Ideal single photon sources can emit single photons in a pure, deterministic and indistinguishable manner. At present, single-photon sources at telecom-band are widely used in quantum secure communication. They are mainly realized by attenuating laser pulses with randomized phase. However, this kind of single photon source has probabilistic outputs of empty events and multi-photon events, which remains one of the challenges in the application of quantum technology.

In this regard, heralded single photon source (HSPS) is one of the promising solutions, which is based on correlated photon-pairs generated from spontaneous nonlinear parametric processes. The detection of one of the photons in the photon-pair indicates the existence of its twin photon, which make sure that empty events can be avoided from the heralding signals. Moreover, HSPS has great advantage in generating high indistinguishable single photons.

To further improve the performance of HSPS, applying multiplexing several HSPS into a common output is a straightforward way which can enhance the single photon rate as well as maintains the single photon purity and indistinguishability. Multiplexing HSPS in spectral domain (i.e. spectral multiplexing) attracts great interest for their scalability and modular miniaturization. Moreover, to achieve practical application, the HSPS should be compact and operated in the wavelength range of telecom-band at room-temperature. However, an HSPS with all the components integrated on a photonics chip has not yet been demonstrated, much less a multiplexing HSPS. In this regard, a fully on-chip multiplexing HSPS at 1.5 ??m is particularly demanded.

The research group led by Prof. Guang-Can Guo and Prof. Qiang Zhou from the University of Electronic Science and Technology of China together with Prof. Lixing You from Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, proposed a 1.5 ??m chip-scale HSPS on lithium niobite on insulator (LNOI) by employing spectral multiplexing and active feed-forward spectral manipulating, and demonstrated a proof-of-principle experiment with discrete fiber-based components. The research results are published in Photonics Research, Volume 10, No. 6, 2022 (Hao Yu, Chenzhi Yuan, Ruiming Zhang, Zichang Zhang, Hao Li, You Wang, Guangwei Deng, Lixing You, Haizhi Song, Zhiming Wang, Guang-Can Guo, Qiang Zhou. Spectrally multiplexed indistinguishable single photon generation at telecom-band [J]. Photonics Research, 2022, 10(6):1417-29).

An integrated scheme of spectrally multiplexed HSPS based on LNOI platform is proposed as shown in the figure. The realizations of all these modules could benefit from the recent remarkable development in manufacturing technologies of LNOI. Key components including periodically poled lithium niobite waveguide, arrayed waveguide gratings, optical delay lines, phase modulator and waveguide-integrated single photon detectors can be integrated onto a single LNOI-based photonics chip. Monolithic hybrid integration of pump laser and feedforward frequency shifting devices can be further realized through electronic/photonic wire bonding.

Schematic of future chip-scale LNOI-based spectrally multiplexed HSPS.

To validate their proposal, a proof-of-principle experiment was demonstrated with discrete fiber-based components. Based on a periodically poled lithium niobite (PPLN) waveguide module and a lithium niobite based frequency shifting device, the research team realized the generation nad multiplexing of three spectral modes at telecom-band. The demonstrated spectrally multiplexed HSPS has an record-high single photon purity with ??⁽²⁾(0)=0.0006±0.0001 at an HSP rate of 3.1 kHz.

Moreover, the research team for the first time investigated the indistinguishability of the multiplexed HSPS by demonstrating a Hong-Ou-Mandel (HOM) interference between the spectrally multiplexed HSPS and an independent attenuated laser source, and proved a high indistinguishability of the spectrally multiplexed HSPS. This work paves the way for the further application of spectral multiplexing HSPS in quantum science and technology.

"Our proposed scheme has great scalability in terms of multiplexing more spectral modes," said by Mr. Yu and Dr. Yuan, the co-first authors of this work, "Combining with the recently developed low loss and large bandwidth photonic and electronic devices, it is possible to multiplex HSPS in the spectrum range of tens of nanometers, and realize a single photon source that is very close to the ideal one." Prof. Qiang Zhou pointed out that, "thanks to the progress of integrated photonics, quantum information technology has ushered in a new technological leap, continuously enabling the development of the quantum technology industry."

This work reports a proposal for on-chip spectrally multiplexed HSPS and a proof-of-principle demonstration based on discrete fiber-based optical components. Our results show that the proposed on-chip spectrally multiplexed HSPS could be feasible based on the LNOI photonics. The work is supported by National Key Research and Development Program of China, National Natural Science Foundation of China, Sichuan Key Research and Development Program.