Demonstration of dispersion-canceled quantum-optical coherence tomography

Bright ultra-broadband fiber-based biphoton source

In this Letter, we report a first, to the best of our knoqledge, experimental realization of a bright ultra-broadband (180 THz) fiber-based biphoton source with widely spectrally separated signal and idler photons. Such a two-photon source is realized due to the joint use of a broadband two-loop phase-matching of interacting light waves and high optical nonlinearity of a silica-core photonic crystal fiber. The high performance of the developed fiber source identifies it as an important and useful tool for a wide range of optical quantum applications.

Dispersion mismatch correction for evident chromatic anomaly in low coherence interferometry

The applications of ultrafast optics to biomedical microscopy have expanded rapidly in recent years, including interferometric techniques like optical coherence tomography and microscopy (OCT/OCM). The advances of ultra-high resolution OCT and the inclusion of OCT/OCM in multimodal systems combined with multiphoton microscopy have marked a transition from using pseudo-continuous broadband sources, such as superluminescent diodes, to ultrafast supercontinuum optical sources. We report anomalies in the dispersion profiles of low-coherence ultrafast pulses through long and non-identical arms of a Michelson interferometer that are well beyond group delay or third-order dispersions. This chromatic anomaly worsens the observed axial resolution and causes fringe artifacts in the reconstructed tomograms in OCT/OCM using traditional algorithms. We present DISpersion COmpensation Techniques for Evident Chromatic Anomalies (DISCOTECA) as a universal solution to address the problem of chromatic dispersion mismatch in interferometry, especially with ultrafast sources. First, we demonstrate the origin of these artifacts through the self-phase modulation of ultrafast pulses due to focusing elements in the beam path. Next, we present three solution paradigms for DISCOTECA: optical, optoelectronic, and computational, along with quantitative comparisons to traditional methods to highlight the improvements to the dynamic range and axial profile. We explain the piecewise reconstruction of the phase mismatch between the arms of the spectral-domain interferometer using a modified short-term Fourier transform algorithm inspired by spectroscopic OCT. Finally, we present a decision-making guide for evaluating the utility of DISCOTECA in interferometry and for the artifact-free reconstruction of OCT images using an ultrafast supercontinuum source for biomedical applications.

Adaptive optical imaging with entangled photons

Adaptive optics (AO) has revolutionized imaging in fields from astronomy to microscopy by correcting optical aberrations. In label-free microscopes, however, conventional AO faces limitations because of the absence of a guide star and the need to select an optimization metric specific to the sample and imaging process. Here, we propose an AO approach leveraging correlations between entangled photons to directly correct the point spread function. This guide star-free method is independent of the specimen and imaging modality. We demonstrate the imaging of biological samples in the presence of aberrations using a bright-field imaging setup operating with a source of spatially entangled photon pairs. Our approach performs better than conventional AO in correcting specific aberrations, particularly those involving substantial defocus. Our work improves AO for label-free microscopy and could play a major role in the development of quantum microscopes.

Probing exciton dynamics with spectral selectivity through the use of quantum entangled photons

Quantum light is increasingly recognized as a promising resource for developing optical measurement techniques. Particular attention has been paid to enhancing the precision of the measurements beyond classical techniques by using nonclassical correlations between quantum entangled photons. Recent advances in the quantum optics technology have made it possible to manipulate spectral and temporal properties of entangled photons, and photon correlations can facilitate the extraction of matter information with relatively simple optical systems compared to conventional schemes. In these respects, the applications of entangled photons to time-resolved spectroscopy can open new avenues for unambiguously extracting information on dynamical processes in complex molecular and materials systems. Here, we propose time-resolved spectroscopy in which specific signal contributions are selectively enhanced by harnessing nonclassical correlations of entangled photons. The entanglement time characterizes the mutual delay between an entangled twin and determines the spectral distribution of photon correlations. The entanglement time plays a dual role as the knob for controlling the accessible time region of dynamical processes and the degrees of spectral selectivity. In this sense, the role of the entanglement time is substantially equivalent to the temporal width of the classical laser pulse. The results demonstrate that the application of quantum entangled photons to time-resolved spectroscopy leads to monitoring dynamical processes in complex molecular and materials systems by selectively extracting desired signal contributions from congested spectra. We anticipate that more elaborately engineered photon states would broaden the availability of quantum light spectroscopy.

Imaging through scattering media under strong ambient light interference via the lock-in process

Scattered light imaging techniques leveraging memory effects have been extensively investigated, yet most approaches are limited to operating in predominantly dark environments. The introduction of additional optical noise disrupts the fine structure of the original speckle pattern, undermining spatial correlation and resulting in imaging failure. In this study, we present a high-performance imaging method that integrates a lock-in process to overcome this limitation. Our experimental results demonstrate that the proposed technique enables successful imaging of targets in low signal-to-background ratio (SBR) environments, even at SBR levels as low as -28.0 dB. Furthermore, the method allows for the directional separation of targets with distinct modulation frequencies. This innovative approach has the potential to significantly expand the applicability of scattering imaging techniques by eliminating the constraints of dark field environments, thereby enhancing the convenience of in vivo microscopy and daytime astronomical observations.

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.

Rapid nanometer-precision autocorrelator

The precise measurement of a target depth has applications in biophysics and nanophysics, and non-linear optical methods are sensitive to intensity changes on very small length scales. By exploiting the high sensitivity of an autocorrelator's dependency on path length, we propose a technique that achieves ≈30 nm depth precision for each pixel in 30 seconds. Our method images up-converted pulses from a non-linear crystal using a sCMOS (scientific Complementary Metal-Oxide-Semiconductor) camera and converts the intensity recorded by each pixel to a delay. By utilising statistical estimation theory and using the data from a set of 32×32 pixels, the standard error (SE) of the detected delay falls below 1 nm after 30 seconds of measurement. Numerical simulations show that this result is extremely close to what can be achieved with a shot-noise-limited source and is consistent with the precision that can be achieved with a sCMOS camera.

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.

Spectral tailoring of photon pairs from microstructured suspended-core optical fibers with liquid-filled nanochannels

We theoretically study the generation of photon pairs via spontaneous four-wave mixing (SFWM) in a liquid-filled microstructured suspended-core optical fiber. We show that it is possible to control the wavelength, group velocity, and bandwidths of the two-photon states. Our proposed fiber structure shows a large number of degrees of freedom to engineer the two-photon state. Here, we focus on the factorable state, which shows no spectral correlation in the two-photon components of the state, and allows the heralding of a single-photon pure state without the need for spectral post-filtering.

High-speed imaging of spatiotemporal correlations in Hong-Ou-Mandel interference

The Hong-Ou-Mandel interference effect lies at the heart of many emerging quantum technologies whose performance can be significantly enhanced with increasing numbers of entangled modes one could measure and thus utilize. Photon pairs generated through the process of spontaneous parametric down conversion are known to be entangled in a vast number of modes in the various degrees of freedom (DOF) the photons possess such as time, energy, and momentum, etc. Due to limitations in detection technology and techniques, often only one such DOFs can be effectively measured at a time, resulting in much lost potential. Here, we experimentally demonstrate, with the aid of a time tagging camera, high speed measurement and characterization of two-photon interference. With a data acquisition time of only a few seconds, we observe a bi-photon interference and coalescence visibility of ∼64% with potentially up to ∼2 × 10³ spatial modes. These results open up a route for practical applications of using the high dimensionality of spatiotemporal DOF in two-photon interference, and in particular, for quantum sensing and communication.

Randomly aperiodically poled LiNbO3 crystal design by Monte Carlo-Metropolis with simulated annealing optimization for ultrabroadband photon pair generation

We report a scheme for generating ultrabroadband two-photon states by spontaneous parametric downconversion (SPDC) using randomly aperiodically poled crystals designed with an optimization algorithm based on the Monte Carlo-Metropolis method with simulated annealing. A particular SPDC source is discussed, showing results of the spectral and temporal properties of the emitted two-photon states, obtaining almost transform-limited SPDC biphoton wave packets. We also analyze the effect of fabrication errors on the SPDC.

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.

Entangled photon-pair sources based on three-wave mixing in bulk crystals

Entangled photon pairs are a critical resource in quantum communication protocols ranging from quantum key distribution to teleportation. The current workhorse technique for producing photon pairs is via spontaneous parametric down conversion (SPDC) in bulk nonlinear crystals. The increased prominence of quantum networks has led to a growing interest in deployable high performance entangled photon-pair sources. This manuscript provides a review of the state-of-the-art bulk-optics-based SPDC sources with continuous wave pump and discusses some of the main considerations when building for deployment.

Broadband fiber-based entangled photon-pair source at telecom O-band

In this Letter, we report a polarization-entangled photon-pair source based on type-II spontaneous parametric downconversion at telecom O-band in periodically poled silica fiber (PPSF). The photon-pair source exhibits more than 130 nm (∼24THz) emission bandwidth centered at 1306.6 nm. The broad emission spectrum results in a short biphoton correlation time, and we experimentally demonstrate a Hong-Ou-Mandel interference dip with a full width of 26.6 fs at half-maximum. Owing to the low birefringence of the PPSF, the biphotons generated from type-II SPDC are polarization-entangled over the entire emission bandwidth, with a measured fidelity to a maximally entangled state greater than 95.4%. The biphoton source provides the broadest bandwidth entangled biphotons at O-band to our knowledge.

Dispersion cancellation in a quantum interferometer with independent single photons

A key technique to perform proper quantum information processing is to get a high visibility quantum interference between independent single photons. One of the crucial elements that affects the quantum interference is a group velocity dispersion that occurs when single photons pass through a dispersive medium. We theoretically and experimentally demonstrate that an effect of group velocity dispersion on the two-photon interference can be cancelled if two independent single photons experience the same amount of pulse broadening. This dispersion cancellation effect can be applied to a multi-path linear interferometer with multiple independent single photons. As multi-path quantum interferometers are at the heart of quantum communication, photonic quantum computing, and boson sampling applications, our work should find wide applicability in quantum information science.

Two-photon interference: the Hong-Ou-Mandel effect

Nearly 30 years ago, two-photon interference was observed, marking the beginning of a new quantum era. Indeed, two-photon interference has no classical analogue, giving it a distinct advantage for a range of applications. The peculiarities of quantum physics may now be used to our advantage to outperform classical computations, securely communicate information, simulate highly complex physical systems and increase the sensitivity of precise measurements. This separation from classical to quantum physics has motivated physicists to study two-particle interference for both fermionic and bosonic quantum objects. So far, two-particle interference has been observed with massive particles, among others, such as electrons and atoms, in addition to plasmons, demonstrating the extent of this effect to larger and more complex quantum systems. A wide array of novel applications to this quantum effect is to be expected in the future. This review will thus cover the progress and applications of two-photon (two-particle) interference over the last three decades.

Observation of second-order interference beyond the coherence time with true thermal photons

It has recently been shown that counter-intuitive Franson-like second-order interference can be observed with a pair of classically correlated pseudo thermal light beams and two separate unbalanced interferometers (UIs): the second-order interference visibility remains fixed at 1/3 even though the path length difference in each UI is increased significantly beyond the coherence length of the pseudo thermal light [Phys. Rev. Lett.119, 223603 (2017)PRLTAO0031-900710.1103/PhysRevLett.119.223603]. However, as the pseudo thermal beam itself originated from a long-coherence laser (and by using a rotating ground disk), there exists the possibility of a classical theoretical model to account for second-order interference beyond the coherence time on the long coherence time of the original laser beam. In this work, we experimentally explore this counter-intuitive phenomenon with a true thermal photon source generated via quantum thermalization, i.e., obtaining a mixed state from a pure two-photon entangled state. This experiment not only demonstrates the unique second-order coherence properties of thermal light clearly but may also open up remote sensing applications based on such effects.

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.

Exploiting chemistry and molecular systems for quantum information science

The power of chemistry to prepare new molecules and materials has driven the quest for new approaches to solve problems having global societal impact, such as in renewable energy, healthcare and information science. In the latter case, the intrinsic quantum nature of the electronic, nuclear and spin degrees of freedom in molecules offers intriguing new possibilities to advance the emerging field of quantum information science. In this Perspective, which resulted from discussions by the co-authors at a US Department of Energy workshop held in November 2018, we discuss how chemical systems and reactions can impact quantum computing, communication and sensing. Hierarchical molecular design and synthesis, from small molecules to supramolecular assemblies, combined with new spectroscopic probes of quantum coherence and theoretical modelling of complex systems, offer a broad range of possibilities to realize practical quantum information science applications.

Hybrid frequency-time spectrograph for the spectral measurement of the two-photon state

In this Letter, a hybrid frequency-time spectrograph combining a tunable optical filter and a dispersive element is presented for measurement of the spectral properties of the two-photon state. In comparison with the previous single-photon spectrograph utilizing the dispersive Fourier transformation (DFT) technique, this method is advanced since it avoids the need for additional wavelength calibration and the electronic laser trigger for coincidence measurement; therefore, its application is extended to continuous wave (CW) pumped two-photon sources. The achievable precision of the spectrum measurement has also been discussed in theory and demonstrated experimentally with a CW pumped periodically poled lithium niobate (PPLN) waveguide-based spontaneous parametric down-conversion photon source. Such a device is expected to be a versatile tool for the characterization of the frequency entangled two-photon state.

Nonlinear interference in crystal superlattices

Nonlinear interferometers with correlated photons hold promise to advance optical characterization and metrology techniques by improving their performance and affordability. These interferometers offer subshot noise phase sensitivity and enable measurements in detection-challenging regions using inexpensive and efficient components. The sensitivity of nonlinear interferometers, defined by the ability to measure small shifts of interference fringes, can be significantly enhanced by using multiple nonlinear elements, or crystal superlattices. However, to date, experiments with more than two nonlinear elements have not been realized, thus hindering the potential of nonlinear interferometers. Here, we build a nonlinear interferometer with up to five nonlinear elements, referred to as superlattices, in a highly stable and versatile configuration. We study the modification of the interference pattern for different configurations of the superlattices and perform a proof-of-concept gas sensing experiment with enhanced sensitivity. Our approach offers a viable path towards broader adoption of nonlinear interferometers with correlated photons for imaging, interferometry, and spectroscopy.

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.

Entangled Two-Photon Absorption Spectroscopy

The application of quantum states of light such as entangled photons, for example, created by parametric down conversion, has experienced tremendous progress in the almost 40 years since their first experimental realization. Initially, they were employed in the investigation of the foundations of quantum physics, such as the violation of Bell's inequalities and studies of quantum entanglement. They later emerged as basic platforms for quantum communication protocols and, in the recent experiments on single-photon interactions, in photonic quantum computation. These applications aim at the controlled manipulation of the photonic degrees of freedom, and therefore rely on simple models of matter, where the analysis is simpler. Furthermore, quantum imaging with entangled light can achieve enhanced resolution, and quantum metrology can overcome the shot noise limit for classical light. This Account focuses on an entirely different emerging class of applications using quantum light as a powerful spectroscopic tool to reveal novel information about complex molecules. These applications utilize two appealing properties of quantum light: its distinct intensity fluctuations and its nonclassical bandwidth properties. These give rise to new and surprising behavior of nonlinear optical signals. Nonclassical intensity fluctuations can enhance nonlinear optical signals relative to linear absorption. For instance, the two-photon absorption of entangled photon pairs scales linearly (rather than quadratically) in the photon flux, just like a single photon absorption. This enables nonlinear quantum spectroscopy of photosensitive, for example, biological, samples at low light intensities. We will discuss how the two-photon absorption cross section becomes a function of the photonic quantum state, which can be manipulated by properties of the entangled photon pairs. In addition, the quantum correlations in entangled photon states further influence the nonlinear signals in a variety of ways. Apart from affecting the signal's scaling with intensity, they also constitute an entirely new approach to shaping and controlling excitation pathways in molecular aggregates in a way that cannot be achieved with shaped classical pulses. This is because between the two absorption events in entangled two-photon absorption, the light and material system are entangled. Classical constraints for the simultaneous time and frequency resolution can thus be circumvented, since the two are not Fourier conjugates. Here we review the simplest manifestation of quantum light spectroscopy, two-photon absorption spectroscopy with entangled photons. This will allow us to discuss exemplarily the impact of quantum properties of light on a nonlinear optical signal and explore the opportunities for future applications.

Single-shot large field of view imaging with scattering media by spatial demultiplexing

Benefiting from the memory effect (ME) for speckle intensity correlations, only one single-shot speckle pattern can be used for the high-quality recovery of objects. However, ME gives a restriction to the field of view (FOV) for imaging with scattering media. Objects beyond the ME region cannot be recovered and produce unwanted speckle patterns, which cause reduction in the speckle contrast and recovery quality. Nevertheless, all the spatial information from a large object is embedded in a single speckle image. Here, we extract the spatial information from these unavoidable speckle patterns and enlarge the FOV of the imaging system with scattering media. Regional point spreading functions, which are fixed and only need to be recorded once for all-time use, are employed to recover corresponding spatial regions of an object by deconvolution. Then, an automatic weighted averaging in an iterative process is performed to obtain the object with significantly enlarged FOV. Our results present an important advancement of imaging techniques with strongly scattering media.

All-depth dispersion cancellation in spectral domain optical coherence tomography using numerical intensity correlations

In ultra-high resolution (UHR-) optical coherence tomography (OCT) group velocity dispersion (GVD) must be corrected for in order to approach the theoretical resolution limit. One approach promises not only compensation, but complete annihilation of even order dispersion effects, and that at all sample depths. This approach has hitherto been demonstrated with an experimentally demanding ‘balanced detection’ configuration based on using two detectors. We demonstrate intensity correlation (IC) OCT using a conventional spectral domain (SD) UHR-OCT system with a single detector. IC-SD-OCT configurations exhibit cross term ghost images and a reduced axial range, half of that of conventional SD-OCT. We demonstrate that both shortcomings can be removed by applying a generic artefact reduction algorithm and using analytic interferograms. We show the superiority of IC-SD-OCT compared to conventional SD-OCT by showing how IC-SD-OCT is able to image spatial structures behind a strongly dispersive silicon wafer. Finally, we question the resolution enhancement of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sqrt{2}$$\end{document}2 that IC-SD-OCT is often believed to have compared to SD-OCT. We show that this is simply the effect of squaring the reflectivity profile as a natural result of processing the product of two intensity spectra instead of a single spectrum.

Non-invasive imaging through strongly scattering media based on speckle pattern estimation and deconvolution

Imaging through scattering media is still a formidable challenge with widespread applications ranging from biomedical imaging to remote sensing. Recent research progresses provide several feasible solutions, which are hampered by limited complexity of targets, invasiveness of data collection process and lack of robustness for reconstruction. In this paper, we show that the complex to-be-observed targets can be non-invasively reconstructed with fine details. Training targets, which can be directly reconstructed by speckle correlation and phase retrieval, are utilized as the input of the proposed speckle pattern estimation model, in which speckle modeling and constrained least square optimization are applied to estimate the distribution of the speckle pattern. Reconstructions for to-be-observed targets are realized by deconvoluting the estimated speckle pattern from the acquired integrated intensity matrices (IIMs). The qualities of reconstructed results are ensured by the stable statistical property and memory effect of laser speckle patterns. Experimental results show that the proposed method can reconstruct complex targets in high quality and the reconstruction performance is robust even much less data are acquired.

Photon heterodyning

Single-photon interference experiments are attempted in the time domain using true single-photon streams. Self-heterodyning beats are clearly observed by letting the field associated with a single photon interfere with itself on a field-quadratic detector, which is a time analogue of Young's two-slit interference experiment. The temporal first-order coherence of single-photon fields, i.e., transient interference fringes, develops as cumulative detection events are mapped point-by-point onto a virtual capture frame by properly correlating the time-series data. The ability to single out photon counts at a designated timing paves the way for digital heterodyning with faint light for such use as phase measurement and quantum information processing.

High speed color imaging through scattering media with a large field of view

Optical imaging through complex media has many important applications. Although research progresses have been made to recover optical image through various turbid media, the widespread application of the technology is hampered by the recovery speed, requirement on specific illumination, poor image quality and limited field of view. Here we demonstrate that above-mentioned drawbacks can be essentially overcome. The realization of high speed color imaging through turbid media is successfully carried out by taking into account the media memory effect, the point spread function, the exit pupil of the optical system, and the optimized signal to noise ratio. By retrieving selected speckles with enlarged field of view, high quality image is recovered with a responding speed only determined by the frame rates of the image capturing devices. The immediate application of the technique is expected to register static and dynamic imaging under human skin to recover information with a wearable device.

Monolithic semiconductor chips as a source for broadband wavelength-multiplexed polarization entangled photons

Generating entangled photons from a monolithic chip is a major milestone towards real-life applications of optical quantum information processing including quantum key distribution and quantum computing. Ultrabroadband entangled photons are of particular interest to various applications such as quantum metrology and multi-party entanglement distribution. In this work, we demonstrate the direct generation of broadband wavelength-multiplexed polarization entangled photons from a semiconductor chip for the first time. Without the use of any off-chip compensation or interferometry, entangled photons with a signal-idler separation as large as 95 nm in the telecom band were observed. The highest concurrence of 0.98±0.01 achieved in this work is also the highest, to the best of our knowledge, comparing to all previously demonstrated semiconductor waveguide sources. This work paves the way for fully integrated, ultrabroadband sources of polarization entangled photons.

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.

Experimental Demonstration of Spectral Intensity Optical Coherence Tomography

We demonstrate experimentally spectral-domain intensity optical coherence tomography using a Mach-Zehnder interferometer with balanced detection. We show that the technique allows for a point spread function with reduced full-width at half maximum compared to conventional optical coherence tomography. The method further provides benefits similar to those of chirped-pulse interferometry in terms of dispersion cancellation but only requires a broadband incoherent source and standard detectors. The measurements are in excellent agreement with the theoretical predictions. Finally, we propose an approach that enables the elimination of potential artefacts arising from multiple interfaces.

Scattering of entangled two-photon states

We consider the scattering of entangled two-photon states from collections of small particles. We also study the related Mie problem of scattering from a sphere. In both cases, we calculate the entropy of entanglement and investigate the influence of the entanglement of the incident field on the entanglement of the scattered field.

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

State-of-the-art in retinal optical coherence tomography image analysis

Optical coherence tomography (OCT) is an emerging imaging modality that has been widely used in the field of biomedical imaging. In the recent past, it has found uses as a diagnostic tool in dermatology, cardiology, and ophthalmology. In this paper we focus on its applications in the field of ophthalmology and retinal imaging. OCT is able to non-invasively produce cross-sectional volumetric images of the tissues which can be used for analysis of tissue structure and properties. Due to the underlying physics, OCT images suffer from a granular pattern, called speckle noise, which restricts the process of interpretation. This requires specialized noise reduction techniques to eliminate the noise while preserving image details. Another major step in OCT image analysis involves the use of segmentation techniques for distinguishing between different structures, especially in retinal OCT volumes. The outcome of this step is usually thickness maps of different retinal layers which are very useful in study of normal/diseased subjects. Lastly, movements of the tissue under imaging as well as the progression of disease in the tissue affect the quality and the proper interpretation of the acquired images which require the use of different image registration techniques. This paper reviews various techniques that are currently used to process raw image data into a form that can be clearly interpreted by clinicians.

Synchronous position two-photon interference of random-phase grating

By generalizing the phase structure of the random-phase grating we recently designed [Opt. Express21, 14056 (2013)OPEXFF1094-408710.1364/OE.21.014056], we show that non-HBT type (synchronous position) two-photon grating interference can be obtained, which physically relies on groups of multiple indistinguishable two-photon paths modulated by the spatial distributions of phase modes. By properly selecting the random-phase structures, synchronous position subwavelength interference can be obtained, the period of which, in the two-photon interference domain, is decreased by a factor N (=3,4,5,6,…), depending on the slit number and random-phase structure, and the visibility of N-fold subwavelength interference fringes could be improved by increasing the slit number of the grating. The results show that modulation on two-photon paths via spatial arrangements of the phase modes offers the possibility to actively control the optical high-order coherence in the same optical scheme.

Classical imaging with undetected photons

Barreto Lemos et al. [Nature 512, 409-412 (2014)] reported an experiment in which a non-degenerate parametric downconverter and a non-degenerate optical parametric amplifier--used as a wavelength-converting phase conjugator--were employed to image object transparencies in a manner akin to ghost imaging. Their experiment, however, relied on single-photon detection, rather than the photon-coincidence measurements employed in ghost imaging with a parametric downconverter source. More importantly, their system formed images despite the photons that passed through the object never being detected. Barreto Lemos et al. interpreted their experiment as a quantum imager, as assuredly it is, owing to its downconverter's emitting entangled signal and idler beams. We show, however, that virtually all the features of their setup can be realized in a quantum-mimetic fashion using classical-state light, specifically a pair of bright pseudothermal beams possessing a phase-sensitive cross correlation. Owing to its much higher signal-to-noise ratio, our bright-source classical imager could greatly reduce image-acquisition time compared to that of Barreto Lemos et al.'s quantum system, while retaining the latter's ability to image with undetected photons.

Linear optical quantum metrology with single photons: exploiting spontaneously generated entanglement to beat the shot-noise limit

Quantum number-path entanglement is a resource for supersensitive quantum metrology and in particular provides for sub-shot-noise or even Heisenberg-limited sensitivity. However, such number-path entanglement has been thought to be resource intensive to create in the first place--typically requiring either very strong nonlinearities, or nondeterministic preparation schemes with feedforward, which are difficult to implement. Very recently, arising from the study of quantum random walks with multiphoton walkers, as well as the study of the computational complexity of passive linear optical interferometers fed with single-photon inputs, it has been shown that such passive linear optical devices generate a superexponentially large amount of number-path entanglement. A logical question to ask is whether this entanglement may be exploited for quantum metrology. We answer that question here in the affirmative by showing that a simple, passive, linear-optical interferometer--fed with only uncorrelated, single-photon inputs, coupled with simple, single-mode, disjoint photodetection--is capable of significantly beating the shot-noise limit. Our result implies a pathway forward to practical quantum metrology with readily available technology.

Scalable spatial superresolution using entangled photons

N00N states-maximally path-entangled states of N photons-exhibit spatial interference patterns sharper than any classical interference pattern. This is known as superresolution. However, even given perfectly efficient number-resolving detectors, the detection efficiency of all previous measurements of such interference would decrease exponentially with the number of photons in the N00N state, often leading to the conclusion that N00N states are unsuitable for spatial measurements. A technique known as the "optical centroid measurement" has been proposed to solve this and has been experimentally verified for photon pairs; here we present the first extension beyond two photons, measuring the superresolution fringes of two-, three-, and four-photon N00N states. Moreover, we compare the N00N-state interference to the corresponding classical superresolution interference. Although both provide the same increase in spatial frequency, the visibility of the classical fringes decreases exponentially with the number of detected photons. Our work represents an essential step forward for quantum-enhanced measurements, overcoming what was believed to be a fundamental challenge to quantum metrology.

Standoff two-color quantum ghost imaging through turbulence

Recently, a two-color quantum ghost imaging configuration was proposed by Karmakar et al. [Phys. Rev. A81, 033845 (2010)]. By illuminating an object located far away from the source and detector, with a signal beam of long wavelength to avoid absorption of short wavelengths in the atmosphere while a reference beam of short wavelength is detected locally, this imaging configuration can be appropriate for standoff sensing. In practice, the signal beam must propagate through atmosphere in the presence of serious turbulence. We analyzed theoretically the performance of this ghost imaging configuration through turbulence. Based on the Gaussian state source model and extended Huygens-Fresnel integral, a formula is derived to depict the ghost image formed through turbulence of a standoff reflective object. Numerical calculations are also given according to the formula. The results show that the image quality will be degraded by the turbulence, but the resolution can be improved by means of optimizing the wavelengths of the reference and signal beams even when the turbulence is very serious.

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.

Superresolution microscopy with quantum emitters

The optical diffraction limit imposes a bound on imaging resolution in classical optics. Over the last twenty years, many theoretical schemes have been presented for overcoming the diffraction barrier in optical imaging using quantum properties of light. Here, we demonstrate a quantum superresolution imaging method taking advantage of nonclassical light naturally produced in fluorescence microscopy due to photon antibunching, a fundamentally quantum phenomenon inhibiting simultaneous emission of multiple photons. Using a photon counting digital camera, we detect antibunching-induced second and third order intensity correlations and perform subdiffraction limited quantum imaging in a standard wide-field fluorescence microscope.

Poled-fiber source of broadband polarization-entangled photon pairs

We demonstrate broadband polarization-entangled photon pair generation in a poled fiber phase matched for Type II downconversion in the 1.5 μm telecom band. Even with signal-idler separation greater than 100 nm, we observe fringe visibilities greater than 97% and tangle greater than 0.8. A Hong-Ou-Mandel interference experiment is also used to experimentally confirm the broadband nature of the entanglement.

Dispersion-cancelled biological imaging with quantum-inspired interferometry

Quantum information science promises transformative impact over a range of key technologies in computing, communication, and sensing. A prominent example uses entangled photons to overcome the resolution-degrading effects of dispersion in the medical-imaging technology, optical coherence tomography. The quantum solution introduces new challenges: inherently low signal and artifacts, additional unwanted signal features. It has recently been shown that entanglement is not a requirement for automatic dispersion cancellation. Such classical techniques could solve the low-signal problem, however they all still suffer from artifacts. Here, we introduce a method of chirped-pulse interferometry based on shaped laser pulses, and use it to produce artifact-free, high-resolution, dispersion-cancelled images of the internal structure of a biological sample. Our work fulfills one of the promises of quantum technologies: automatic-dispersion-cancellation interferometry in biomedical imaging. It also shows how subtle differences between a quantum technique and its classical analogue may have unforeseen, yet beneficial, consequences.

Spatial second-order interference of pseudothermal light in a Hong-Ou-Mandel interferometer

The spatial second-order interference of two independent pseudothermal light beams in a Hong-Ou-Mandel interferometer is studied experimentally and theoretically. The similar cosine modulation in the second-order coherence function as the one with entangled-photon pairs in a Hong-Ou-Mandel interferometer is observed. Two-photon interference based on Feynman's path integral theory is employed to interpret the results. The experimental results and theoretical simulations agree with each other very well.

Resolution improvement in spectral-domain optical coherence tomography based on classical intensity correlations

We describe a resolution-improved spectral-domain (SD) optical coherence tomography (OCT) based on classical intensity correlations as a counterpart of the recently proposed intensity-based time-domain OCT. Similarly to quantum OCT, the setup employs the Hong-Ou-Mandel interferometer at the output of the conventional SD-OCT. It is shown theoretically that a factor-of-√2 improvement in axial resolution is obtained with this arrangement. Some unique and useful features of our proposed system are addressed.

Shaping the ultrafast temporal correlations of thermal-like photons

We show that the temporal correlations between two light beams arising from a broadband thermal-like source can be controlled in the femtosecond regime. Specifically, by introducing spectral phase-only masks in the path of one of the beams, we show that the timing and strength of the photon correlations can be programmed on demand. This example demonstrates that the interbeam second-order coherence function propagates as a phase-sensitive ultrafast wave packet in the path towards the detectors, and is thus, susceptible to be modified by acting on just one of the beams. For quite some time, it has been thought that this could only happen with sources showing time-energy entanglement. Our work shows that such a property is due to the existence of a certain type of correlation, but not necessarily the entanglement.