Quantum Optical Coherence Tomography Research Articles

Integrated, bright broadband, two-colour parametric down-conversion source.

Broadband quantum light is a vital resource for quantum metrology and spectroscopy applications such as quantum optical coherence tomography or entangled two photon absorption. For entangled two photon absorption in particular, very high photon flux combined with high time-frequency entanglement is crucial for observing a signal. So far these conditions could be met by using high power lasers driving degenerate, type 0 bulk-crystal spontaneous parametric down conversion (SPDC) sources. This naturally limits the available wavelength ranges and precludes deterministic splitting of the generated output photons. In this work we demonstrate an integrated two-colour SPDC source utilising a group-velocity matched lithium niobate waveguide, reaching both exceptional brightness 1.52⋅106 p a i r s s m W G H z and large bandwidth (7.8 THz FWHM) while pumped with a few mW of continuous wave (CW) laser light. By converting a narrow band pump to broadband pulses the created photon pairs show correlation times of Δτ ≈ 120 fs while maintaining the narrow bandwidth Δωp ≪ 1 MHz of the CW pump light, yielding strong time-frequency entanglement. Furthermore our process can be adapted to a wide range of central wavelengths.

Non-collinear generation of ultra-broadband parametric fluorescence photon pairs using chirped quasi-phase matching slab waveguides.

Many optical quantum applications rely on broadband frequency correlated photon pair sources. We previously reported a scheme for collinear emission of high-efficiency and ultra-broadband photon pairs using chirped quasi-phase matching (QPM) periodically poled stoichiometric lithium tantalate (PPSLT) ridge waveguides. However, collinearly emitted photon pairs cannot be directly adopted for applications that are based on two-photon interference, such as quantum optical coherence tomography (QOCT). In this work, we developed a chirped QPM device with a slab waveguide structure. This device was designed to produce spatially separable (photon pair non-collinear emission) parametric fluorescence photon pairs with an ultra-broadband bandwidth in an extremely efficient manner. Using a non-chirped QPM slab waveguide, we observed a photon pair spectrum with a full-width-at-half-maximum (FWHM) bandwidth of 26 nm. When using a 3% chirped QPM slab waveguide, the FWHM bandwidth of the spectrum increased to 190 nm, and the base-to-base width is 308 nm. We also confirmed a generation efficiency of 2.4×106 pairs/(μW·s) using the non-chirped device, and a efficiency of 8×105 pairs/(μW·s) using the 3% chirped device under non-collinear emission conditions after single-mode fiber coupling. This is, to the best of our knowledge, the first report of frequency correlated photon pairs generation using slab waveguide device as a source. In addition, using slab waveguides as photon pair sources, we performed two-photon interference experiments with the non-chirped device and obtained a Hong-Ou-Mandel (HOM) dip with a FWHM of 7.7 μm and visibility of 98%. When using the 3% chirped device as photon pair source, the HOM measurement gave a 2 μm FWHM dip and 74% visibility.

Quantum microscopy of cells at the Heisenberg limit

Entangled biphoton sources exhibit nonclassical characteristics and have been applied to imaging techniques such as ghost imaging, quantum holography, and quantum optical coherence tomography. The development of wide-field quantum imaging to date has been hindered by low spatial resolutions, speeds, and contrast-to-noise ratios (CNRs). Here, we present quantum microscopy by coincidence (QMC) with balanced pathlengths, which enables super-resolution imaging at the Heisenberg limit with substantially higher speeds and CNRs than existing wide-field quantum imaging methods. QMC benefits from a configuration with balanced pathlengths, where a pair of entangled photons traversing symmetric paths with balanced optical pathlengths in two arms behave like a single photon with half the wavelength, leading to a two-fold resolution improvement. Concurrently, QMC resists stray light up to 155 times stronger than classical signals. The low intensity and entanglement features of biphotons in QMC promise nondestructive bioimaging. QMC advances quantum imaging to the microscopic level with significant improvements in speed and CNR toward the bioimaging of cancer cells. We experimentally and theoretically prove that the configuration with balanced pathlengths illuminates an avenue for quantum-enhanced coincidence imaging at the Heisenberg limit.

Quantum Optical Coherence Microscopy for Bioimaging 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.

High-depth-resolution imaging of dispersive samples using quantum optical coherence tomography.

Quantum optical coherence tomography (QOCT) is a promising approach to overcome the degradation of the resolution in optical coherence tomography (OCT) due to dispersion. Here, we report on an experimental demonstration of QOCT imaging in the high-resolution regime. We achieved a depth resolution of 2.5 μm, which is the highest value for QOCT imaging, to the best of our knowledge. We show that the QOCT image of a dispersive material remains clear whereas the OCT image is drastically degraded.

Quantum Wiener-Khinchin Theorem for Spectral-Domain Optical Coherence Tomography

The Wiener-Khinchin theorem, the fact that the autocorrelation function of a time process has a spectral decomposition given by its power-spectrum intensity, can be used in many disciplines. However, the applications based on a quantum counterpart of the Wiener-Khinchin theorem that provides a translation between time-energy degrees of freedom of biphoton wave function still remains relatively unexplored. Here, we use a quantum Wiener-Khinchin theorem (QWKT) to state that two-photon joint spectral intensity and the cross-correlation of two-photon temporal signal can be connected by making a Fourier transform. The mathematically defined QWKT is experimentally demonstrated in frequency-entangled two-photon Hong-Ou-Mandel (HOM) interference with the assistance of spectrally resolved detection. We apply this method to spectral-domain quantum optical coherence tomography that detects thickness-induced optical delays in a transparent sample, and show that our method suffices to achieve great advantages in measurement precision within a wide dynamic range and capturing time over the conventional HOM interferometric schemes. These results may significantly facilitate the use of QWKT for quantum information processing and quantum interferometric spectroscopy.

Denoising of Fourier domain quantum optical coherence tomography spectrums based on deep-learning methods

One of the promising optical coherence tomography schemes using quantum light is the Fourier domain quantum optical coherence tomography. It combines the advantage of quantum metrology and the optical coherence tomography, providing a great detection with a high axial resolution. However, the application of the Fourier domain quantum optical coherence tomography is hard to implement since various types of noise would affect the quality and finally eliminate the advantages. In this paper, we quantitively analyze the affection of the noise induced by quantum fluctuation on Fourier domain quantum optical coherence tomography and propose to suppress the effect by deep-learning method. Our simulation shows that it could severely lower the accuracy of the detection, and can be removed by our deep-learning model. We believe that our results will promote the application of similar quantum optical coherence tomography strategies to real scenarios.

Artefact-removal algorithms for Fourier domain quantum optical coherence tomography

Quantum Optical Coherence Tomography (Q-OCT) is a non-classical equivalent of Optical Coherence Tomography and is able to provide a twofold axial resolution increase and immunity to resolution-degrading dispersion. The main drawback of Q-OCT are artefacts which are additional elements that clutter an A-scan and lead to a complete loss of structural information for multilayered objects. Whereas there are very practical and successful methods for artefact removal in Time-domain Q-OCT, no such scheme has been devised for Fourier-domain Q-OCT (Fd-Q-OCT), although the latter modality—through joint spectrum detection—outputs a lot of useful information on both the system and the imaged object. Here, we propose two algorithms which process a Fd-Q-OCT joint spectrum into an artefact-free A-scan. We present the theoretical background of these algorithms and show their performance on computer-generated data. The limitations of both algorithms with regards to the experimental system and the imaged object are discussed.

Birefringence and scattering characterization using polarization sensitive quantum optical coherence tomography.

Polarization sensitive quantum optical coherence tomography (PS-QOCT) is used to characterize birefringence, scattering/absorption and path length differences in a flat transparent plastic sample. In this Letter, we present the first, to the best of our knowledge, experimental technique of PS-QOCT imaging and characterization of a birefringent material. Polarization sensitive coincidence rate of the entangled photons measured for each pixel and the 2D cross-sectional images are reconstructed. The technique described in this work has many potential applications in the characterization of micro and nano structures, including biological samples.

Intensity correlation OCT is a classical mimic of quantum OCT providing up to twofold resolution improvement

Quantum Optical Coherence Tomography (Q-OCT) uses quantum properties of light to provide several advantages over its classical counterpart, OCT: it achieves a twice better axial resolution with the same spectral bandwidth and it is immune to even orders of dispersion. Since these features are very sought-after in OCT imaging, many hardware and software techniques have been created to mimic the quantum behaviour of light and achieve these features using traditional OCT systems. The most recent, purely algorithmic scheme—an improved version of Intensity Correlation Spectral Domain OCT named ICA-SD-OCT—showed even-order dispersion cancellation and reduction of artefacts. The true capabilities of this method were unfortunately severely undermined, both in terms of its relation to Q-OCT and its main performance parameters. In this work, we provide experimental demonstrations as well as numerical and analytical arguments to show that ICA-SD-OCT is a true classical equivalent of Q-OCT, more specifically its Fourier domain version, and therefore it enables a true two-fold axial resolution improvement. We believe that clarification of all the misconceptions about this very promising algorithm will highlight the great value of this method for OCT and consequently lead to its practical applications for resolution- and quality-enhanced OCT imaging.

Bright 547-dimensional Hilbert-space entangled resource in 28-pair modes biphoton frequency comb from a reconfigurable silicon microring resonator

High-dimensional entanglement provides valuable resources for quantum technologies, including quantum communication, quantum optical coherence tomography, and quantum computing. Obtaining a high brightness and dimensional entanglement source has significant value. Here we utilize a tunable asymmetric Mach–Zehnder interferometer coupled silicon microring resonator with 100 GHz free spectral range to achieve this goal. With the strategy of the tunable coupler, the dynamical and extensive tuning range of quality factors of the microring can be obtained, and then the biphoton pair generation rate can be optimized. By selecting and characterizing 28 pairs from a more than 30-pair modes biphoton frequency comb, we obtain a Schmidt number of at least 23.4 and on-chip pair generation rate of 19.9 MHz/mW2 under a low on-chip pump power, which corresponds to 547 dimensions Hilbert space in frequency freedom. These results will prompt the wide applications of quantum frequency comb and boost the further large density and scalable on-chip quantum information processing.

Cross-sectional imaging through scattering media by quantum-mimetic optical coherence tomography with wavefront shaping

Quantum-mimetic (QM) optical coherence tomography (OCT) combined with wavefront shaping is proposed as a novel approach to achieve high-resolution cross-sectional imaging through a strongly scattering medium. Specifically, QM-OCT is a recently devised OCT technique mimicking quantum OCT to acquire its main characteristic features, most notably immunity to group-velocity dispersion and enhancement of axial resolution, without quantum light. To validate the proposed method, we demonstrate experimentally that a one-dimensional cross-sectional image of a sample located behind a diffuser plate is obtained successfully and efficiently with QM-OCT by shaping the wavefront of light incident onto the diffuser.

Fourier domain quantum optical coherence tomography.

Quantum optical coherence tomography (Q-OCT) is the non-classical counterpart of optical coherence tomography (OCT), a high-resolution 3D imaging technique based on white-light interferometry. Because Q-OCT uses a source of frequency-entangled photon pairs, not only is the axial resolution not affected by dispersion mismatch in the interferometer but is also inherently improved by a factor of two. Unfortunately, practical applications of Q-OCT are hindered by image-scrambling artefacts and slow acquisition times. Here, we present a theoretical analysis of a novel approach that is free of these problems: Fourier domain Q-OCT (Fd-Q-OCT). Based on a photon pair coincidence detection as in the standard Q-OCT configuration, it also discerns each photon pair by their wavelength. We show that all the information about the internal structures of the object is encoded in the joint spectrum and can be easily retrieved through Fourier transformation. No depth scanning is required, making our technique potentially faster than standard Q-OCT. Finally, we show that the data available in the joint spectrum enables artefact removal and discuss prospective algorithms for doing so.

Spectrally resolved Hong–Ou–Mandel interferometry for quantum-optical coherence tomography

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 well-known 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 effect, 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.

Experimental demonstration of full-field quantum optical coherence tomography

We present, to the best of our knowledge, the first implementation of full-field quantum optical coherence tomography (FF-QOCT). In our system, we are able to obtain full three-dimensional (3D) information about the internal structure of a sample under study by relying on transversely resolved Hong–Ou–Mandel (HOM) interferometry with the help of an intensified CCD (ICCD) camera. Our system requires a single axial scan, obtaining full-field transverse single-photon intensity in coincidence with the detection of the sibling photon for each value of the signal-idler temporal delay. We believe that this capability constitutes a significant step forward toward the implementation of QOCT as a practical biomedical imaging technique.

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

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.

Practical implementation of spectral-intensity dispersion-canceled optical coherence tomography with artifact suppression

Dispersion-canceled optical coherence tomography (OCT) based on spectral intensity interferometry was devised as a classical counterpart of quantum OCT to enhance the basic performance of conventional OCT. In this paper, we demonstrate experimentally that an alternative method of realizing this kind of OCT by means of two optical fiber couplers and a single spectrometer is a more practical and reliable option than the existing methods proposed previously. Furthermore, we develop a recipe for reducing multiple artifacts simultaneously on the basis of simple averaging and verify experimentally that it works successfully in the sense that all the artifacts are mitigated effectively and only the true signals carrying structural information about the sample survive.

Classical realization of dispersion-canceled, artifact-free, and background-free optical coherence tomography.

Quantum-optical coherence tomography (Q-OCT) provides a dispersion-canceled axial-imaging method, but its practical use is limited by the weakness of the light source and by artifacts in the images. A recent study using chirped-pulse interferometry (CPI) has demonstrated dispersion-canceled and artifact-free OCT with a classical system; however, unwanted background signals still remain after removing the artifacts. Here, we propose a classical optical method that realizes dispersion-canceled, artifact-free, and background-free OCT. We employ a time-reversed system for Q-OCT with transform-limited input laser pulses to achieve dispersion-canceled OCT with a classical system. We have also introduced a subtraction method to remove artifacts and background signals. With these methods, we experimentally demonstrated dispersion-canceled, artifact-free, and background-free axial imaging of a coverglass and cross-sectional imaging of the surface of a coin.

0.54 μm resolution two-photon interference with dispersion cancellation for quantum optical coherence tomography.

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

Intensity-interferometric spectral-domain optical coherence tomography with dispersion cancellation

We describe a technique for cancelling group-velocity dispersion in spectral-domain (SD) optical coherence tomography (OCT) based on classical intensity correlations. As a classical analogue of quantum OCT, a Hong-Ou-Mandel interferometer is combined with a conventional SD-OCT setup, and correlations between different spectral intensities are calculated. It is shown theoretically that a simple computational procedure used in SD-OCT enables scanless cross-sectional imaging with both dispersion cancellation and a factor-of-√2 resolution enhancement. The method involves no ultrafast detectors and works with common light sources.