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arXiv:1212.0567 · Optics Express

Near-infrared Hong-Ou-Mandel interference on a silicon quantum photonic chip

Xinan Xu, Zhenda Xie, Jiangjun Zheng, Junlin Liang, Tian Zhong, Mingbin Yu, Serdar Kocaman, Guo‐Qiang Lo, Dim‐Lee Kwong, Dirk Englund, Franco N. C. Wong, Chee Wei Wong

Published Feb 21, 2013• 16 citations
arXivHTMLPDF

Abstract

Near-infrared Hong-Ou-Mandel quantum interference is observed in silicon nanophotonic directional couplers with raw visibilities on-chip at 90.5%. Spectrally-bright 1557-nm two-photon states are generated in a periodically-poled KTiOPO₄ waveguide chip, serving as the entangled photon source and pumped with a self-injection locked laser, for the photon statistical measurements. Efficient four-port coupling in the communications C-band and in the high-index-contrast silicon photonics platform is demonstrated, with matching theoretical predictions of the quantum interference visibility. Constituents for the residual quantum visibility imperfection are examined, supported with theoretical analysis of the sequentially-triggered multipair biphoton, towards scalable high-bitrate quantum information processing and communications. The on-chip HOM interference is useful towards scalable high-bitrate quantum information processing and communications.

DOI
10.1364/oe.21.005014

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Near-infrared Hong-Ou-Mandel quantum interference is observed in silicon nanophotonic directional couplers with raw visibilities on-chip at 90.5%. Spectrally-bright 1557-nm two-photon states are generated in a periodically-poled KTiOPO₄ waveguide chip, serving as the entangled photon source and pumped with a self-injection locked laser, for the photon statistical measurements. Efficient four-port coupling in the communications C-band and in the high-index-contrast silicon photonics platform is demonstrated, with matching theoretical predictions of the quantum interference visibility. Constituents for the residual quantum visibility imperfection are examined, supported with theoretical analysis of the sequentially-triggered multipair biphoton, towards scalable high-bitrate quantum information processing and communications. The on-chip HOM interference is useful towards scalable high-bitrate quantum information processing and communications.
Interference between single photons is key for many quantum optics experiments and applications in quantum technologies, such as quantum communication or computation. It is advantageous to operate the systems at telecommunication wavelengths and to integrate the setups for these applications in order to improve stability, compactness and scalability. A new promising material platform for integrated quantum optics is lithium niobate on insulator (LNOI). Here, we realise Hong-Ou-Mandel (HOM) interference between telecom photons from an engineered parametric down-conversion source in an LNOI directional coupler. The coupler has been designed and fabricated in house and provides close to perfect balanced beam splitting. We obtain a raw HOM visibility of (93.5 ± 0.7) %, limited mainly by the source performance and in good agreement with off-chip measurements. This lays the foundation for more sophisticated quantum experiments in LNOI.
Summary form only given. Quantum computing technology is at a crossroads. The technological route we choose must scale without limit. For quantum optics to play a part, optical loss, miniaturisation, and manufacturability must be simultaneously addressed. In quantum optics, a loss of a single photon is a loss of irreplaceable quantum information. Macroscale optics offers exceptionally low loss, and high performance, but has no hope to scale to thousands or millions of modes. Microscale optics, and silicon photonics in particular, offer exceptional miniaturisation and large-scale manufacturability, but pay a high price in loss [1]. We propose -and realise -a new platform for quantum optics: we retain the scale and manufacturability of silicon photonics, but extirpate its loss, both linear and nonlinear. We shift silicon quantum photonics to the short-wave infrared (SWYR), centred on 2.1 microns wavelength, on the optical end of the mid-infrared spectrum [2]. The minimum requirements for any quantum optical platform are: to source quantum-correlated light, and to demonstrate high-quality quantum interference. Here, we do both: we engineer an on-chip photon-pair source, and interfere the light from two such sources in a time-reversed Hong-Ou-Mandel (HOM) measurement [3]. In so doing, we show the first photon-pair source with all fields in the mid-infrared, and the first quantum interference between mid-infrared photons. We coherently pump both on-chip sources and thermally vary the on-chip phase, (j) to perform a time-reversed HOM interference measurement [3] with an accidental-subtracted visibility of V = 99.1 ± 0.9%.
The generation of pure single-photon states and their characterization are fundamental in quantum information processing applications like quantum computing. In this work, we design integrated optical directional couplers on a silicon nitride platform for heralded single-photon characterization. We assume the generation of such photons in silicon nitride waveguides through the spontaneous four-wave mixing process. We show that manipulating the waveguide geometry and dielectric contrast makes it possible to obtain nearly wavelength-independent directional coupler designs that perform as 50:50 beamsplitters over a broad bandwidth around the central design wavelength. Assuming two independent but identical heralded single-photon sources, we compare the visibility obtained by simulating the Hong-Ou-Mandel interference for three different 50:50 beamsplitters: an ideal one (perfectly wavelength-independent), an engineered one (nearly wavelength-independent), and an unengineered one (significantly wavelength-dependent). The purity obtained with the engineered beamsplitter closely matches the purity obtained with the ideal one, for which the interference visibility is a direct measure of the quantum state purity. This finding implies that the proposed engineered beamsplitter is suitable for characterizing heralded single-photon states, provided its bandwidth is 23 nm.
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.