SSR) strategies, finding that SSR is advantageous for long interaction times and resource-limited trials, despite current limitations due to imperfect sensor initialization fidelity (0.8).
- Foundational Impact: These protocols are foundational for high-efficiency, high-resolution nanoscale NMR spectroscopy and quantum communication applications.
Technical Specifications
Section titled âTechnical SpecificationsâThe following hard data points were extracted from the experimental results concerning optimal frequency discrimination and estimation:
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Frequency Discrimination Time (Topt) | 44 | ”s | Single coherent measurement |
| Frequency Separation (ÎÏ) | 2 | kHz | Discriminated frequency difference |
| Fourier Limit (1/T) | ~23 | kHz | Standard discrimination limit |
| Frequency Sensitivity (ÎÏ) | 1.6 | ”Hz/Hz2 | Achieved sensitivity for estimation |
| Quantum Limit (QFI) | 0.9 | ”Hz/Hz2 | Theoretical limit for estimation |
| Signal Amplitude (B) | 1.7 | ”T | Magnetic field amplitude |
| External Magnetic Field | 400 | G | Used to lift ground state degeneracy |
| Interpulse Spacing (XY8-N) | 500 | ns | Used in the control sequence |
| Readout Fidelity (SSR) | 0.8 | N/A | Limited by imperfect initialization/T1 lifetime |
| Photon Count (State | 0>) | 0.084 | N/A |
| Photon Count (State | 1>) | 0.07 | N/A |
Key Methodologies
Section titled âKey MethodologiesâThe experimental success relies on precise control over the NV center spin state and the use of high-quality diamond material.
- Sensor Platform: Utilized a single Nitrogen-Vacancy (NV) center embedded in ultrapure Single Crystal Diamond (SCD).
- Qubit Definition: Applied a 400 G magnetic field aligned along the NV symmetry axis to define the |0> and |1> ground spin states as the working qubit.
- Qubit Initialization: Initialized the qubit into a coherent superposition state prior to sensing.
- Optimal Control Sequence: Implemented the optimal frequency discrimination protocol using XY8-N sequences with a fixed interpulse spacing of 500 ns.
- Phase Acceleration: Applied control Ï-pulses whenever the sign of the differential Hamiltonian (H1 - H2) changed. This strategy ensures the maximal angle between the two possible states (orthogonality), leading to the desired quadratic (t2) phase accumulation.
- Readout Mapping: Mapped the accumulated sensor phase (Ï) into a population difference using a 90° phase-shifted pulse.
- Readout Strategies Benchmarked:
- Ensemble Averaging (EA): Repeated the sensing and optical readout sequence Nens times.
- Single-Shot Readout (SSR): Mapped the NV state onto a weakly coupled 13C ancilla spin (quantum memory), allowing NRR repetitive, quantum non-demolition (QND) readouts.
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & CapabilitiesâThe demonstrated research in optimal quantum sensing requires materials with exceptional purity, precise defect control, and advanced surface engineeringâall core competencies of 6CCVD. We provide the necessary diamond substrates and customization services to replicate, extend, and commercialize this nanoscale NMR technology.
| Research Requirement | 6CCVD Solution & Capability | Technical Advantage |
|---|---|---|
| Ultrapure Diamond Substrates (Essential for long T2/T2* coherence times) | Optical Grade Single Crystal Diamond (SCD). MPCVD grown with nitrogen concentration < 1 ppb. Available in thicknesses from 0.1 ”m to 500 ”m, and substrates up to 10 mm thick. | Guarantees the long spin coherence times (T2) required to achieve the demonstrated sub-Fourier limit sensitivity and quadratic (t2) phase accumulation. |
| Custom NV Center Engineering (Required for single-spin sensing) | Custom Doping and Defect Creation. We offer precise control over NV density via post-growth irradiation and annealing processes, optimizing for single-spin or high-density ensemble applications. | Allows researchers to tailor the NV concentration to match the specific requirements of their nanoscale NMR setup (e.g., single NV for ultimate spatial resolution or high-density ensemble for enhanced signal-to-noise ratio). |
| Integrated Qubit Control (Need for precise microwave delivery, e.g., copper wire in Fig. 2a) | Custom Metalization Services. Internal capability for depositing high-purity Au, Pt, Pd, Ti, W, and Cu layers. | Enables the integration of on-chip microwave antennas and control lines directly onto the diamond surface, facilitating the high-speed Ï-pulses necessary for optimal control protocols. |
| Scaling and Array Development (Moving beyond single-NV proof-of-concept) | Large Diameter PCD/SCD Wafers. Plates/wafers available up to 125 mm (PCD). | Supports the scaling of quantum sensors into large arrays for high-throughput magnetic imaging or commercial NMR applications. |
| Surface Quality (Critical for coupling external spins and optical readout) | Ultra-Low Roughness Polishing. SCD surfaces polished to Ra < 1 nm; inch-size PCD polished to Ra < 5 nm. | Minimizes optical scattering losses during fluorescence readout and ensures optimal proximity and coupling efficiency for surface-based nanoscale NMR experiments. |
Engineering Support
Section titled âEngineering Supportâ6CCVDâs in-house PhD team specializes in MPCVD diamond growth and post-processing for quantum applications. We can assist researchers and engineers with material selection, defect engineering, and substrate preparation for similar nanoscale NMR spectroscopy and quantum metrology projects.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.