In this paper, we revisit the well-known Hong-Ou-Mandel (HOM) effect in which two photons, which meet at a beamsplitter, can interfere destructively, leading to null in coincidence counts. In a standard HOM measurement, the coincidence counts across the two output ports of the beamsplitter are monitored as the temporal delay between the two photons prior to the beamsplitter is varied, resulting in the wellknown HOM dip. We show, both theoretically and experimentally, that by leaving the delay fixed at a particular value while relying on spectrally-resolved coincidence photon-counting, we can reconstruct the HOM dip, which would have been obtained through a standard delay-scanning, non-spectrally-resolved HOM measurement. We show that our numerical reconstruction procedure exhibits a novel dispersion cancellation effects, to all orders. We discuss how our present work can lead to a drastic reduction in the time required to acquire a HOM interferogram, and specifically discuss how this could be of particular importance for the implementation of efficient quantum-optical coherence tomography devices.

Optical-coherence tomography (OCT) is a technique that employs light in order to measure the internal structure of semitransparent, e.g. biological, samples. It is based on the interference pattern of low-coherence light. Quantum-OCT (QOCT), instead, employs the correlation properties of entangled photon pairs, for example, generated by the process of spontaneous parametric downconversion (SPDC). The usual QOCT scheme uses photon pairs characterised by a joint-spectral amplitude with strict spectral anti-correlations. It has been shown that, in contrast with its classical counterpart, QOCT provides resolution enhancement and dispersion cancellation. In this paper, we revisit the theory of QOCT and extend the theoretical model so as to include photon pairs with arbitrary spectral correlations. We present experimental results that complement the theory and explain the physical underpinnings appearing in the interference pattern. In our experiment, we utilize a pump for the SPDC process ranging from continuous wave to pulsed in the femtosecond regime, and show that cross-correlation interference effects appearing for each pair of layers may be directly suppressed for a sufficiently large pump bandwidth. Our results provide insights and strategies that could guide practical implementations of QOCT.

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Quantum-optical coherence tomography (QOCT) is an optical sectioning modality based on the quantum interference of photon pairs [Nasr et al., Phys. Rev. Lett. 91, 083601 (2003)], obtained from a spontaneous parametric downconversion (SPDC) source. The promise of QOCT derives from two quantumconferred advantages when compared to equivalent classical optical coherence tomography (OCT) systems: a factor of 2 axial resolution enhancement, as well as dispersion cancellation. Despite its promise, the technique is far from being competitive with current OCT devices due to the long required acquisition times, derived from the low photon-pair emission rates. In this work, we on the one hand demonstrate a quantum optical coherence microscopy (QOCM) technique that is designed to overcome some of the limitations of previous QOCT implementations, and on the other hand test it on representative samples, including glass layers with manufactured transverse patterns and metal-coated biological specimens. In an effort to maintain as large a flux as possible, we use a collinear SPDC source, so that the entire emitted photon pair flux may contribute to the measurements, together with a multi-mode detection design. Consistent with the collinear design, we employ a Michelson interferometer with the sample placed as end-mirror in one of the interferometer arms, instead of the more typical Hong-Ou-Mandel used in QOCT implementations. In order to probe biological samples we transition from a Michelson to a Linnik interferometer by placing a microscope objective in the sample arm. In our setup, while the idler photon is collected with a multi-mode fiber, the signal photon is detected by an ICCD camera, leading to full-field transverse reconstruction through a single axial acquisition sequence. Interestingly, our setup permits concurrent OCT and QOCT trace acquisition, the former with greater counts and the latter with the benefit of quantum-conferred advantages. We 1

We present the first frequency-domain implementation of quantum optical coherence tomography, which eliminates the need for axial scanning, thus significantly speeding up data acquisition.