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Spectrally multiplexed and ultrabright entangled photon pairs in a lithium niobate microresonator

Boyu Xu, Likun Chen, Jintian Lin, Lan‐Tian Feng, Rui Niu, Zhi‐Yuan Zhou, Renhong Gao, Chun‐Hua Dong, Guang‐Can Guo, Qihuang Gong, Ya Cheng, Yun‐Feng Xiao, Xi‐Feng Ren

Published Aug 17, 2021• 2 citations
Publisher

Abstract

Abstract On-chip bright quantum sources with multiplexing ability are extremely high in demand for the integrated quantum networks with unprecedented scalability and complexity. Here, we demonstrate an ultrabright and broadband biphoton quantum source generated in a lithium niobate microresonator system. Without introducing the conventional domain poling, the on-chip microdisk produces entangled photon pairs covering a broad bandwidth promised by natural phase matching in spontaneous parametric down conversion. Experimentally, the multiplexed photon pairs are characterized by 30 nm bandwidth limited by the filtering system, which can be furthered enlarged. Meanwhile, the generation rate reaches 5.13 MHz/μW with a coincidence-to- accidental ratio up to 804. Besides, the quantum source manifests the prominent purity with heralded single photon correlation g(2)H(0)=0.0098±0.0021 and energy-time entanglement with excellent interference visibility of 96.5%±1.9%.

DOI
10.21203/rs.3.rs-744512/v1

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Abstract On-chip bright quantum sources with multiplexing ability are extremely high in demand for the integrated quantum networks with unprecedented scalability and complexity. Here, we demonstrate an ultrabright and broadband biphoton quantum source generated in a lithium niobate microresonator system. Without introducing the conventional domain poling, the on-chip microdisk produces entangled photon pairs covering a broad bandwidth promised by natural phase matching in spontaneous parametric down conversion. Experimentally, the multiplexed photon pairs are characterized by 30 nm bandwidth limited by the filtering system, which can be furthered enlarged. Meanwhile, the generation rate reaches 5.13 MHz/μW with a coincidence-to- accidental ratio up to 804. Besides, the quantum source manifests the prominent purity with heralded single photon correlation g(2)H(0)=0.0098±0.0021 and energy-time entanglement with excellent interference visibility of 96.5%±1.9%.
On-chip bright quantum sources with multiplexing ability are extremely high in demand for the integrated quantum networks with unprecedented scalability and complexity. Here, we demonstrate an ultrabright and broadband biphoton quantum source generated in a lithium niobate microresonator system.Without introducing the conventional domain poling, the on-chip microdisk produces entangled photon pairs covering a broad bandwidth promised by natural phase matching in spontaneous parametric down conversion.Experimentally, the multiplexed photon pairs are characterized by 30 nm30\ \rm nm30 nm bandwidth limited by the filtering system, which can be furthered enlarged.Meanwhile, the generation rate reaches $5.13\ {\rm MHz}/\upmu \rm W$ with a coincidence-to-accidental ratio up to 804804804.Besides, the quantum source manifests the prominent purity with heralded single photon correlation gH(2)(0)=0.0098±0.0021g_H^{(2)}(0)=0.0098\pm0.0021gH(2)​(0)=0.0098±0.0021 and energy-time entanglement with excellent interference visibility of 96.5%±1.9%96.5\%\pm1.9\%96.5%±1.9%. Such quantum sources at the telecommunication band pave the way for high-dimensional entanglement and future integrated quantum information systems.
Quantum networks based on wavelength-multiplexed entanglement enable parallel distribution of quantum correlations, increasing channel capacity for secure communication and distributed quantum information processing. However, broadband integrated sources capable of generating multipartite entanglement beyond photon pairs remain scarce. Here we report on-chip generation of telecom-band four-photon entanglement in a periodically poled thin-film lithium niobate on insulator (LNOI) waveguide. Type-0 spontaneous parametric down-conversion provides a phase-matching bandwidth exceeding 200 nm, enabling spectrally separable generation of multi-photon entanglement across the telecom band. The generated photons are encoded in time bins for robust fiber compatibility, and a coherent interface enabling reversible conversion between time-bin and polarization degrees of freedom allows complete quantum state tomography. We measure two-photon entanglement with a brightness of 6.7 MHz/mW/nm and a fidelity of 0.874±0.0020.874 \pm 0.0020.874±0.002. At a pump power of 0.08 mW, the four-photon state exhibits a fourfold coincidence rate of 1 Hz and a fidelity of 0.74±0.010.74 \pm 0.010.74±0.01, representing a threefold improvement over previous integrated platforms. Our results establish LNOI as a scalable platform for broadband multi-photon entanglement and provide a practical route toward dense wavelength-multiplexed quantum networks.
Nonlinear wavelength conversion is essential for many classical and quantum pho-tonic applications. The underlying second-order nonlinear optical processes, however, generally exhibit limited spectral bandwidths that impact their application potential. Here we use a high-Q X-cut lithium niobate microdisk resonator to demonstrate both second-harmonic generation and spontaneous parametric down-conversion on chip. In particular, our lithium niobate microresonator, with its wide-range cyclic phase matching and rich optical mode structures, is able to achieve ultra-broadband spontaneous parametric down-conversion, with a bandwidth over 400 nm, inferred from recorded spectra of the down-converted photons. The produced biphoton pairs exhibit strong temporal correlation, with a coincidence-to-accidental ratio measured to be 43.1. Our device is promising for integrated quantum photonics where optical frequency could be used as a degree of freedom for signal processing.
A high-flux entangled-photon source is a key resource for quantum optical study and application. Here, it is realized in a lithium niobate on isolator (LNOI) chip, with 2.79 \ifmmode\times\else\texttimes\fi{} 1011{10}^{11}1011 Hz/mW photon-pair rate and 1.53 \ifmmode\times\else\texttimes\fi{} 109{10}^{9}109 Hz/(nm mW) spectral brightness. These data are boosted by over 2 orders of magnitude compared with existing technologies. A 160-nm-broad bandwidth is engineered for eight-channel multiplexed energy-time entanglement. Harnessed by high-extinction-frequency correlation and Franson interferences up to 99.17% visibility, such energy-time-entanglement multiplexing further enhances the high-flux data rate and warrants broad application in quantum-information processing on a chip.
Quantum photon sources of high rate, brightness, and purity are increasingly desirable as quantum information systems are quickly scaled up and applied to many fields. Using a periodically poled lithium niobate microresonator on chip, we demonstrate photon-pair generation at high rates of 8.5 and 36.3 MHz using only 3.4 and 13.4 μW pump power, respectively, marking orders of magnitude improvement over the state of the art, across all material platforms. These results constitute the first direct measurement of the device's giant single photon nonlinearity. The measured coincidence to accidental ratio is well above 100 at those high rates and reaches 14682±4427 at a lower pump power. The same chip enables heralded single-photon generation at tens of megahertz rates, each with low autocorrelation g_{H}^{(2)}(0)=0.008 and 0.097 for the microwatt pumps, which marks a new milestone. Such distinct performance, facilitated by the chip device's noiseless and giant optical nonlinearity, will contribute to the forthcoming pervasive adoption of quantum optical information technologies.