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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

The First Quantum Interference in the Mid-Infrared is on a Silicon Chip

Lawrence M. Rosenfeld, Dominic A. Sulway, Mark G. Thompson, John Rarity, Joshua W. Silverstone

Published Jun 1, 2019• 1 citation
Publisher

Abstract

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%.

DOI
10.1109/cleoe-eqec.2019.8872818

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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%.
Applied quantum optics stands to revolutionise many aspects of information technology, provided performance can be maintained when scaled up. Silicon quantum photonics satisfies the scaling requirements of miniaturisation and manufacturability, but at 1.55 µm it suffers from problematic linear and nonlinear loss. Here we show that, by translating silicon quantum photonics to the mid-infrared, a new quantum optics platform is created which can simultaneously maximise manufacturability and miniaturisation, while reducing loss. We demonstrate the necessary platform components: photon-pair generation, single-photon detection, and high-visibility quantum interference, all at wavelengths beyond 2 µm. Across various regimes, we observe a maximum net coincidence rate of 448 ± 12 Hz, a coincidence-to-accidental ratio of 25.7 ± 1.1, and, a net two-photon quantum interference visibility of 0.993 ± 0.017. Mid-infrared silicon quantum photonics will bring new quantum applications within reach.
Quantum silicon photonics must overcome considerable loss engineering challenges before it can scale to the millions of physical qubits required for fault tolerant quantum computation [1] . The first such challenge is the intrinsic two-photon absorption (TPA), present in silicon at telecommunications wavelengths. This places fundamental limits on the heralding or Klyshko efficiency of silicon photon-pair sources based on spontaneous four-wave mixing (SFWM) [2] . We have recently shown that by moving to a pump wavelength beyond 2-microns, in the mid-infrared (MIR), TPA in silicon can be curtailed, and by using 340-nm thick silicon waveguides, phase matching can be achieved for efficient SFWM [2] .
Photon-pair sources are essential for complex and scalable quantum photonics circuits. Silicon photonics offers a scalable and low cost platform with a mature fabrication technology for realising this [1] . Recent demonstrations of photon-pair generation and quantum interference in the 2.1 µm band have paved the way towards mid-infrared integrated photon sources [2] , where many gases can be sensed with unique ‘fingerprint’ absorption lines. Producing long wavelength photons with large spectral detunings from the pump, utilising anomalous group-velocity dispersion and type-0 phase matching, is hard due to the highly dispersive nature of nanoscale silicon waveguides.
Abstract Although holding great promise for low noise, ease of operation and networking 1 , useful photonic quantum computing has been precluded by the need for beyond-state-of-the-art components, manufactured by the millions 2–6 . Here we introduce a manufacturable platform 7 for quantum computing with photons. We benchmark a set of monolithically integrated silicon-photonics-based modules to generate, manipulate, network and detect heralded photonic qubits, demonstrating dual-rail photonic qubits with 99.98% ± 0.01% state preparation and measurement fidelity, Hong–Ou–Mandel (HOM) quantum interference between independent photon sources with 99.50% ± 0.25% visibility, two-qubit fusion with 99.22% ± 0.12% fidelity and a chip-to-chip qubit interconnect with 99.72% ± 0.04% fidelity, conditional on photon detection and not accounting for loss. We preview a selection of next-generation technologies: low-loss silicon nitride (SiN) waveguides and components to address loss, as well as fabrication-tolerant photon sources, high-efficiency photon-number-resolving detectors (PNRDs), low-loss chip-to-fibre coupling and barium titanate (BTO) electro-optic phase shifters for high-performance fast switching.
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.
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.