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Applied Optics and Photonics China (AOPC)

An on-chip quantum splitter of degenerate photon pairs using a spiral waveguide

Haoran Ma, Ye Hui Liao, Fanjie Ruan, Yuehai Wang, Jianyi Yang

Published Dec 13, 2024• 1 citation
Publisher

Abstract

Integrated sources of indistinguishable photons have garnered significant attention due to their applications in optical quantum computing, quantum communication and quantum networks. However, photons generated from spontaneous four-wave mixing (SFWM) always occur in pairs, which need to be split into different modes for further applications. Here, we present a quantum splitter for degenerate photon pairs based on a silicon photonic chip. This device incorporates a Sagnac loop, two asymmetrical Mach-Zehnder interferometers (AMZIs), and a spiral waveguide. The spiral waveguide generates pairs of photons, the Sagnac loop facilitates bidirectional pumping of the source, and the two AMZIs enables the extraction of photons from the pump light. The photons generated from two directions interfere at a balanced multi-mode interferemeter (MMI) to enable the reversed Hong-Ou-Mandel (HOM) effect to take place. Our experimental results demonstrated an on-chip interference visibility of 96.63% and an off-chip HOM dip visibility of 90.93%, indicating that the generated photons are well-suited for further integration with other components in quantum applications.

DOI
10.1117/12.3047784

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Integrated sources of indistinguishable photons have garnered significant attention due to their applications in optical quantum computing, quantum communication and quantum networks. However, photons generated from spontaneous four-wave mixing (SFWM) always occur in pairs, which need to be split into different modes for further applications. Here, we present a quantum splitter for degenerate photon pairs based on a silicon photonic chip. This device incorporates a Sagnac loop, two asymmetrical Mach-Zehnder interferometers (AMZIs), and a spiral waveguide. The spiral waveguide generates pairs of photons, the Sagnac loop facilitates bidirectional pumping of the source, and the two AMZIs enables the extraction of photons from the pump light. The photons generated from two directions interfere at a balanced multi-mode interferemeter (MMI) to enable the reversed Hong-Ou-Mandel (HOM) effect to take place. Our experimental results demonstrated an on-chip interference visibility of 96.63% and an off-chip HOM dip visibility of 90.93%, indicating that the generated photons are well-suited for further integration with other components in quantum applications.
Large-scale quantum photonic circuits require integrating multiple single-photon sources, which are typically based on spontaneous four-wave mixing (SFWM) in spiral waveguides or microring resonators (MRRs). Photons can be generated in both clockwise (CW) and counterclockwise (CCW) orientations from a single source in a Sagnac configuration, showing promise for improving scalability. In this work, we propose a fully integrable scheme for bidirectional creation and usage of single photons. This concept is based on two asymmetric Mach-Zehnder interferometers (AMZIs) integrated within the Sagnac loop. As a proof of concept, we fabricated a device with a spiral waveguide on a silicon chip and demonstrated its application as a quantum splitter and two multi-wavelength quantum light sources.
Integrated sources of indistinguishable photons have attracted a lot of attention because of their applications in quantum communication and optical quantum computing. Here, we demonstrate an ultra-compact quantum splitter for degenerate single photons based on a monolithic chip incorporating Sagnac loop and a micro-ring resonator with a footprint of 0.011 mm2, generating and deterministically splitting indistinguishable photon pairs using time-reversed Hong-Ou-Mandel interference. The ring resonator provides enhanced photon generation rate, and the Sagnac loop ensures the photons travel through equal path lengths and interfere with the correct phase to enable the reversed HOM effect to take place. In the experiment, we observed a HOM dip visibility of 94.5 +- 3.3 %, indicating the photons generated by the degenerate single photon source are in a suitable state for further integration with other components for quantum applications, such as controlled-NOT gates.
In this talk, I will discuss two applications of nonlinear quantum photonics in silicon nanowires: correlated photon pair sources and a single-photon wavelength converter. Correlated photon pairs can be generated in a nonlinear silicon waveguide through the spontaneous four-wave mixing (SFWM) process, where two photons from a bright pump laser are annihilated, and a two photons are spontaneously created at different wavelengths (the signal and idler). However, indistinguishable photon pairs are of particular interest for two-photon interference experiments, meaning that the signal and idler should have the same wavelength and be entirely identical to each other. Indistinguishable pairs can be generated with the SFWM process. where two pump lasers create a degenerate signal and idler at the frequency mid-way between them. Since the two photons are indistinguishable, it is not trivial to divide them into separate waveguides so as to manipulate them individually. I will present recent results concerning a silicon quantum splitter, where degenerate pairs are created in a ring resonator which is pumped in both directions. The ring is coupled to a sagnac loop interferometer, which combines the two directions at a 50 : 50 coupler. Here, a quantum interference effect (reverse-Hong Ou Mandel interference) splits the photons to two output waveguides. This device is ultra-compact and the use of a sagnac loop means that the phase is automatically set correctly for the interference. I will also discuss progress on single photon wavelength conversion in silicon nanowires using the four-wave mixing Bragg scattering (FWMBS) process. Here, two intense pump beams are used to modulate a weak signal, causing it to shift to the idler by the frequency difference between the pumps. In principle, this can allow coherent, noiseless frequency conversion of single photon signals. Fig. 1(b) shows output spectra from a 6mm silicon nanowire when the frequency shift is varied 100 GHz to 400 GHz. Multiple peaks are visible, corresponding to up-conversion and down-conversion of the input signal.
Developing a high-quality, bright source of polarization-entangled photon pairs on CMOS-compatible integrated platforms, such as silicon or silicon nitride, remains a significant challenge. Here, a novel silicon chip-based nonlinear Sagnac interferometer, which mainly integrates a bidirectionally pumped microring resonator with a polarization-splitting rotator, was proposed to generate polarization-entangled photon pairs. By pumping the system with a pulsed laser at 1550 nm and a power of 78 μW, polarization-entangled photon pairs in the telecom C-band were produced through spontaneous four-wave mixing, achieving a state-of-the-art brightness of 1.6 × 107 pairs s−1 GHz−1 mW−2. Using an off-chip polarization analyzer, the visibility of polarization-correlation interference fringe was measured to be 96.6%. Quantum state tomography revealed a fidelity of 0.958 for the maximally entangled state produced by the source. These exceptional performance metrics, combined with the integrability of the device, make it highly suitable for advanced quantum applications, particularly in multi-user entanglement-based quantum communication networks.
Integrated silicon micro-ring resonator (MRR) has been widely used as on-chip single photon source. In this paper, we introduce an approach to generate frequency-degenerate photons by bidirectionally pumping an add-drop micro-ring resonator (MRR) within a Sagnac loop configuration. This scheme facilitates the concurrent generation of photon pairs from both clockwise (CW) and counterclockwise (CCW) directions, transforming a single MRR into a dual-source system. Through CMOS fabrication techniques, we realized the proposed device and conducted measurements of key parameters including the single side count rate (SSCR), coincidence count rate (CCR), and coincidence-to-accidental coincidence ratio (CAR) for both directional outputs. Notably, the CCRs exhibit remarkable similarity between the two sources, reaching approximately 550 Hz at an waveguide power of 0.59 mW. Furthermore, we observed a differential CAR, with the CCW direction yielding a lower value compared to the CW direction, with estimated maxima of 161 and 387, respectively. These findings underscore the viability of utilizing a singular MRR as a dual-source entity. Additionally, we outline the design of an on-chip structure for heralded Hong-Ou-Mandel (HOM) interference, necessitating 4-fold coincidences.Our work holds the promise of advancing the realm of large-scale integrated quantum photonic chips.