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Bulletin of the Lebedev Physics Institute

Application of a Tunable Fiber Optic Source of Biphotons to Determine the Spectral Dependence of the Second-Order Cross-Correlation Function

Maksim A. Smirnov, A. F. Khairullin, Sergey A. Moiseev

Published Dec 1, 2025• 0 citations
Publisher

Abstract

Abstract Quantum-entangled two-photon fields (biphotons) are attracting increasing interest in practical applications based on high photon correlation. Using a tunable fiber optic biphoton source, it was demonstrated that the performance of a quantum optical system at various wavelengths can be evaluated by correlation measurements. It was established that the results of such measurements, particularly those related to the efficiency of the detectors used, depend significantly on the optical losses in the system.

DOI
10.3103/s1068335625604728

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•Jan 1, 2015•0 citations
Abstract Quantum-entangled two-photon fields (biphotons) are attracting increasing interest in practical applications based on high photon correlation. Using a tunable fiber optic biphoton source, it was demonstrated that the performance of a quantum optical system at various wavelengths can be evaluated by correlation measurements. It was established that the results of such measurements, particularly those related to the efficiency of the detectors used, depend significantly on the optical losses in the system.
Fibre optics is a mature technology that revolutionized telecommunication. Quantum photonics is an emerging technology still under development. The fusion of the two can bring technologically-robust solutions for quantum applications. Here we give one concrete example of a fiber-optic entangled photon source. Free of alignment, fiber coupling or any birefringence compensating schemes, it is arguably world’s simplest polarization-entangled photon source, and perfectly compatible with the vast fibre-optic network infrastructure. Furthermore, using it to form a fibre-based nonlinear interferometer results in the coupling of the spectral domain with the polarization domain, and tunability in both domains. By manipulating the dispersion and birefringence properties of the linear medium in the interferometer, we experimentally demonstrate altering the biphoton spectrum through biphoton interference, as well as tuning the degree of polarization entanglement. Due to the low-birefringence nature of both the nonlinear and linear media in the interferometer, we can preserve the high-quality polarization entanglement while shaping the biphoton spectrum. Additionally, the coupling of the polarization degree of freedom with the spectral degree of freedom, allowing us to tune the degree of entanglement by simply tuning wavelength. Our fibre-based quantum entanglement sources demonstrate a great deal of flexibility in entanglement engineering for future fibre-optic quantum networks.
We demonstrate a scheme for frequency correlated biphoton spectroscopy using a strongly non-degenerate downconversion source and a tunable upconversion detector. In this scheme, the spectral function at one wavelength range of a remote object can be reproduced by locally measuring another wavelength range using an upconversion detector and monitoring the coincidence counts. The spectral resolution of the system is better than 0.1 nm, corresponding to the acceptance width of the upconversion detector, while the measurement range is determined by the linewidth of the source.
Nonlinear spectroscopy with quantum entangled photons is an emerging field of research that holds the promise to achieve superior signal-to-noise ratio and effectively isolate many-body interactions. Photon sources used for this purpose, however, lack the frequency tunability and spectral bandwidth demanded by contemporary molecular materials. Here, we present design strategies for efficient spontaneous parametric downconversion to generate biphoton states with adequate spectral bandwidth and at visible wavelengths. Importantly, we demonstrate, by suitable design of the nonlinear optical interaction, the scope to engineer the degree of spectral correlations between the photons of the pair. We also present an experimental methodology to effectively characterize such spectral correlations. Importantly, we believe that such a characterization tool can be effectively adapted as a spectroscopy platform to optically probe system-bath interactions in materials.
Advancements in the field of compact quantum photonics have enabled researchers to reliably generate and detect spectral and temporal correlated biphotons, opening avenues for diverse applications in quantum technology. Although many routine quantum interferometry and entanglement generation experiments are performed worldwide, only a few studies discuss how different experimental detection and correlation measurement parameters can significantly influence the raw outcomes of these experiments. In this work, we analytically and experimentally elucidate the impact of quantum efficiency and dead time of single-photon detectors, and bin width of the time correlator on the coincidence-to-accidental ratio (CAR) and hence the quantum temporal cross-correlations (gsi2(0)g_{si}^{2}(0)gsi2​(0)) of photon pairs generated at 1560 nm via a type-II MgO:ppLN ridge waveguide-based source. We experimentally achieved a high cross-correlation of up togsi2(0)=∼104g_{si}^{2}(0)=\sim 10^{4}gsi2​(0)=∼104at different detection settings, thus revealing the high-quality of photon-pair correlations, surpassing several state-of-the-art biphoton sources. Additionally, we demonstrate how this analysis anticipates the effect of afterpulsing, temporal distinguishability between signal and idler photons, and temporal resolution of the correlation analysis. We further use this analysis for application in Hong-Ou-Mandel (HOM) interferometry and achieve raw visibility∼99±1 %\sim 99\pm 1\ \%∼99±1 %. To the best of our knowledge, such high raw HOM visibility has not been previously reported in a fiber-based type-II SPDC configuration. This study is important for understanding the quantum source and finding the optimal detection settings to operate it on for its real-time practical implementation in various quantum optical experiments.
The second-order correlation function which characterizes the quantum correlation between the optical and terahertz photons generated under spontaneous parametric down-conversion has been measured for the first time. A scheme of experiment based on the analysis of analog signals from terahertz and optical detectors has been proposed. Various approaches to exclude the effect of electronic and thermal noise on the measured correlation level have been discussed. The results make it possible to expand quantum optical technologies such as quantum calibration of detectors, fabrication of single-photon sources, and imaging with single-pixel detectors to the terahertz range.
The creation and manipulation of biphotons is important for many applications in quantum optics and quantum information. Topics that benefit from efficient biphoton sources range from the most fundamental quantum science experiments to the highly applied fields of quantum communication and quantum computation. Biphoton sources have long been hailed as one of the leading methods for creating entangled photon pairs for tests of Bell's inequality, creating heralded photon pairs that are used in on-demand single-photon sources and heralded measurement techniques, and for quantum communication protocols to name a few. Specifically for quantum communication, biphoton sources are commonly used for cutting edge quantum key distribution (QKD) protocols. In the first part of the thesis, I focus on realizing an efficient biphoton source that produces high yield photon pairs. More specifically, I develop an optimized biphoton source using the nonlinear optical process of spontaneous parametric down-conversion in a second-order nonlinear crystal. I develop a formalism for predicting the two important metrics of a biphoton source: the heralding efficiency and joint count rate. I show how, from a large parameter space, one can tailor the phase matching of the nonlinear interaction to create a high quality biphoton source that produces both high heralding efficiency and high joint count rate. I achieve heralding efficiencies of 86$\\pm555\\%$ and joint count rates of 2.58$\\pm0.6kHzpermWpumppower.Ishowthatusingacollinearnondegenerategeometryallowsforheraldingefficienciesofupto99.70.6 kHz per mW pump power. I show that using a collinear nondegenerate geometry allows for heralding efficiencies of up to 99.70.6kHzpermWpumppower.Ishowthatusingacollinearnondegenerategeometryallowsforheraldingefficienciesofupto99.7\\%$ assuming no loss in the system. I verify the theoretical model with experimental results and find good agreement. In the second part of the thesis I turn to manipulating the single photons born from the biphoton source for applications in creating single-photon spectrometers and time-frequency QKD systems. The security of QKD is only guaranteed if the two parties have access to a set of states called mutually unbiased states. I create a set of these states in time and their conjugate states in the frequency basis and show that I can manipulate single photon correlations in time and frequency so that an eavesdropper can be detected if she localizes a photon to a 1 ns time interval. Additionally, in these experiments, I stretch a single photon wavepacket of 5-ps-width to a wavepacket of 8.3-ns-width and subsequently recompress it to at least the resolution of the detectors (sim\\simsim 300 ps). This demonstrates a stretch factor of >1600 for a single-photon pulse using a group velocity dispersive material. To my knowledge, this is the largest reported stretch factor for a single-photon wavepacket produced by a biphoton source. The ability to stretch and recompress a single photon by this amount has applications in creating high-resolution, high-efficiency, single-photon spectrometers as well as advancing time-frequency QKD systems and other temporal pulse shaping applications.