New generation of optical integrated devices for future quantum computers

(a) Schematic representation of a 4×4 MMI integrated chip. (b) Simulation of classical light propagation in the device shown schematically in a.

A research group led by scientists from the University of Bristol has demonstrated the quantum operation of new components that will enable compact circuits for future photonic quantum computers.

Nature Communications – Multimode quantum interference of photons in multiport integrated devices

Photonics is a leading approach in realizing future quantum technologies and recently, optical waveguide circuits on silicon chips have demonstrated high levels of miniaturization and performance. Multimode interference (MMI) devices promise a straightforward implementation of compact and robust multiport circuits. Here, we show quantum interference in a 2×2 MMI coupler with visibility of V=95.6±0.9%. We further demonstrate the operation of a 4×4 port MMI device with photon pairs, which exhibits complex quantum interference behaviour. We have developed a new technique to fully characterize such multiport devices, which removes the need for phase-sensitive measurements and may find applications for a wide range of photonic devices. Our results show that MMI devices can operate in the quantum regime with high fidelity and promise substantial simplification and concatenation of photonic quantum circuits.

The parametric down-conversion source includes two Filter A (2nm FWHM in the 2×2 MMI measurements, 0.5nm FWHM in the 4×4 MMI measurements) to ensure single photons are indistinguishable.

Recently, the researchers from the University of Bristol’s Centre for Quantum Photonics showed, in several important breakthroughs, that quantum information can be manipulated with integrated photonic circuits. Such circuits are compact (enabling scalability) and stable (with low noise) and could lead in the near future to mass production of chips for quantum computers.

Now the team, in collaboration with Dr Terry Rudolph at Imperial College London, shows a new class of integrated divides that promise further reduction in the number of components that will be used for building future quantum circuits.

These devices, based on optical multimode interference (and therefore often called MMIs) have been widely employed in classical optics as they are compact and very robust to fabrication tolerances. “While building a complex quantum network requires a large number of basic components, MMIs can often enable the implementation with much fewer resources,” said Alberto Peruzzo, PhD student working on the experiment.

Until now it was not clear how these devices would work in the quantum regime. Bristol researchers have demonstrated that MMIs can perform quantum interference at the high fidelity required.

Scientists will now be able to implement more compact photonics circuits for quantum computing. MMIs can generate large entangled states, at the heart of the exponential speedup promised by quantum computing.

“Applications will range from new circuits for quantum computation to ultra precise measurement and secure quantum communication,” said Professor Jeremy O’Brien, director of the Centre for Quantum Photonics.

The team now plans to build new sophisticated circuits for quantum computation and quantum metrology using MMI devices.

Quantum technologies aim to harness superposition and entanglement to enhance communication security, provide exponential computational advantage for particular tasks including factoring, database search and simulation of important quantum systems, and reach the ultimate limits of precision in measurement. Photons are an appealing information carrier for their inherently low-noise, high-speed transmission, and the fact that entangling interactions between photons can be achieved using only linear optical circuits or mediated by atom-like systems. A photonics approach to these technologies requires complex, multiport quantum circuits—essentially multipath, multiphoton interferometers—that exhibit high fidelity quantum interference. Circuits fabricated from 2×2 directional couplers have demonstrated high performance; however, construction of more sophisticated multiport circuits would require their decomposition into a very large number of 2×2 directional couplers. For example, an arbitrary N×N mode unitary would require a sequence of O(n2) individual 2×2 directional couplers.

Multimode interference (MMI) devices are based on the self-imaging principle, by which an input field profile is reproduced in single or multiple images at periodic intervals along the propagation direction of a multimode waveguide. The effect is based on the propagation properties of a guide with a large number of lateral modes that see different effective refractive indices. Each mode propagates at a specific velocity accumulating different phases that results in constructive and destructive interference along the multimode region. At the position in which all the modes re-phase the total electromagnetic field is the same as the input, resulting in a self-imaged condition. MMI devices allow the design of N×M splitters with superior performances, excellent tolerance to polarization and wavelength variations, and relaxed fabrication requirements compared with the other main beam-splitting technology, the directional couplers. Consequently, MMI couplers have found applications in a broad range of photonic systems, including phase diversity networks, light switching and modulators, in laser architectures and for optical-sensing applications. In the context of photonic quantum circuits, they promise to dramatically reduce the complexity of such circuits, including for example those required to generate maximally entangled path or ‘NOON’ states, W states and the implementation of N×N unitaries.

In contrast to directional couplers, the self-imaging effect in MMIs allows the flexibility to directly realize symmetric N×N multiport devices with several input and output ports. Multiport circuits are particularly promising for quantum optics and information purposes, and fundamental experiments have been conducted to study the behaviour of non-classical interference of single photons in bulk optics and fibre circuits. However, their performance is limited by stability and control of the splitting ratios. The implementation of multiport splitters in MMI devices should allow higher performances because of the monolithic and scalable architecture. However, it is not clear that the multimode nature of MMI devices will allow quantum operations, in particular quantum interference.

In this paper, after a description of the devices fabrication and the experimental setup, we report our results on a 2×2 MMI. Then we show operation of a 4×4 MMI with single photon pairs injected in all possible pairs of input ports. Finally, we describe and apply a technique to characterize the 4×4 device based on the measured HOM dip visibilities.

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