Quantum enhanced radio detection and ranging with solid spins

Abstract

The accurate radio frequency (RF) ranging and localizing of objects has benefited the researches including autonomous driving, the Internet of Things, and manufacturing. Quantum receivers have been proposed to detect the radio signal with ability that can outperform conventional measurement. As one of the most promising candidates, solid spin shows superior robustness, high spatial resolution and miniaturization. However, challenges arise from the moderate response to a high frequency RF signal. Here, by exploiting the coherent interaction between quantum sensor and RF field, we demonstrate quantum enhanced radio detection and ranging. The RF magnetic sensitivity is improved by three orders to 21 [Formula: see text], based on nanoscale quantum sensing and RF focusing. Further enhancing the response of spins to the target's position through multi-photon excitation, a ranging accuracy of 16 μm is realized with a GHz RF signal. The results pave the way for exploring quantum enhanced radar and communications with solid spins.

Fig. 1. The scheme of high accuracy RF ranging with a quantum sensor.

a Conceptual setup for the radio ranging with quantum sensor. Two RF paths with the same frequency serve as the reference (RF A) and ranging signal (RF B). The ranging signal is reflected by a target with a distance of L. Then, the free space interference signal between the two paths is confined in a microscale volume and interacts with the NV center quantum sensor. b Principle of extracting the target’s distance information. The phase of the back scattered RF pulse changes with the position of the target, as φ(L). It determines the amplitude (B_RF) of the interference between the backscattered and the reference RF pulses. Subsequently, the Rabi oscillation rate of quantum sensor will change with the position of target, as Ω(L). The position of target is finally estimated by measuring the electron spin of NV center ensemble in diamond.

Fig. 2. The quantum sensing of free space RF signal.

a Optically detected magnetic resonance (ODMR) spectrum of NV center ensemble. The circles are experimental results. The solid line is the Lorentz fit. b The Rabi oscillation is visualized by initialization and detecting the spin state with a green laser (0.7 mW power). The pulse duration is $\frac{t_{\det }}{2}$ (= 550 ns) for both the initializing and detecting. The dashed line indicates a perfect spin transition without decoherence and inhomogeneous broadening. High fluorescence emission means the spin state of m_s = 0, while low emission rate represents the spin state of m_s = ± 1. The error bars represent the standard error of measurements. c The photograph illustrated the center of nanowire-bowtie structure. d The distribution of RF field, which is imaged by detecting the spin state transition under continuous-wave RF and laser excitation. e Real time measurement of RF amplitude modulation with NV center ensemble. The amplitude of RF source is changed with a step of 0.95 nT. Each experimental data point is obtained by repeating the spin detection pulse sequence 2 × 10⁵ times. Only the reference RF is turned on here, and the power is approximately 2.3 mW.

Fig. 3. The high accuracy radio ranging with quantum sensor.

a The pulse sequence for RF ranging. Fluorescence signals of NV center ensemble before and after the RF-spin interaction are recorded as a reference I₀ and a ranging signal I(L). b The normalized ranging signal is plotted as the function of target’s position. The number N is modified by changing the RF pulse duration here. The smallest FWHM is obtained at the position where φ(L) = π (B_RF = 0). c FWHM of the ranging signal is reduced as $\frac{1}{N}$. The green squares are experimental results, while solid and dashed lines represent the estimation of 1/N and $1/\sqrt{N}$, respectively. d The optical response of NV center ensemble to the target’s position is deduced from the data in (b). e The maximum optical response dI/dL increases with the RF π pulse number beyond $\sqrt{N}$, due to the coherence of RF-spin interaction. The blue circles are experimental results, while solid and dashed lines represent the estimation of N and $\sqrt{N}$, respectively. The data of optical response in (d, e) have been normalized by I₀. Error bars in (c, e) represent the standard error of several measurements.

Fig. 4. The estimation of RF ranging accuracy.

a Time trace of the normalized ranging signal with N = 4. The configuration of RF pulse is set to obtain a local maximum optical response dI/dL, as that in Fig. 3d. b Allan deviation of the data in (a) decreases with the measurement time t_m. The deviation of ranging is calculated by dividing the deviation of fluorescence signal by the optical response dI/dL. Circles are experimental results, while dashed line represents the shot-noise limit of $1/\sqrt{t_{\mathrm{m}}}$.

Fig. 5. The schematic diagram of experimental setup for the radio ranging with NV center.

DM, long-pass dichroic mirror with an edge wavelength of 552 nm; AOM acousto-optic modulator, SPCM single-photon-counting modulator, PBS polarizing beam splitter, LP longpass filter with an edge wavelength of 633 nm; HWP half-wave plate.