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IEEE Journal of Quantum Electronics

Generation of Time-Bin Entanglement in a Silicon Waveguide With Variable Channel Bandwidths

Yan Xu, Rui Wang, Meng Xue Deng, Yu Yu, Yi Wang

Published Jun 14, 2023• 3 citations
Publisher

Abstract

Entangled photon pairs based on silicon straight waveguides have been applied in the area of quantum information owing to the compactness of the devices and high nonlinearity of the silicon material. In order to fully take advantage of the broad emission spectrum of photon pairs for quantum-key-distribution, dense wavelength division multiplexer (DWDM) is often required. However, the relationship between the channel bandwidth of DWDM and the two-photon interference visibility remains unambiguous, in which the high visibility of quantum interference fundamentally reduces quantum bit error rate in quantum-key-distribution system. In this paper, time-bin entangled photon pairs with a raw visibility of 94.3% ± 3.3% are generated in a silicon chip with a channel bandwidth of 0.1 nm, showing a violation of Bell’s inequality by more than three standard deviations, and a fidelity of 0.942 ± 0.02 is obtained by quantum state tomography. Applying variable channel bandwidths, the corresponding two-photon interference visibilities are measured, fitting well with the theoretical model incorporating multiple parameters such as optical loss, dark count probability, and averaged photon-pair number in DWDM channel.

DOI
10.1109/jqe.2023.3286035

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Entangled photon pairs based on silicon straight waveguides have been applied in the area of quantum information owing to the compactness of the devices and high nonlinearity of the silicon material. In order to fully take advantage of the broad emission spectrum of photon pairs for quantum-key-distribution, dense wavelength division multiplexer (DWDM) is often required. However, the relationship between the channel bandwidth of DWDM and the two-photon interference visibility remains unambiguous, in which the high visibility of quantum interference fundamentally reduces quantum bit error rate in quantum-key-distribution system. In this paper, time-bin entangled photon pairs with a raw visibility of 94.3% ± 3.3% are generated in a silicon chip with a channel bandwidth of 0.1 nm, showing a violation of Bell’s inequality by more than three standard deviations, and a fidelity of 0.942 ± 0.02 is obtained by quantum state tomography. Applying variable channel bandwidths, the corresponding two-photon interference visibilities are measured, fitting well with the theoretical model incorporating multiple parameters such as optical loss, dark count probability, and averaged photon-pair number in DWDM channel.
Optical quantum states based on entangled photons are the key resource in quantum-information science. The realization of multiplexed multiple entanglement are necessary for developing high-capacity quantum information process. Silicon-on-insulator (SOI) has recently become a leading platform for generating and processing of non-classical optical states. In this work, by combining the wavelength- and time-division multiplexing technologies, we demonstrate a multiplexing time-bin entangled photon pair source based on a silicon nanowire waveguide and distribute entangled photons into 3(time) × 14(wavelength) channels independently. The indistinguishability of photon pairs in each time channel is confirmed by a fourfold Hong-Ou-Mandal quantum interference. Our work paves a new and promising way to achieve a high capacity quantum communication and to generate a multiple-photon non-classical state.
We report the fully telecom compliant generation of photon pairs on a silicon micro-ring resonator chip. The ring cavity is designed such as to demultiplex the emitted frequency-comb like spectrum using standard off-the-shelves telecom 100 GHz dense wavelength division multiplexers (DWDM). We measure energy-time entanglement in different channel pairs with raw visibilities exceeding 97%, which paves the way for silicon photonics based quantum key distribution (QKD) with either increased bit rates, or more users.
Integrated time-bin-entangled photon-pair source with cavity-enhanced nonlinear optical processes is essential for quantum information technologies. However, microcavities with a high quality factor inherently introduce a trade-off between generation efficiency and photon bandwidth, which hinders the development of high-speed quantum networks with an integrated source. Here, we address this challenge by optimizing the nonlinearity property of the material and the geometry of the integrated microring resonator with a 4H-silicon carbide platform. Operating at a loaded quality factor of 1.9 ×\times× 10^5 - spectral bandwidth of 1.0 GHz and pumped with 300-ps double pulses separated by 1.25 ns at a repetition rate of 160 MHz, the device achieves a time-bin-entangled photon-pair generation rate of 1.35 ×\times× 10^7 s^-1 mW^-2. A raw visibility of 95.55 ±\pm± 0.18% is measured, showing a violation of Bell's inequality by more than 138 standard deviations, and a fidelity of 94.37 ±\pm± 0.22% is obtained by quantum state tomography. These results provide a scalable pathway to an efficient and broadband time-bin entangled quantum light source, overcoming intrinsic limitations of cavity-based designs and advancing integrated platforms for future quantum communication networks.
Abstract Optical losses and environmental noise present significant challenges for the coherent propagation of entangled states of light over large distances in quantum communication protocols. To cope with optical losses, one possible strategy is to increase the emission rate as much as possible. Decoherence, related to thermal and pressure fluctuation and to the presence of parasitic processes, can be overcome by encoding information via hyper-entanglement, i.e. entanglement over multiple degrees of freedom (DoFs). In this work we experimentally demonstrate a silicon-integrated device implementing both strategies, demonstrating the emission of spectrally multiplexed and hyper-entangled photon pairs. The pair generation rate exceeds 10 9 pairs per second at the output of the device over an 8 THz-wide comb of multiplexed frequency modes. Hyperentanglement is proved by showing entanglement in the frequency-bin and time-energy DoFs, along with high purity of the states when tracing over the time-energy degree of freedom. Quantum state tomography shows both fidelity and purity to be greater than 95% in the frequency domain, while, in the time-domain encoding, Bell’s inequality is violated by more than 10 standard deviations.
Silicon quantum photonics promises to take quantum optics to the large scale, where single photons carry the quantum information which can accelerate difficult computational problems, provide perfectly secure communication, or allow high-accuracy measurements. The silicon photonics platform offers miniature, high-yield, and high-performance photonics which are integrable with CMOS electronics and telecommunications-band optics. To date, only non-scalable photon-pair sources, based on spontaneous four-wave mixing (SFWM) in straight silicon waveguides have been shown to interfere [1]. Until now, single-photon experiments in silicon [2] have exclusively used an off-chip apparatus, such as a silica arrayed waveguide grating, to control and manipulate photon frequency. We present a silicon-on-insulator quantum photonic device (Fig. 1a) able to generate and analyse two maximally entangled qubits. We use this device to demonstrate high-visibility quantum interferencebetween resonant SFWM sources (analogous to [1]), phase-stable frequency-selection, and quantum state tomography. We varied the phase  to obtain the fringes shown in Fig. 1b, with a visibility of 96.8%. This high visibility indicates that the two microring sources are substantially identical. We setup the entangled state via the on-chip 17-dB extinction filters, and used quantum state tomography to analyse it (Fig. 1c). We measured a fidelity of 0.8. This is the first time an entangled state of photonic qubits has been both generated and analysed on-chip, and this generation has been done in a way which is scalable to larger photon number. These results represent a new step towards fully silicon-integrated quantum photonics.