Two-photon quantum correlations are a resource for quantum communication, sensing, and imaging. Coincidence-based quantum imaging leverages spatiotemporal quantum correlations to form images from two-photon coincidences rather than intensity alone. To be useful beyond free-space laboratory setups, such as in access-constrained, realistic settings, quantum correlations must be distributed through compact waveguides while remaining usable after propagation across many spatial modes and at practical rates.

This thesis develops methods for waveguide-based quantum imaging by integrating and advancing three main technology components: engineered quantum light sources based on spontaneous parametric down-conversion (SPDC), optical waveguides (including disordered and multicore optical fibre), and detection with single-photon avalanche diode (SPAD) array cameras, through both time-tagging and rolling-delay compensation. We demonstrate and quantify transport of quantum spatiotemporal correlations, and implement quantum imaging in a waveguided geometry using optical fibre. In addition, we show real-time operation with continuous coincidence image frames. The main specific achievements are:

- Foundations for correlation transport.

    We model the propagation of quantum correlated SPDC two-photon states in media (continuous and discrete pictures), identifying measurable signatures of correlation conservation after transport.

    We develop a protocol that measures the waveguide point-spread function via coincidence imaging with a time-tagging SPAD array and timestamp post-processing.

    Using these tools, we define metrics to certify and quantify correlation transport through waveguides, and map its dependence on waveguide parameters.

- Waveguide-based transport of SPDC correlations

    We validate transmission of SPDC spatiotemporal correlations through a custom-fabricated transverse Anderson localization optical fibre and a commercially available multicore fibre (MCF), showing high-dimensional, mode-parallel delivery of correlations necessary for coincidence imaging protocols. We inject position anticorrelated photon pairs into the fibres, extract coincidences from SPAD array timestamps, characterize the waveguide point-spread function through the coincidence-imaging protocol and quantify preservation of correlations.

- Waveguided quantum ghost imaging.

    Building on the established quantum correlation distribution, we implement waveguided quantum ghost imaging (QGI) through a MCF using a non-degenerate signal-idler photon pair SPDC source. The idler illuminates the sample through the waveguide and is detected with a single SPAD, while the signal, which has not interacted with the sample, is imaged with spatial resolution on a SPAD array camera. We report image quality and imaging rates, and identify limits of optical resolution.

- Real-time quantum ghost imaging.

    We realize real-time, low-latency QGI (free-space and waveguided) by leveraging in-pixel asynchronous coincidence extraction in the SPAD array camera, eliminating external post-processing latency. Coincidence image frames are produced continuously, supporting live alignment and use.

Together, these results provide methods for waveguided, mode-parallel delivery and measurement of quantum correlations using optical fibre transport and SPAD array camera coincidence imaging, and thus extend quantum imaging technology towards practical waveguided settings. This may outline routes to applications in constrained environments, low photon flux regimes, and imaging at unusual wavelengths, potentially enabling new use-cases for quantum imaging, for example in the life sciences.

Tuesday December 02, 10:00 h. ICFO Auditorium

Thesis Director: Prof. Dr. Valerio Pruneri