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arXiv:2005.09842 · Scientific Reports

Interference effects in quantum-optical coherence tomography using spectrally engineered photon pairs

Pablo Yepiz-Graciano, Alí Michel Angulo Martínez, Dorilian Lopez-Mago, Gustavo Castro‐Olvera, Martha Rosete-Aguilar, Jesús Garduño-Mejı́a, Roberto Ramírez-Alarcón, Héctor Cruz Ramírez, Alfred B. U’Ren

Published Jun 20, 2019• 41 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1038/s41598-019-45088-0

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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 (Q-OCT) presents many advantages over its classical counterpart, Optical Coherence Tomography (OCT): it provides an increased axial resolution and is immune to even orders of dispersion. The core of Q-OCT is quantum interference of negatively correlated entangled photon pairs obtained in a Hong-Ou-Mandel configuration. This two-photon interference can be observed in the time domain in the form of dips or in the Fourier domain by means of a joint spectrum. The latter approach proved to be practical in the sense that it alleviated the requirement posed on light in Q-OCT to exhibit strict negative correlations, since the negative correlations can be easily extracted in the Fourier domain as the main diagonal of the joint spectrum. In this work, we investigate the use of this spectral approach in which quantum interference is obtained with classical low-intensity light pulses. We report theoretical calculations and their experimental validation and show that although such classical light is much easier to launch into an experimental system, it offers limited benefits as compared to Q-OCT based on entangled light. We analyse the differences in the characteristics of the joint spectrum obtained with entangled photons and with classical light and explain the origins of these differences.
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.
Quantum optical coherence tomography (Q-OCT) presents many advantages over its classical counterpart, optical coherence tomography (OCT), provides an increased axial resolution, and is immune to even orders of dispersion. The core of Q-OCT is the quantum interference of negatively correlated entangled photon pairs which, in the Fourier domain, are observed by means of a joint spectrum measurement. In this work, we explore the use of a spectral approach in a novel configuration where classical light pulses are employed instead of entangled photons. The intensity of these light pulses is reduced to a single photon level. We report theoretical analysis along with its experimental validation to show that although such a classical light is much easier to launch into an experimental system, it offers limited benefits compared to Q-OCT based on the entangled light. We analyze the differences in the characteristics of the joint spectrum obtained with entangled photons and with classical optical pulses and point out to the differences' source: the lack of the advantage-bringing term in the signal.
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.
Quantum information technologies harness the intrinsic nature of quantum theory to beat the limitations of the classical methods for information processing and communication. Recently, the application of quantum features to metrology has attracted much attention. Quantum optical coherence tomography (QOCT), which utilizes two-photon interference between entangled photon pairs, is a promising approach to overcome the problem with optical coherence tomography (OCT): As the resolution of OCT becomes higher, degradation of the resolution due to dispersion within the medium becomes more critical. Here we report on the realization of 0.54 μm resolution two-photon interference, which surpasses the current record resolution 0.75 μm of low-coherence interference for OCT. In addition, the resolution for QOCT showed almost no change against the dispersion of a 1 mm thickness of water inserted in the optical path, whereas the resolution for OCT dramatically degrades. For this experiment, a highly-efficient chirped quasi-phase-matched lithium tantalate device was developed using a novel 'nano-electrode-poling' technique. The results presented here represent a breakthrough for the realization of quantum protocols, including QOCT, quantum clock synchronization, and more. Our work will open up possibilities for medical and biological applications.
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.