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We compare the signal-to-noise ratio for different measurements that could be used for stellar interferometry. We find that single-photon sources with number-resolved detection outperform other weak local oscillator states. 

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. 

We report results of very-long-baseline interferometric imaging using distributed single photons. We demonstrate source autocorrelation reconstruction, and increased signal-to-noise ratio per detected coincidence compared to using classical states as phase reference. 

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. 

When a low flux of time-frequency-entangled photon pairs (EPP) illuminates a two-photon transition, the rate of two-photon absorption (TPA) can be enhanced considerably by the quantum nature of photon number correlations and frequency correlations. We use a quantum-theoretic derivation of entangled TPA (ETPA) and calculate an upper bound on the amount of quantum enhancement that is possible in such systems. The derived bounds indicate that in order to observe ETPA the experiments would need to operate at a combination of significantly higher rates of EPP illumination, molecular concentrations, and conventional TPA cross sections than are achieved in typical experiments. 

Recent proposals suggest that a distributed single-photon would outperform weak coherent or thermal states as a phase reference for long-baseline interferometry of dim sources. We demonstrate experimental results toward confirming this prediction. 

The theory of sum-frequency generation (SFG) as a two-photon measurement process is used to infer the role of two-photon entanglement in this process, and an experimental setup and preliminary data are presented as a way towards quantifying the dependence of SFG on entanglement.