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arXiv:1905.13395 · Photonics Research

Narrowband photonic quantum entanglement with counterpropagating domain engineering

Yichen Liu, Dongjie Guo, Ran Yang, Changwei Sun, Jia-Chen Duan, Yan-Xiao Gong, Zhenda Xie, Shining N. Zhu

Published Jul 23, 2021• 16 citations
arXivHTMLPDF

Abstract

Narrowband photonic entanglement is a crucial resource for long-distance quantum communication and quantum information processing, including quantum memories. We demonstrate the first polarization entanglement with 7.1 GHz inherent bandwidth by counterpropagating domain engineering, which is also confirmed by Hong–Ou–Mandel interference with 155-ps base-to-base dip width and ( 97.1 ± 0.59 ) % high visibility. The entanglement is harnessed with 18.5-standard-deviations Bell inequality violation, and further characterized with state tomography of ( 95.71 ± 0.61 ) % fidelity. Such narrowband entanglement sets a cornerstone for practical quantum information applications.

DOI
10.1364/prj.413075

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Narrowband photonic entanglement is a crucial resource for long-distance quantum communication and quantum information processing, including quantum memories. We demonstrate the first polarization entanglement with 7.1 GHz inherent bandwidth by counterpropagating domain engineering, which is also confirmed by Hong–Ou–Mandel interference with 155-ps base-to-base dip width and ( 97.1 ± 0.59 ) % high visibility. The entanglement is harnessed with 18.5-standard-deviations Bell inequality violation, and further characterized with state tomography of ( 95.71 ± 0.61 ) % fidelity. Such narrowband entanglement sets a cornerstone for practical quantum information applications.
Photonic entanglement is central resource to quantum information sciences, such as quantum communication and quantum computation. The entangled photons generated in conventional spontaneous parametric down-conversion usually yield THz bandwidth which becomes very dim in many applications call for narrow-band entanglement sources. Here we demonstrate the polarization-entanglement photon source with counterpropagating phase matching, which results in an inherent bandwidth of 7.1 GHz at telecom wavelength. The entanglement is measured to violate the Bell inequality by up to 18.5 standard deviations, with Clauser–Horne–Shimony–Holt S-parameter of 2.720±0.039. The quantum state tomography further characterizes the entanglement, with fidelity F=(95.71±0.61)%.
Quantum entanglement is a fundamental resource for secure information processing and communications. The canonical optical quantum information processing encodes a single qubit per photon, with remarkable demonstrations of secure quantum communications, linear optical quantum computing amongst others. Often the polarization qubit, a discrete variable with entangled Bell states, is used. Continuous variables such as energy-time modes and spatial modes, however, present a dramatically larger Hilbert space for quantum information processing. The high-dimensional entanglement capture more qubits per photon, enabling dramatically higher secure key rates over longer distances and with better error resilience. Here I will describe our results on high-dimensional entanglement in quantum frequency combs for dense quantum communications. We first demonstrate revival of the Hong-Ou-Mandel interferences, long postulated by theorists more than a decade ago, up to 19-dimensions and with visibilities up to 96.5%. The mode-locked two-photon state in high-dimensions is further witnessed through a stabilized Franson interferometer, as a generalized Bell’s inequality test and hyperentangled through multiple degrees-of-freedom. Entanglement revivals of the non-local interference at discrete time-bins are uncovered, up to 97.8% visibility, as a fundamental resource for dense secure information processing.
Hong-Ou-Mandel (HOM) interferometry with entangled photons exhibits distinctive quantum features. By introducing frequency entanglement (discrete-color entangled states) into HOM interference, the characteristic HOM dip is modulated by sinusoidal fringes, which significantly enhance the sensitivity of HOM sensors. The frequency-entangled photon sources demonstrated to date rely on non-resonant parametric down-conversion (PDC), which limits the photon coherence length and, consequently, restricts the sensing dynamic range to the sub-millimeter scale. In this work, we demonstrate narrowband frequency-entangled photon source based on resonant PDC in a crystalline whispering gallery mode resonator. The MHz-level spectral bandwidth of photons enables a meter-scale dynamic range. With highly nondegenerate frequency-entangled photon pairs featuring a 96 THz frequency detuning, we observe high-contrast quantum beating with sub-picosecond resolution in the HOM experiment. Our WGMR-based frequency-entangled photon source has potential applications in quantum metrology and quantum information processing.
A high-quality narrowband polarization-entangled source in the telecom band is preferred to avoid frequency dispersion for long-distance transmission in optical fibers and to efficiently couple with telecom-band quantum memories. Here, we report narrowband, telecom-band, polarization-entangled photon pair generation based on the superposition of single-longitudinal-mode photon pairs from two monolithic nonlinear crystal cavities in a passively stable interferometer based on beam displacers. The photon pairs generated from the cavities exhibit a high coincidence to accidental coincidence ratio of 20 000 and a bandwidth below 500 MHz. Two-photon polarization interference, Bell inequality, and quantum state tomography are performed to indicate the high quality of the entangled source. The current configuration demonstrates greater stability than traditional free space cavity-enhanced polarization-entangled state generation, which is promising for quantum communication applications.
Hong–Ou–Mandel (HOM) interferometry with entangled photons exhibits distinctive quantum features. By introducing frequency entanglement (discrete-color entangled states) into HOM interference, the characteristic HOM dip is modulated by sinusoidal fringes, which significantly enhances the sensitivity of HOM sensors. The frequency-entangled photon sources demonstrated to date rely on non-resonant parametric down-conversion (PDC), which limits the photon coherence length and, consequently, restricts the sensing dynamic range to the millimeter scale. In this work, we demonstrate narrowband frequency-entangled photon source based on resonant PDC in a crystalline whispering gallery mode resonator. The MHz-level spectral bandwidth of photons enables a meter-scale dynamic range. With highly nondegenerate frequency-entangled photon pairs featuring a 96 THz frequency detuning, we observe high-contrast quantum beating with sub-picosecond resolution in the HOM experiment. Our WGMR-based frequency-entangled photon source has potential applications in quantum metrology and quantum information processing.
Entanglement has evolved from being a spooky interaction towards a key resource of real-life applications such as absolute secure communication. Today, quantum networking devices are commercially available, typically taking advantage of time-bin or polarisation entanglement, the latter one doubtlessly being easier to analyse thanks to interferometer free set-ups. The communication distance of such systems is generally limited to a few hundreds of kilometres due to intrinsic fibre transmission losses and imperfect detectors. It was shown that the communication distance can be greatly increased when combining entangled pairs of photons and quantum memories. But the narrow absorption bandwidth of current quantum memories is in contrary to the widely used broadband entanglement sources based on non linear inter- actions. More precisely, there is a lack of ultra narrowband sources in the telecom C-band, where fibre losses are minimal and high performance guided-wave optics are available. Here we introduce a high quality polarisation entanglement engineering scheme based on a birefringent de- lay line (BDL). This BDL is applied to generate polarization entanglement from a high efficiency ultra narrowband tele- com photon pair source where no polarisation entanglement is available initially.