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OSA Advanced Photonics Congress

Are different types of photon-pair sources equally indistinguishable in silicon photonics?

Alessio Baldazzi, Jong-Moo Lee, Matteo Sanna, Stefano Azzini, Joon Tae Ahn, Myung‐Lae Lee, Young-Ik Sohn, Lorenzo Pavesi

Published Jan 1, 2023• 0 citations
Publisher

Abstract

We quantified the indistinguishability between two photon-pair sources using on-chip Hong-Ou-Mandel interference making a direct comparison between two different kinds of sources: spiral waveguides and microring resonators.
DOI
10.1364/iprsn.2023.itu3a.4

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•Jan 1, 2019•0 citations
In the same silicon photonic integrated circuit, we compare two types of integrated degenerate photon-pair sources (microring resonators and waveguides) using Hong–Ou–Mandel (HOM) interference experiments. Two nominally identical microring resonators are coupled to two nominally identical waveguides, which form the arms of a Mach–Zehnder interferometer. This is pumped by two lasers at two different wavelengths to generate, by spontaneous four-wave mixing, degenerate photon pairs. In particular, the microring resonators can be thermally tuned in or out of resonance with the pump wavelengths, thus choosing either the microring resonators or the waveguides as photon-pair sources, respectively. In this way, an on-chip HOM visibility of 94% with microring resonators and 99% with straight waveguides is measured upon filtering. We compare our experimental results with theoretical simulations of the joint spectral intensity and the purity of the degenerate photon pairs. We verify that the visibility is connected to the sources’ indistinguishability, which can be quantified by the overlap between the joint spectral amplitudes (JSA) of the photon pairs generated by the two sources. We estimate a JSA overlap of 98% with waveguides and 89% with microring resonators.
Quantum interference between identical photons lies at the heart of our plan to construct a silicon photonic quantum computer. Several ground-breaking experiments have demonstrated the on-chip interference between photons generated in spiral [1] and ring resonator [2] sources. However, these two-photon experiments have relied on detecting the simultaneous arrival of a pair of photons from either source. In order that such experiments can be scaled-up, we require the multiplexing of probabilistic photon pair sources into an on-demand single photon source [3]. A first step towards this is to demonstrate heralded single photons from different sources will retain the purity and indistinguishability required to interfere with high visibility [4]. In our work we examine photon pair production by spontaneous four-wave mixing in a silicon waveguide source and determine the factors that limit the heralded two-photon interference visibility. We present a novel approach to modelling this generation process using the joint- temporal amplitude and compare our results with experimental data. [1] On-chip quantum interference between silicon photon-pair sources, Nature Photonics 8, 104, (2014). [2] Qubit entanglement between ring-resonator photon-pair sources on a silicon chip, Nature Communications 6, 7948, (2015). [3] Active temporal and spatial multiplexing of photons, Optica 3, 127, (2016) [4] Indistinguishable photon pair generation using two independent silicon wire waveguides, New Journal of Physics 13, 065005, (2011).
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.
One of the key building blocks of quantum photonic systems is a source of indistinguishable photons. Silicon ring resonators can be used as photon pair sources by taking advantage of silicon's large third order nonlinearity with a process known as spontaneous four wave mixing. These sources are capable of producing pairs of indistinguishable photons but typically suffer from an effective 50% loss. By slightly decoupling the input waveguide from the ring, the desired photons generated in the ring can be directed to the drop port. Thus, the ratio between the coincidences from the drop port and the total number of coincidences from all ports (coincidence efficiency) can be significantly increased, with the trade-off being that the pump is less efficiently coupled into the ring. This asymmetric coupling scheme was used to demonstrate a dual-bus microring source with a coincidence efficiency of ~96%. Unfortunately, the penalty to pump power was quite large, with a required ~10x increase in the pump power to maintain the same pair generation rate.\nTo further improve upon this result, a microring source was designed with multipoint evanescent couplers to induce a strong spectral dependence in the coupling coefficients. This coupler design can be used on both sides of the ring resonator so that resonances supported by one of the couplers are suppressed by the other. This is the ideal configuration for a ring-based photon-pair source as it can only support the pump photons at the input side while only allowing the generated photons to leave through the output side. Asymmetric coupling was also used in this configuration but in a different way than in the previous source. In this case, the input side of the ring was configured to be critically coupled for maximal resonance enhancement while the output side was configured to be over coupled to minimize photon-pair loss within the ring. This device design was used to demonstrate a photon-pair source with coincidence efficiencies of 0.9981+-6.425x10^-4 and 0.9979+-1.665x10^-3 for degenerate and non-degenerate pump configuration respectively.\nIn addition to the source development, work was done to move some of the pump rejection filtering onto the photonic chip. While the fabricated device only exhibited an extinction of 50 dB, the root cause of the limited extinction was explored and a new filter design was proposed to overcome these limitations. With the combination of the microring source with its greatly improved efficiency and the movement toward reducing the required off-chip filtering, this work represents a significant advancement in the scalability of integrated quantum photonic systems which can enable high performance quantum computing and communication systems.