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Quantum Communications and Quantum Imaging

Table-top demonstration of interferometric imaging using path-entangled single photons

Matthew R. Brown, Markus Allgaier, Valérian Thiel, John D. Monnier, Michael G. Raymer, Brian J. Smith

Published Oct 3, 2023• 0 citations
Publisher

Abstract

Very-long baseline interferometry has been one of the major astronomical imaging techniques used in the last century for tasks ranging from measuring diameters of stars to imaging black holes at the center of galaxies. However, the usual heterodyne technique is typically limited to radio wavelengths for the longest baselines due to fundamental noise from the local oscillator, which is used to measure the collected electric field in time at each aperture. Further, the visible and near-infrared (V-NIR) wavelengths do not easily allow such measurements due to their higher frequency; so, for optimal performance, the collected fields must be directly interfered with each other to measure the spatial correlation of the stellar light between each aperture. This implies, at V-NIR wavelengths, a practical limitation on the distance between the receivers and the brightness of stellar sources since bringing the fields together is lossy. Several theoretical proposals have promised reduction of this loss by using single photons along with quantum networks and/or quantum memories. We demonstrate a proof-of-principle, table-top experiment of one proposal by interfering path-entangled single photons generated from parametric down conversion and the light collected from a quasi-thermal source occupying a single spectral-temporal mode representing light from a star. The interference signal was then used to recover the spatial autocorrelation of two source distributions: 1 and 2 mm separated double slits. We compare this to a theoretical model and see good agreement. This model allows further comparison to other weak, non-single-photon, local-oscillator sources such as coherent states.

DOI
10.1117/12.2677724

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Very-long baseline interferometry has been one of the major astronomical imaging techniques used in the last century for tasks ranging from measuring diameters of stars to imaging black holes at the center of galaxies. However, the usual heterodyne technique is typically limited to radio wavelengths for the longest baselines due to fundamental noise from the local oscillator, which is used to measure the collected electric field in time at each aperture. Further, the visible and near-infrared (V-NIR) wavelengths do not easily allow such measurements due to their higher frequency; so, for optimal performance, the collected fields must be directly interfered with each other to measure the spatial correlation of the stellar light between each aperture. This implies, at V-NIR wavelengths, a practical limitation on the distance between the receivers and the brightness of stellar sources since bringing the fields together is lossy. Several theoretical proposals have promised reduction of this loss by using single photons along with quantum networks and/or quantum memories. We demonstrate a proof-of-principle, table-top experiment of one proposal by interfering path-entangled single photons generated from parametric down conversion and the light collected from a quasi-thermal source occupying a single spectral-temporal mode representing light from a star. The interference signal was then used to recover the spatial autocorrelation of two source distributions: 1 and 2 mm separated double slits. We compare this to a theoretical model and see good agreement. This model allows further comparison to other weak, non-single-photon, local-oscillator sources such as coherent states.
Recent proposals suggest that distributed single photons serving as a ‘non-local oscillator’ can outperform coherent states as a phase reference for long-baseline interferometric imaging of weak sources [1,2]. Such nonlocal quantum states distributed between telescopes can, in-principle, surpass the limitations of conventional interferometric-based astronomical imaging approaches for very-long baselines such as: signal-to-noise, shot noise, signal loss, and faintness of the imaged objects. Here we demonstrate in a table-top experiment, interference between a nonlocal oscillator generated by equal-path splitting of an idler photon from a pulsed, separable, parametric down conversion process and a spectrally single-mode, quasi-thermal source. We compare the single-photon nonlocal oscillator to a more conventional local oscillator with uncertain photon number. Both methods enabled reconstruction of the source’s Gaussian spatial distribution by measurement of the interference visibility as a function of baseline separation and then applying the van Cittert-Zernike theorem [3,4]. In both cases, good qualitative agreement was found with the reconstructed source width and the known source width as measured using a camera. We also report an increase of signal-to-noise per ‘faux’ stellar photon detected when heralding the idler photon. 1593 heralded (non-local oscillator) detection events led to a maximum visibility of ~17% compared to the 10412 unheralded (classical local oscillator) detection events, which gave rise to a maximum visibility of ~10% – the first instance of quantum-enhanced sensing in this context.
There have been several versions of optical wavelength astronomical imaging interferometers over the years, with the preferred form being Michelson direct detection (though Hanbury-Brown-Twiss is currently in revival). Even though it is prevalent in radio astronomy, using a common reference (e.g., a laser) is known to have a poor signal-to-noise ratio at visible wavelengths as the shot noise introduced by the reference overwhelms the considerably weaker signal collected by the telescopes. In 2012, a team of quantum physicists (Gottesman, Jennewein and Croke (GJC)) proposed a novel method for using a common optical reference that would abate the shot-noise issue: a path-entangled single-photon reference (i.e., a single photon that is split on a beam splitter). Transported to the various telescopes using a quantum network to overcome loss, the distributed single photon is then interfered with the optical field collected by the telescopes. Previously, we successfully demonstrated a proof-of-principle table-top experiment that implements the GJC protocol where we recovered the spatial autocorrelation of quasi-thermal double-slit sources in a single spectral-temporal mode where the single photon was produced by heralded parametric down conversion. Using quantum optics theory, we modeled our system and found good agreement allowing us to extend our model, and compare and contrast with similarly weak, non-single-photon reference sources (e.g., coherent states). Using the knowledge gained from this experiment, we document the plausibility of an on-sky measurement of the sun utilizing a similar phase reference.
Quantum entanglement-based imaging promises significantly increased resolution by extending the spatial separation of optical collection apertures used in very-long-baseline interferometry for astronomy and geodesy. We report a tabletop entanglement-based interferometric imaging technique that utilizes two entangled field modes serving as a phase reference between two apertures. The spatial distribution of a simulated thermal light source is determined by interfering light collected at each aperture with one of the entangled fields and performing joint measurements. This experiment demonstrates the ability of entanglement to implement interferometric imaging.
Observation of the universe demands telescopes with high resolution. In the optical band, traditional interference requires bringing interfering fields together, which limits the resolution due to the restricted length of baseline. Here we demonstrate the very long-baseline interferometer (VLBI) in optical band, where two interfering fields never met each other. In particular, we report the first quantum interference observation when the input of VLBI is single-photon state. Interference is recovered after measuring the amplitudes of photon fields and digitally processing the signals of quantum receivers. Moreover, we analyze interference in time and spectral domains for broadband thermal light input and show that the ultrahigh spectral resolution can improve the precision of radial velocity to 0.08 centimeters per second, which is 2 orders of magnitude better than that achievable at the current stage. Further, we apply the spectrally resolved interference in distinguishing two independent sources with angular resolutions beyond diffraction limit. Our investigations have a profound effect on the VLBI, quantum optics, and precision measurement.
Microarcsecond resolutions afforded by an optical-NIR array with kilometer-baselines would enable breakthrough science. However significant technology barriers exist in transporting weakly coherent photon states over these distances: primarily photon loss and phase errors. Quantum telescopy, using entangled states to link spatially separated apertures, offers a possible solution to the loss of photons. We report on an initiative launched by NSF NOIRLab in collaboration with the Center for Quantum Networks and Arizona Quantum Initiative at the University of Arizona, Tucson, to explore these concepts further. A brief description of the quantum concepts and a possible technology roadmap towards a quantum-enhanced very long baseline optical-NIR interferometric array is presented. An on-sky demonstration of measuring spatial coherence of photons with apertures linked through the simplest Gottesman protocol over short baselines and with limited phase fluctuations is envisaged as the first step.
In astronomy, interferometry of light collected by separate telescopes is often performed by physically bringing the optical paths together in the form of Young's double-slit experiment. Optical loss severely limits the efficiency of this so-called direct detection method, motivating the fundamental question of whether one can achieve a comparable performance using separate optical measurements at the two telescopes before combining the measurement results. Using quantum mechanics and estimation theory, here I show that any such spatially local measurement scheme, such as heterodyne detection, is fundamentally inferior to coherently nonlocal measurements, such as direct detection, for estimating the mutual coherence of bipartite thermal light when the average photon flux is low. This surprising result reveals an overlooked signature of quantum nonlocality in a classic optics experiment.