QUANTUM SENSING
Diamond quantum sensors for near-wall flow measurements
Many fluid-mechanics problems, from slip at a solid–fluid interface to the smallest scales of wall turbulence, are limited by how close to the wall measurements can reach. This project develops diamond quantum sensors to close that gap.
Nitrogen-vacancy (NV) centers are atomic-scale defects in diamond. Their spin states respond to temperature, strain, and electric and magnetic fields, and can be read out optically at room temperature. A layer of NV centers just below the surface makes the diamond wall a sensor in direct contact with the flow.
THE PLATFORM
A diamond sensor in the wall of a flow channel, read out with light and microwaves
Sensor and flow cell. A 3 × 3 × 0.25 mm diamond with a 0.5 µm NV layer sits nearly flush in the wall of an 80 mm channel (10 × 1 mm cross-section). The NV face is in contact with the water. A peristaltic pump circulates deionized water at 1–300 mL/min; PT100 sensors measure its temperature (Fig. 1).
Optics and microwaves. A pulsed 532 nm laser excites the NV layer through the back of the diamond. The red photoluminescence returns through the same 100× objective to a single-photon counter. An IQ-modulated generator drives the spins through a ring antenna; light passes through its central hole (Fig. 2).
No bias magnet. Only Earth’s field (≈46 µT, estimated) is present. The two NV spin transitions are then ≈3.55 MHz apart, so each microwave pulse drives both.
Fig. 1. Exploded view of the flow cell (Fu & Dabiri 2026, Fig. 2c).
Fig. 2. Timing, optical, and microwave subsystems, with a cross-section of the sample. Redrawn from Fu & Dabiri (2026), Fig. 1. Click to enlarge.
MEASUREMENT PROTOCOL
Light prepares the spins, microwave pulses steer them, and light reads them out
A green laser pulse initializes the spins and records a reference signal, PL₁. Three microwave pulses (π/2, π, π/2) follow, separated by free-evolution times τ. A second laser pulse reads out the result, PL₂ (Fig. 3). As 2τ is swept, PL₂/PL₁ oscillates at the echo frequency, the difference between the spin splitting D(T) and the microwave frequency, so a temperature change appears as a frequency change.
Stepping the phase of the last pulse through 0°, 180°, 90°, and 270° gives in-phase and quadrature signals. Whether the quadrature signal leads or lags gives the sign of the shift (Fig. 4).
Fig. 3. Pulse sequence with four-phase readout (schematic; nominal pulse widths 125, 250, and 125 ns). After Fu & Dabiri (2026), Fig. 5a.
Fig. 4. Measured echo signals with the microwave 0.37 MHz below (left) and 0.35 MHz above (right) D₀: the echo frequency flips sign with the microwave offset. Replotted from Fu & Dabiri (2026), Fig. 5b,c (three-point moving mean, as published).
RESULTS
Pulsed NV thermometry without a bias magnet
In short: without a bias magnet, the standard thermal-echo sequence barely responds to temperature, but a spin-echo timing does, and its response is consistent with the literature value.
The NV spin splitting D (≈2.87 GHz) shifts by about −75 kHz per °C. The study tested which pulse timings detect this shift when the microwave drive (Rabi frequency ≈2 MHz) is comparable to the ≈3.55 MHz splitting between the two transitions.
The standard thermal-echo timing (1:4:1) gave almost no temperature signal. The spin-echo timing (1:2:1) gave a clear signal, peaking at the nominal π/2 pulse (Fig. 5). An ensemble model identifies mixing between microwave-bright and dark spin states as a plausible mechanism, termed a mixing-enabled thermal echo (METE).
The echo frequency tracked the microwave offset with the expected slope of −1 (R² = 0.9995). Over 21–25 °C water temperature, the shift was −73.50 ± 2.06 kHz/°C, consistent with the literature value of −75.0 kHz/°C (Fig. 6).
These findings could support pulsed NV thermometry where bias magnets are impractical and microwave power is limited, as in biological or mobile applications.
Fig. 5. Echo amplitude versus pulse-duration scale for the spin-echo (left) and thermal-echo (right) timings; bars are ±1 standard error and the dotted line marks the nominal π/2 duration. Replotted from Fu & Dabiri (2026), Fig. 4.
Fig. 6. Echo frequency versus microwave offset (left) and shift of D versus fluid temperature (right). Replotted from Fu & Dabiri (2026), Fig. 6.
PUBLICATIONS AND PRESENTATIONS
M. K. Fu and J. O. Dabiri, arXiv:2609.22661 [quant-ph], Sep. 2026.
M. K. Fu and J. O. Dabiri, APS Global Physics Summit (March Meeting), Denver, CO, Mar. 2026; 78th Annual Meeting of the APS Division of Fluid Dynamics, Houston, TX, Nov. 2025.
M. K. Fu and J. O. Dabiri, APS Global Physics Summit (March Meeting), Anaheim, CA, Mar. 2025; 77th Annual Meeting of the APS Division of Fluid Dynamics, Salt Lake City, UT, Nov. 2024.
M. K. Fu and J. O. Dabiri, 74th Annual Meeting of the APS Division of Fluid Dynamics, Nov. 2021.