Entanglement-inspired frequency-agile rangefinding

Abstract

Entanglement, a key feature of quantum mechanics, is recognized for its non-classical correlations which have been shown to provide significant noise resistance in single-photon rangefinding and communications. Drawing inspiration from the advantage given by energy-time entanglement, we developed an energy-time correlated source based on a classical laser that preserves the substantial noise reduction typical of quantum illumination while surpassing the quantum brightness limitation by over six orders of magnitude, making it highly suitable for practical remote sensing applications. A frequency-agile pseudo-random source is realized through fiber chromatic dispersion and pulse carving using an electro-optic intensity modulator. Operating at a faint transmission power of 48 μW, the distance between two buildings 154.8182 m apart can be measured with a precision better than 0.1 mm, under varying solar background levels and weather conditions with an integration time of only 100 ms. These trials verified the predicted noise reduction of this system, demonstrating advantages over quantum illumination-based rangefinding and highlighting its potential for practical remote sensing applications.

Fig. 1. Experimental setup.

a Experimental setup for the rangefinding field trial. EO modulator: electro-optic intensity modulator; FPGA: field-programmable gate array. b The front view shows the transmitter and receiver on the balcony, connected to the lab by two single-mode fibers. c The back view depicts the black box housing the transmitter and receiver aimed at the target (the external wall of the Wills Memorial Building (WMB)). d A 3D Google map shows the distance between the balcony (Queen’s Building) and the target (WMB), with the bright white spot in the inset indicating the illumination region of the target. Underlying map from Google Earth, captured on 19 July 2024 (Data: SIO, NOAA, U.S. Navy, NGA, GEBCO, Airbus, Landsat/Copernicus, IBCAO; © Google).

Fig. 2. Energy-time source characterization.

a Time-domain pulse traces showing the separation of three selected channels (colored regions) from the stretched pulse (gray area with black line) originating from the initial fs-laser pulse. b Spectral separation of the three energy channels (colored regions) derived from the original classical light source (gray area with black line).

Fig. 3. Nighttime characterization results.

a Return photon-counting histograms for each channel (A, B, and C) triggered at the source repetition rate of 50 MHz, displayed within one source period of 20 ns with one second integration time. Inset: magnified view of the primary signal peak. b A 200 ns excerpt of a return photon-counting histogram triggered at the pattern repetition rate of 100 kHz, integrated over one second. Peaks 1, 2 and 3 correspond to echo signal photons, modulator leaky photons, and dark counts, respectively. c Coincidence-based cross-correlation between the normalized reference electrical signal and the detected echo photons back from WMB, with an integration time of one second. The highest peak at 154.81822(5) m marks the measured range. Inset: expanded view of the main peak, showing periodic pulses from the original source. d The main peak, further magnified from the inset of c, fitted with a Gaussian function (red line).

Fig. 4. Daytime enhancement to SNR.

a Coincidence-based cross-correlation peaks C_ij between the normalized reference channels i and detected photon channels j (i, j ∈ A, B, C) collected in bright daylight (sunshine falling on WMB) and one second integration time. b Signal-to-noise ratio for one-channel (n = 1), two-channel (n = 2), and three-channel (n = 3) platforms, integrated over one second, as a function of the detected total solar background. Symbols represent the experimental data with error bars showing the standard deviation measured from 60 independent range measurements, with the fitted theoretical model shown as colored curves. The purple dashed line shows the simulated results for 80 channels using commercial dense wavelength division multiplexing (DWDM). c, d Correlation tomography extracted from the orange signal region and blue noise region in a, respectively.

Fig. 5. Field trial rangefinding results.

a–d Images of the target building (WMB) under various weather conditions (BG: Total background counts measured by three single-photon detectors). e–h Echo photon-counting histograms for each channel (A, B, and C), displayed within single source period of 20 ns for the corresponding weather scenarios, with an integration time of 100 ms. i–l Coincidence-based cross-correlated peaks under different weather conditions using one channel (n = 1), two channels (n = 2), and three channels (n = 3), integrated over 100 ms. m–p Simulation results of signal-to-noise ratio (SNR) under corresponding background level as a function of channel number, n, and the detection range. These results demonstrate that significant noise reduction can be achieved by increasing channel numbers, particularly in strong daytime solar background.

Fig. 6. Extended rangefinding field trial to over 400 m.

a A 3D Google map shows the distances between the balcony (Queen’s Building) and the targets (both Wills Memorial Building and Cabot Tower). Underlying map from Google Earth, captured on 19 July 2024 (Data: Airbus, SIO, NOAA, U.S. Navy, NGA, GEBCO; © Google). b Return photon-counting histograms for each channel (A, B, and C) from Cabot Tower triggered at the source repetition rate of 50 MHz, displayed within one source period of 20 ns with one second integration time. c Coincidence-based cross-correlated peaks using n channels (n = 1, 2 and 3, respectively), with an integration time of one second. The highest peak at 413.1151 m marks the measured range, fitted with a Gaussian function. Underlying map in a from Google Earth.