Reactive near-field subwavelength microwave imaging with a non-invasive Rydberg probe

TL;DR

The paper tackles the challenge that metal probes perturb reactive near fields in microwave imaging. It introduces a non-invasive, fibre-integrated Rydberg probe that uses velocity-selective EIT-AT spectroscopy to map RNF with subwavelength resolution. The authors demonstrate high-fidelity RNF maps of a horn antenna (SSIM ~0.97) and imaging of subwavelength targets, achieving ~0.62 mm resolution and significantly improved SNR compared with metal probes. This quantum-enabled sensing approach enables accurate field characterization without perturbation, with strong potential for chip diagnostics and IC testing.

Abstract

Non-invasive microwave field imaging--accurately mapping field distributions without perturbing them--is essential in areas such as aerospace engineering, biomedical imaging and integrated-circuit diagnostics. Conventional metal probes, however, inevitably perturb reactive near fields: they act as strong scatterers that drive induced currents and secondary radiation, remap evanescent components and thereby degrade both accuracy and spatial resolution, particularly in the reactive near-field regime that is most relevant to these applications. Here we demonstrate, to our knowledge for the first time, reactive near-field subwavelength imaging of microwave fields using the quantum non-demolition properties of Rydberg atoms, realized with a compact, non-invasive single-ended fibre-integrated Rydberg probe engineered to minimize field disturbance. The probe achieves an imaging resolution of {\unboldmath$λ/56$}, and the measured field distributions agree with full-wave simulations with structural similarity approaching unity, confirming both its subwavelength spatial resolution and its genuinely non-invasive character compared with conventional metal-based probes. Because the atomic sensor is intrinsically isotropic, the same device can faithfully image multi-dimensional field structures without orientation-dependent calibration. Our results therefore establish a general, non-invasive route to high-accuracy, subwavelength reactive near-field microwave imaging, with particular promise for applications such as chip-defect detection and integrated-circuit diagnostics, where even small perturbations by the probe can mask the underlying physics of interest.

Figures (5)

• Figure 1: Schematic of the RNF imaging platform.a Cesium level scheme and optical geometry. The 852 nm probe drives $|6S_{1/2}\rangle \rightarrow |6P_{3/2}\rangle$ with detuning $\Delta_{p}$; the 510 nm coupling drives $|6P_{3/2}\rangle \rightarrow |44D_{5/2}\rangle$ with detuning $\Delta_{c}$; a microwave at 8.556 GHz couples $|44D_{5/2}\rangle \rightarrow |45P_{3/2}\rangle$. b Antennas under test (AUT) and scan geometry. A standard horn (aperture $138~\mathrm{mm}\times 107~\mathrm{mm}$) and a UWB chip antenna (elliptical patch, major axis $19~\mathrm{mm}$, minor axis $9~\mathrm{mm}$) are driven; the near-field scan trajectory and Cartesian axes are indicated. c Non-invasive, single-ended fibre-integrated Rydberg probe. The 852 nm probe (orange) and 510 nm coupling (green) are delivered via two cascaded multimode circulators to the probe head. An 852 nm high-reflectivity coating (HR) on the inner surface of the cell’s rear window reflects the probe to generate a counter-propagating beam (magenta), which returns via the PBS, collimators and circulators—thereby enabling both co- and counter-propagating configurations with the coupling beam. d Robotic-arm three-dimensional imaging platform. The AUT and probe head operate inside a microwave-absorbing enclosure, while the optical setup remains outside.
• Figure 2: Microwave field mapping in the RNF of a standard-gain horn.a Schematic bounds of the reactive near field, radiating near field and far field for a rectangular horn. b Theoretical (CST-simulated) field distribution at the $\mathrm{Z}=17.5~\mathrm{mm}$ plane, used as the undisturbed-field reference. c,d Measured two-dimensional field-strength maps obtained with the Rydberg probe and, for comparison, a compact omnidirectional metal antenna with an elliptical aperture comparable to that of the cesium vapour cell (Fig. \ref{['fig1']}b) ($\mathrm{XY}$ plane at $\mathrm{Z}=17.5~\mathrm{mm}$; step sizes $\Delta \mathrm{X}=\Delta \mathrm{Y}=1~\mathrm{mm}$). e One-dimensional profiles along $\mathrm{X}$ at $\mathrm{Y}=80~\mathrm{mm}$ (step $\Delta \mathrm{X}=2~\mathrm{mm}$). f,g Pointwise difference maps relative to the simulated reference for the Rydberg probe and the metal antenna, respectively.
• Figure 3: Microwave near--field characterization and RNF--RDNF evolution of a horn antenna.a One-dimensional electric-field profiles along $\mathrm{X}$ at $\mathrm{Y}=80~\mathrm{mm}$ extracted from the Rydberg-probe measurements in b–d. b–d Measured electric-field maps obtained with the Rydberg probe at $\mathrm{Z}=17.5~\mathrm{mm}$ (RNF) and at $\mathrm{Z}=73.5~\mathrm{mm}$ and $\mathrm{Z}=123.5~\mathrm{mm}$ (RDNF). e–h Corresponding CST-simulated field maps serving as undisturbed-field references for panels a–d.
• Figure 4: Non-invasive imaging of a subwavelength target in the microwave reactive near field.a Schematic of the target-imaging configuration in the reactive near field. b–d Rydberg-atom images: background without the target (b), image with the target present (c) and background-subtracted image (d); step sizes $\Delta \mathrm{X} = 0.1~\mathrm{mm}$ and $\Delta \mathrm{Y} = 0.1~\mathrm{mm}$. e–g Corresponding images obtained with the traditional metal probe (Fig. \ref{['fig1']}b): background (e), target present (f) and background-subtracted image (g).
• Figure 5: Imaging performance metrics.a One-dimensional microwave electric-field profiles at $\mathrm{Y}=38~\mathrm{mm}$ extracted from Fig. \ref{['fig4']}b–d (Rydberg probe). b One-dimensional profiles at $\mathrm{X}=38~\mathrm{mm}$ from Fig. \ref{['fig4']}b–d. c One-dimensional profiles at $\mathrm{Y}=38~\mathrm{mm}$ from Fig. \ref{['fig4']}e–g (metal probe). d One-dimensional profiles at $\mathrm{X}=38~\mathrm{mm}$ from Fig. \ref{['fig4']}e–g.