Single-Spin Nitrogen-Vacancy Magnetometer with Enhanced Static Field Sensitivity

Abstract

Precision sensing and imaging of weak static magnetic fields are crucial for a variety of emerging nanoscale applications. While nitrogen-vacancy (NV) centers in diamond provide exceptional AC magnetic field sensitivity with nanoscale spatial resolution, their sensitivity to static (DC) magnetic fields is fundamentally limited by the short dephasing time (T2*) due to spin-spin interactions. In this work, we present a novel hybrid sensing approach that integrates a soft ferromagnetic microwire with a single near-surface NV center to amplify its response to external static magnetic fields. This hybrid configuration achieves a DC magnetic field sensitivity of 63 nT/sqrt(Hz) for a single NV center - about 500 times greater than conventional inhomogeneous broadening- or T2*-limited magnetometry, with potential for further enhancement. The compact and highly sensitive nature of this sensor opens new opportunities for quantum sensing applications involving the detection of static or slowly varying magnetic fields across diverse scientific and technological domains.

Figures (6)

• Figure 1: (a) A simplified schematic of the GMI–NV-based hybrid sensor setup is shown. A soft ferromagnetic (GMI) wire is positioned on top of a diamond wafer containing individual shallow NV centers (indicated by a red sphere with an arrow). A microscope objective, aligned through the wafer, is used to focus a green excitation laser and collect the resulting NV fluorescence. Microwave pulses, used to manipulate the NV spin states, and a radio-frequency waveform, used to activate the GMI effect, are applied simultaneously through the GMI wire. A nearby coil generates the DC test magnetic field to be detected.
• Figure 2: Electron spin resonance measurements of an individual shallow NV center in the presence of stray magnetic field from the magnetic wire. (a) ODMR spectrum. Due to the presence of stray magnetic field from the wire ($\sim$ 0.5 mT), NV center's otherwise degenerate $m_s=\pm1$ spin sub levels are split from the zero field line $D$ = 2870 MHz. Red solid line is a Lorentzian fit to the data. The two dips at a separation of $\sim$3 MHz on either side of $D$ indicate the presence unpolarised $^{15}$N nuclear spin of the NV center. (b) Ramsey interference signal exhibiting a decay with a characteristic timescale of $\text{T}_2^* = 0.69\,\mu\text{s}$. The red line represents an exponential fit to the envelope of the oscillations. Inset: Fast Fourier transform (FFT) of the Ramsey signal as a function of detuning frequency, showing a hyperfine splitting (3 MHz) corresponding to $^{15}$N nuclear spin, as identified in (a). (c) Hahn-echo measurement. The top and bottom blue traces correspond to echo signals with the phase of the initial $\frac{\pi}{2}$ pulse shifted by 180$^\circ$ between the two traces. The red lines are fits to extract coherence decay. The average coherence time $\text{T}_2$ from both fits is 21 $\mu\text{s}$.
• Figure 3: Demonstration of DC magnetic field sensitivity of the NV-GMI hybrid sensor. (a) Schematic representation of the DC magnetometry. A Hahn-echo sequence with a fixed $\tau$ was applied to the NV. The GMI wire was simultaneously driven by a RF signal whose frequency was matched to the echo period ($f_\text{ac}=1/2\tau$) (b) Oscillatory response of the magnetometer signal as a function of the amplitude of the external DC magnetic field. The $f_\text{ac}$ was set to 100 kHz. The data was fitted to a sinusoidal function. The sensitivity of the sensor is calculated from the maximum slope (black straight line) of the response.
• Figure 4: (a) NV-GMI hybrid magnetometer signal for different RF drive frequency (equivalently $1/\text{2}\tau$). Decreasing the $\tau$ gives higher signal contrast but at the expense of halved phase acquired by the NV and hence the reduced $\eta_{\text{dc}}$. (b) Number of oscillations of the magnetometer signal vs. amplitude of the RF drive voltage across the GMI wire. (c) Magnetometer signal as a function of external $\text{B}_\text{dc}$ amplitude for three different conditions: no RF drive applied to the GMI wire (magenta), RF drive with asynchronous frequency (black), and RF drive with asynchronous phase (red). For clarity, the traces are vertically offset. Note that $\text{B}_\text{dc}$ values are represented in terms of the raw current applied to the field-generating coil.
• Figure 5: Noise spectral density of the hybrid NV-GMI magnetometer. The sensor was operated at maximum sensitivity by applying the RF drive signal at optimal frequency and maximum amplitude, with the Hahn-echo sequence synchronized to the drive. The magnetically sensitive trace was recorded under the application of a small static test field.