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European Quantum Electronics Conference (EQEC)

Six-photon experiments using an on-chip array of heralded, pure single photon sources

Justin B. Spring, Paolo L. Mennea, Benjamin J. Metcalf, Peter C. Humphreys, M.W. Moore, Stefanie Barz, James C. Gates, W. Steven Kolthammer, Brian J. Smith, Peter G. R. Smith, Ian A. Walmsley

Published Jun 21, 2015• 0 citations
OpenAlex

Abstract

Heralded single photon sources, based on probabilistic non-linear interactions, currently serve as the de-facto standard for performing multi-photon quantum interference experiments. Already, interesting phenomena may be explored with a handful of such sources [1], whilst in the future, large arrays of identical sources could be multiplexed with a switching network to achieve a truly scalable, near-deterministic emission [2]. However, fabricating such a large number of indistinguishable, pure, heralded single photon sources, operating with low-loss, remains an open challenge. Spontaneous four-wave mixing (SFWM) in a silica waveguide is a promising route to building such a source. The low-loss and excellent mode-matching to optical fibre of silica waveguides allows high preparation efficiency. Guided-wave photonics also allows the precise control of the optical modes needed to engineer emission of photons into a single pair of Schmidt modes: allowing heralded photons of high purity without narrowband filtering [3].

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Heralded single photon sources, based on probabilistic non-linear interactions, currently serve as the de-facto standard for performing multi-photon quantum interference experiments. Already, interesting phenomena may be explored with a handful of such sources [1], whilst in the future, large arrays of identical sources could be multiplexed with a switching network to achieve a truly scalable, near-deterministic emission [2]. However, fabricating such a large number of indistinguishable, pure, heralded single photon sources, operating with low-loss, remains an open challenge. Spontaneous four-wave mixing (SFWM) in a silica waveguide is a promising route to building such a source. The low-loss and excellent mode-matching to optical fibre of silica waveguides allows high preparation efficiency. Guided-wave photonics also allows the precise control of the optical modes needed to engineer emission of photons into a single pair of Schmidt modes: allowing heralded photons of high purity without narrowband filtering [3].
Interference between independent single photons is perhaps the most fundamental interaction in quantum optics. It has become increasingly important as a tool for optical quantum information science, as one of the rudimentary quantum operations, together with photon detection, for generating entanglement between non-interacting particles. Despite this, demonstrations of large-scale photonic networks involving more than two independent sources of quantum light have been limited due to the difficulty in constructing large arrays of high-quality single photon sources. Here, we solve the key challenge, reporting a novel array of more than eighteen near-identical, low-loss, high-purity, heralded single photon sources achieved using spontaneous four-wave mixing (SFWM) on a silica chip. We verify source quality through a series of heralded Hong-Ou-Mandel experiments, and further report the experimental three-photon extension of the entire Hong-Ou-Mandel interference curves, which map out the interference landscape between three independent single photon sources for the first time.
Single photon sources are a key building block for quantum information processing. A common approach uses spontaneous emitters based on parametric down conversion and four wave mixing. In 2005 experiments on the first spectrally engineered four wave mixing sources in fibre were published [1,2]. A primary obstacle for photonics to realize quantum-enhanced applications is loss. Designing single photon sources matched to single mode optical fibres greatly reduces coupling losses [3]. In addition, tailoring the phase matching properties of the sources to generate factorable photon pairs makes strong spectral filtering obsolete and thereby avoids the degradation of photon number correlations due to loss.
While integrated photonics is a robust platform for quantum information processing, architectures for photonic quantum computing place stringent demands on high quality information carriers. Sources of single photons that are highly indistinguishable and pure, that are either near-deterministic or heralded with high efficiency, and that are suitable for mass-manufacture, have been elusive. Here, we demonstrate on-chip photon sources that simultaneously meet each of these requirements. Our photon sources are fabricated in silicon using mature processes, and exploit a dual-mode pump-delayed excitation scheme to engineer the emission of spectrally pure photon pairs through inter-modal spontaneous four-wave mixing in low-loss spiralled multi-mode waveguides. We simultaneously measure a spectral purity of 0.9904 ± 0.0006, a mutual indistinguishability of 0.987 ± 0.002, and >90% intrinsic heralding efficiency. We measure on-chip quantum interference with a visibility of 0.96 ± 0.02 between heralded photons from different sources.
A key obstacle to the experimental realization of many photonic quantum-enhanced technologies is the lack of low-loss sources of single photons in pure quantum states. We demonstrate a promising solution: generation of heralded single photons in a silica photonic chip by spontaneous four-wave mixing. A heralding efficiency of 40%, corresponding to a preparation efficiency of 80% accounting for detector performance, is achieved due to efficient coupling of the low-loss source to optical fibers. A single photon purity of 0.86 is measured from the source number statistics without narrow spectral filtering, and confirmed by direct measurement of the joint spectral intensity. We calculate that similar high-heralded-purity output can be obtained from visible to telecom spectral regions using this approach. On-chip silica sources can have immediate application in a wide range of single-photon quantum optics applications which employ silica photonics.
Single photon sources based on spontaneous photon-pair generation have enabled pioneering experiments in quantum optics. However, their non-determinism presents a bottleneck to scaling up photonic and hybrid quantum-enhanced technologies. Furthermore, photon pairs are typically emitted into many correlated frequency modes, producing an undesirable mixed state on heralding. Here we present a complete fibre-integrated heralded single photon source that addresses both these difficulties simultaneously. We use active switching to provide a path to deterministic operation by multiplexing separate spontaneous sources, and dispersion engineering to minimise frequency correlation for high-purity single photon generation. All the essential elements -- nonlinear material with dispersion control, wavelength isolation, optical delay, and fast switching -- are incorporated in a low-loss alignment-free package that heralds photons in telecoms single-mode fibre. Our results demonstrate a scalable approach to delivering pure single photons directly into guided-wave photonic devices.
Single-photon sources based on spontaneous photon-pair generation have enabled pioneering experiments in quantum physics. However, next-generation photonic quantum technologies require higher generation probabilities of photons in well-controlled pure states capable of high-visibility interference. We have harnessed bespoke fiber technology to develop virtually alignment-free sources that deliver high-purity heralded single photons in telecoms single-mode fiber. The resulting access to low-loss optical delay enabled us to actively route the heralded output from two almost identical sources to enhance the delivery probability of single photons relative to one individual source. Our results indicate how the scale of photonic quantum technologies might be increased via guided-wave multiplexing of high-purity photons.