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Quantum Computing, Communication, and Simulation

Two routes towards fibre-integrated optical switching with single photons

Cameron McGarry, Tabijah Wasawo, W.O. Davis, Kerrianne Harrington, Kristina R. Rusimova, J. Nunn, T. A. Birks, Peter J. Mosley

Published Mar 13, 2024• 0 citations
Publisher

Abstract

A fast, low-loss optical switch would be a powerful tool for photonic quantum technologies, allowing the implementation of, for example, rapid quantum logic gates, loop memories, and improved heralded single photon sources. Integration into optical fibre promises an avenue to future scalability, but existing optical switches cannot achieve both high speed and high efficiency. Here I present two routes to fibre-integrated, low -loss and fast optical switching. The first uses the acousto-optic effect in a tapered fibre. We demonstrate that light in the tapered region of the fibre can be coupled between optical modes by an acoustic wave, which introduces a phase-shift. The second route is by a two-photon transition in rubidium vapour. A strong control field modulates the mode of a weak signal field. We demonstrate this effect with low loss (1dB) and on short timescales (100MHz) in bulk optics, and work towards a fibre-integrated vapour cell.

DOI
10.1117/12.2692959

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A fast, low-loss optical switch would be a powerful tool for photonic quantum technologies, allowing the implementation of, for example, rapid quantum logic gates, loop memories, and improved heralded single photon sources. Integration into optical fibre promises an avenue to future scalability, but existing optical switches cannot achieve both high speed and high efficiency. Here I present two routes to fibre-integrated, low -loss and fast optical switching. The first uses the acousto-optic effect in a tapered fibre. We demonstrate that light in the tapered region of the fibre can be coupled between optical modes by an acoustic wave, which introduces a phase-shift. The second route is by a two-photon transition in rubidium vapour. A strong control field modulates the mode of a weak signal field. We demonstrate this effect with low loss (1dB) and on short timescales (100MHz) in bulk optics, and work towards a fibre-integrated vapour cell.
The ability to dynamically route and manage optical signals at high speeds with low-loss is a substantial challenge in quantum optics. Existing optical switches, such as electro-optic phase modulators either achieve high-speed or low insertion losses, but rarely both. However, optically-controlled phase modulation in atomic vapour promises a pathway for efficient, high-bandwidth switching [1]. These characteristics are critical for single photon multiplexing in quantum technologies, implementing loop memory schemes and addressing scalability challenges in photonic quantum computation [2]. We present a cavity based system that can deliver high-speed all-optical switching of a signal field with minimal loss by accessing the strong two photon absorption in warm rubidium vapour via a counter-propagating control field.
Abstract Low-loss high-speed switches are an integral component of future photonic quantum technologies, with applications in state generation, multiplexing, and the implementation of quantum gates. Phase modulation is one method of achieving this switching; however, existing optical phase modulators offer either high bandwidth or low loss—not both. We demonstrate fast (100 MHz bandwidth), low-loss ( 83 ( 2 ) % transmission) phase shifting ( Δ ϕ = ( 0.90 ( 5 ) ) π ) in a signal field, induced by a control field, and mediated by the two-photon 5 S 1 / 2 → 5 P 3 / 2 → 5 D 5 / 2 transition in 87 Rb vapour. The all-optical nature of the scheme circumvents restrictions of electronic phase modulators, where bandwidth and repetition rate can be limited by the requirement to rapidly modulate high voltages. We discuss routes to enhance both performance and scalability for application to a range of quantum and classical technologies.
A key resource for quantum optics experiments is an on-demand source of single and multiple photon states at telecommunication wavelengths. This letter presents a her- alded single photon source based on a hybrid technology approach, combining high efficiency periodically poled lithium niobate waveguides, low-loss laser inscribed circuits, and fast (>1 MHz) fibre coupled electro-optic switches. Hybrid inter- facing different platforms is a promising route to exploiting the advantages of existing technology and has permitted the demonstration of the multiplexing of four identical sources of single photons to one output. Since this is an integrated tech- nology, it provides scalability and can immediately leverage any improvements in transmission, detection and photon production efficiencies.
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.
Efficient single-photon routing and switching are crucial for optical quantum computing and communication. For this purpose, all-optical switches are designed for gigahertz bandwidths. The switching mechanism is based on the optical Kerr effect via cross-phase modulation (CPM) of the single-photon signal by a strong 1550-nm pump pulse. For energy-efficient switching, this nonlinear effect is exploited in a microresonator that can either be used directly as an intensity switch in a typical add–drop configuration or as a phase shifter in a Mach–Zehnder interferometer (MZI) structure. To speed up resonance build-up and quenching, a pre-emphasis build and an off-resonance wipe pulse are used. The proposed designs are verified by traveling-wave simulations which demonstrate that 0.1 dB insertion loss and ~1 ns switching windows can be achieved. For a scalable out-of-the-lab transfer, we investigate the feasibility of the proposed switch designs for fabrication in a mature photonic integrated circuit (PIC) platform. In particular, silicon nitride PICs have demonstrated record-low losses which makes them suitable for single-photon applications. By parametric modelling of the microresonator’s directional couplers based on Lumerical EME and 2.5-dimensional varFDTD simulations, the required power transmission coefficients for both signal and pump wavelength can be achieved. This results in an all-optical switch design ready for fabrication in a commercial PIC foundry which can potentially enable scalable architectures for quantum photonic applications.