Utilising NV based quantum sensing for velocimetry at the nanoscale
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-03-24 |
| Journal | Scientific Reports |
| Authors | D. Cohen, Ramil Nigmatullin, Oded Kenneth, Fedor Jelezko, Maxim Khodas |
| Institutions | Hebrew University of Jerusalem, Technion â Israel Institute of Technology |
| Citations | 20 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive Summaryâ- Core Innovation: Development of a non-intrusive, nanoscale velocimetry technique utilizing Nitrogen-Vacancy (NV) centers in diamond for quantum sensing.
- Mechanism: The NV centers measure the magnetic noise power spectrum (S(omega)) generated by the motion of unpolarized nuclear spins in the flowing liquid (nano-NMR spectroscopy).
- Key Advantage (Diffusion Dominance): The method remains effective and highly sensitive even when diffusion noise is more dominant than the drift velocity noise, a major limitation for classic velocimetry techniques.
- Performance Scaling: The scheme leverages the non-Lorentzian behavior of the power spectrum at low frequencies, achieving a sensitivity scaling proportional to the square root of the velocity (S proportional to 1 - square root(v)).
- Sensitivity Improvement: Achieves an improvement of more than three orders of magnitude in fractional uncertainty compared to estimations based on the standard Lorentzian model, particularly for low-viscosity fluids like water.
- Microfluidic Relevance: Provides crucial sensitivity to surface effects and flow profiles near boundaries, addressing the need for better understanding of flow characteristics in microfluidic and nanofluidic infrastructures.
- Implementation: The technique relies only on statistical polarization, simplifying implementation compared to classic NMR velocimetry methods requiring strong, stable magnetic field gradients and efficient nuclear spin polarization.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| NV Center Depth (d) | 5 to 15 | nm | Range used for optimal sensitivity analysis (shallow NVs). |
| Water Nuclei Density (n) | 33 | nm-3 | Used in sensitivity calculations (equivalent to 3.3 x 1028 m-3). |
| Water Self-Diffusion Coefficient (D) | 2.3 x 103 | nm2/”s | Standard experimental parameter (2.3 x 10-9 m2/s). |
| NV Ensemble Density (Upper Bound) | 1012 | cm-2 | Assumed density for ensemble sensitivity calculations. |
| Assumed Coherence Time (T2) | 100 | ”s | Coherence time assumed after dynamical decoupling. |
| BRMS (5 nm NV, Water) | ~250 | kHz/gammae | Magnetic field noise magnitude (gammae is the electron gyromagnetic ratio). |
| Dimensionless Velocity (v=1) | 150 | mm/s | Corresponds to the drift velocity where the drift time scale equals the diffusion time scale (for water, d=15 nm). |
| Optimal Depth (Water, Intermediate Freq) | 15 | nm | Depth where the power spectrum equals T2-1. |
| Fractional Velocity Uncertainty (Low Freq Regime, Ensemble) | 0.15 * square root(s / (T * V * A)) | Dimensionless | Achieved sensitivity scaling, where T is total experiment time, V is velocity, and A is NV area. |
| MD Simulation Particle Count | 31,710 | Particles | Used the Lennard Jones (LJ) fluid model for verification. |
Key Methodologies
Section titled âKey Methodologiesâ- Quantum Sensor Setup: An ensemble of NV centers is located near the diamond surface, which forms the boundary of a microfluidic flow channel.
- Sensing Principle: The NV centers act as two-level systems whose relaxation rate (Gamma) is proportional to the power spectrum (S(omega)) of the magnetic noise generated by the nuclear spins in the flowing liquid.
- Measurement Protocols:
- Decoherence Spectroscopy/Spin Relaxation: Measures the relaxation rate (Gamma) by optically probing the NV center population decay.
- T0 Term Probing (Zero Frequency): Measures the classical noise term (related to thermal polarization fluctuations) via the NVâs pure dephasing time (T2).
- T±1 Term Probing (Larmor Frequency): Measures the rotating noise term (used in nano-NMR) by introducing NV rotation at the Larmor frequency using pulsed dynamical decoupling sequences.
- Velocity Estimation Strategy:
- Correlation Function Method: Preferred for highly viscous fluids (e.g., oil) or deeper NVs where the NV coherence time (T2) is shorter than the signal correlation time (tauD).
- Power Spectrum Method: Preferred for low-viscosity fluids (e.g., water) and shallow NVs, utilizing the non-Lorentzian (square root) dependence on velocity at low frequencies for maximum sensitivity.
- Molecular Dynamics (MD) Simulation: A Lennard Jones (LJ) fluid model was used to simulate the dipole bath. The system was initialized thermally, and a constant drift velocity (v) was added. The z-component of the magnetic field Bz(t) at the NV position was computed over time to generate and fit the power spectra, validating the analytic scaling arguments.
Commercial Applications
Section titled âCommercial Applicationsâ- Microfluidics and Nanofluidics: Essential for characterizing flow profiles, especially near channel boundaries, where the no-slip boundary condition may break down in nanoscale regimes.
- Bio-medical Research (Lab-on-a-Chip): Provides high temporal and spatial resolution measurements required for advanced chemical and biological analysis platforms.
- Quantum Sensing Technology: Advances the application of NV centers as nanoscale magnetometers and NMR spectrometers for complex dynamic systems.
- Surface Science and Tribology: Enables non-intrusive investigation of fluid dynamics and boundary layer effects at solid-liquid interfaces.
- High-Resolution Nano-NMR: Extends the capability of NV-based NMR to measure physical parameters (like velocity and diffusion coefficients) in minute liquid samples.