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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. 

Errors are the fundamental barrier to the development of quantum systems. Quantum networks are complex systems formed by the interconnection of multiple components and suffer from error accumulation. Characterizing errors introduced by quantum network components becomes a fundamental task to overcome their depleting effects in quantum communication. Quantum Network Tomography (QNT) addresses end-to-end characterization of link errors in quantum networks. It is a tool for building error-aware applications, network management, and system validation. We provide an overview of QNT and its initial results for characterizing quantum star networks. We apply a previously defined QNT protocol for estimating bit-flip channels to estimate depolarizing channels. We analyze the performance of our estimators numerically by assessing the Quantum Cramèr-Rao Bound (QCRB) and the Mean Square Error (MSE) in the finite sample regime. Finally, we provide a discussion on current challenges in the field of QNT and elicit exciting research directions for future investigation. 

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. 

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.