Two-photon interference is a cornerstone of quantum optics, enabling imaging, sensing, and precision measurements that surpass classical limits. By harnessing the quantum interference of photon pairs, as demonstrated by the Hong-Ou-Mandel effect, this approach offers superior axial resolution and intrinsic dispersion cancellation, along with strong noise suppression arising from the precise temporal correlations between photons. Building on these principles, we present the theoretical framework and experimental realization of phase-dependent quantum optical coherence tomography (QOCT), a technique that employs phase-modulated two-photon interference for noninvasive morphological analysis of multilayered samples. We demonstrate that introducing controlled phase shifts to photon pairs in a Hong-Ou-Mandel interferometer effectively eliminates artifacts caused by reflections at different sample layers, thereby greatly improving the accuracy and reliability of QOCT measurements. This work advances the fundamental understanding and practical deployment of two-photon interference, overcoming a key limitation in applying QOCT to real-world applications such as biomedical imaging and materials characterization.
Two-photon interference is a cornerstone of quantum optics, enabling imaging, sensing, and precision measurements that surpass classical limits. By harnessing the quantum interference of photon pairs, as demonstrated by the Hong-Ou-Mandel effect, this approach offers superior axial resolution and intrinsic dispersion cancellation, along with strong noise suppression arising from the precise temporal correlations between photons. Building on these principles, we present the theoretical framework and experimental realization of phase-dependent quantum optical coherence tomography (QOCT), a technique that employs phase-modulated two-photon interference for noninvasive morphological analysis of multilayered samples. We demonstrate that introducing controlled phase shifts to photon pairs in a Hong-Ou-Mandel interferometer effectively eliminates artifacts caused by reflections at different sample layers, thereby greatly improving the accuracy and reliability of QOCT measurements. This work advances the fundamental understanding and practical deployment of two-photon interference, overcoming a key limitation in applying QOCT to real-world applications such as biomedical imaging and materials characterization.
The use of quantum properties of light has allowed researchers to create technologies that can outperform classical setups. In quantum imaging and sensing, the cross-correlation of photon pairs enables precise time synchronization of photons reaching the photodetectors, effectively suppressing environmental noise, and improving measurement accuracy. Building on this concept, we theoretically introduce and experimentally explore phase-dependent Hong-Ou-Mandel (HOM) interferometry. This technique employs phase-shifted indistinguishable photon pairs in a modified HOM interferometer for detection of microwave signals. We demonstrate that applying a phase shift to photon pairs via electro-optic modulation in the modified HOM interferometer causes a change in the middle interference structure, which can be used to gain information about the microwave signal causing the phase shift. In this work, we explore the quantization of the electromagnetic field, as well as nonclassical states of light. Phase estimation and metrics in quantum metrology are explored to gain insight into the benefits of utilizing these nonclassical states. An understanding of optical interferometry is built by investigating different types of interferometers, before introducing the HOM interferometer. Theory for the modified HOM interferometer is introduced, followed by an exploration of the interference structure that arises from the different path lengths in the modified system. We gathered data experimentally, showing the addition of a phase through electro-optic modulators (EOMs) leads to a change in the height of the structure. From this data, we calculate the Fisher information (FI), and find a maximum sensitivity of 58 mradians for phase difference, and 25 mV for amplitude. Additionally, we theoretically explore the FI to find optimal parameters to increase the system's sensitivity.
Quantum-optical coherence tomography (QOCT) is an optical sectioning modality based on the quantum interference of entangled photon pairs in the well-known Hong-Ou-Mandel interferometer. Despite its promise, the technique is far from being competitive with current classical 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 technique, based on full-field QOCT, which employs entangled collinear photon pairs in a Linnik interferometer 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 our setup, while the idler photon is collected with a multimode fiber, the signal photon is detected by an intensified charge-coupled device 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 hope that our current results will represent a significant step forward towards the actual applicability of QOCT, e.g., in clinical settings.
Random phase perturbations in dynamic media fundamentally limit a broad class of conventional optical imaging, causing severe motion artifacts and resolution degradation in applications from in vivo biomedical tomography to remote sensing. Here, we demonstrate that quantum optical coherence tomography (QOCT), leveraging Hong–Ou–Mandel interference of entangled photons, achieves unprecedented resilience against these disturbances and provides quantitative formulas characterizing this robustness. Theoretically, QOCT exhibits >10-fold slower visibility decay than classical optical coherence tomography (OCT) under random phase fluctuations. Experimentally, it maintains >90 % baseline visibility through turbulent media—conditions where OCT fails. Crucially, this robustness stems from intrinsic quantum properties: interference detection between indistinguishable paths of frequency-entangled identical photons eliminates random path length perturbations and provides native axial resolution doubling (versus classical OCT at identical bandwidth), with neither post-processing nor system modifications required. We validate QOCT’s capability by performing micron-scale 3D tomography of structured targets through dynamic turbid liquids, achieving high-fidelity reconstructions under strong flow turbulence (Reynolds number >5600). This work establishes a paradigm for quantum-enhanced imaging, enabling high-precision volumetric tomography in scenarios from motion artifact-free biological diagnosis to non-contact subaquatic mapping through turbulent air–water interfaces.
Quantum optical coherence tomography (QOCT) is the non-classical counterpart of optical coherence tomography (OCT), by which quantum interference of entangled photon pairs is used to show some advantages over classical OCT. Currently, almost all research on the QOCT focuses on the case with two photons and a twofold improvement in axial resolution has been demonstrated. Here, we introduce the multiple reflections strategy into the QOCT scheme, referencing the multi-pass strategy of the sequential scheme in quantum metrology, and provide a theoretical framework for multi-reflection quantum-optical coherence tomography (MQOCT). According to such a scheme, when the incident light interacts with the sample N times, the axial resolution of MQOCT is theoretically enhanced by a factor of N compared to that of QOCT under the same spectral bandwidth while preserving the dispersion cancellation capability of QOCT. Furthermore, we present a classical optical analogy of MQOCT using P -beam correlated multi-mode broadband beams and experimentally validate it. The classical optical analogy of MQOCT theoretically has the same resolution enhancement capability and dispersion cancellation capability as MQOCT while avoiding the fragility issues associated with the quantum light source which is used in MQOCT.
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.
Quantum optical coherence tomography (QOCT) offers a simple way to cancel dispersion broadening in a sample while also providing twice the resolution compared to classical OCT. However, to achieve these advantages, a bright and broadband source of entangled photon pairs is required. A simple implementation uses collinear spontaneous parametric down-conversion in a Michelson interferometer (MI), yet this autocorrelation scheme suffers from parasitic terms and sensitivity to phase noise. Here, we introduce a cross-correlation MI-based QOCT that overcomes these drawbacks, significantly advancing QOCT toward practical applications.