We demonstrate quantum walks of correlated photons in a two-dimensional network of directly laser written waveguides coupled in a "swiss cross" arrangement. The correlated detection events show high-visibility quantum interference and unique composite behavior: strong correlation and independence of the quantum walkers, between and within the planes of the cross. Violations of a classically defined inequality, for photons injected in the same plane and in orthogonal planes, reveal nonclassical behavior in a nonplanar structure.
Multi-mode interference (MMI) devices fabricated in silicon oxynitride (SiON) with a refractive index contrast of 2.4% provide a highly compact and stable platform for multi-photon non-classical interference. MMI devices can introduce which-path information for photons propagating in the multi-mode section which can result in degradation of this non-classical interference. We theoretically derive the visibility of quantum interference of two photons injected in a MMI device and predict near unity visibility for compact SiON devices. We complement the theoretical results by experimentally demonstrating visibilities of up to 97.7% in 2×2 MMI devices without the requirement of narrow-band photons.
Quantum reservoir computing (QRC) has been strongly emerging as a time series prediction approach in quantum machine learning (QML). This work is the first to apply optimization to resource noise in a quantum reservoir to effectively improve time series prediction. Based on this development, we propose a new approach to quantum circuit optimization.We build on the work of Suzuki et al., who used quantum hardware noise as an essential resource in quantum noise-induced reservoir (QNIR) computer for generating non-trivial output sequences, and we achieve a novel, optimized QNIR, in which the artificial noise channels are parameterized. To optimize the parameterized resource noise, we use dual annealing and evolutionary optimization. Another essential component of our approach is reducing quantum resources in the number of artificial noise models, number of qubits, entanglement scheme complexity, and circuit depth. A key result is the reduction from a general multi-component noise model to a single reset noise model.Reservoir computers are especially well-suited for modelling nonlinear dynamical systems. In this paper we consider NARMA and Mackey-Glass systems, which are common univariate time series benchmarks for reservoir computers and recurrent neural networks. Recently QRCs have demonstrated good prediction capability with small numbers of qubits. QNIR simulations based on our optimization approach demonstrate high performance on NARMA benchmarks while using only a 12-qubit reservoir and a single noise model. Good prediction performances over 100 timesteps ahead for the Mackey-Glass system are demonstrated in the chaotic regime. In addition, these results provide valuable insight into resource noise requirements for the QNIR.
Integrated photonics is required to fully exploit the capabilities of Optical Quantum Information science. We demonstrate new components that take full advantage of the integrated architecture; we show quantum interference in MMI couplers and two-particle quantum walks in coupled waveguides.Quantum information science takes advantage of the fundamental laws of quantum mechanics to perform particular tasks better than the classical counterpart. It promises new paradigms of communication, computation and measurement; such as perfectly secure quantum key distribution, intrinsic parallel computation and increased precision by beating the standard quantum limit. Of the various physical systems being pursued, photons are often the logical choice [1], thanks to their low noise properties, ease of manipulation and transmission. In addition to single photon sources and detectors, photonic quantum technologies will rely on sophisticated optical circuits involving high-visibility classical and quantum interference. Already a number of optical quantum circuits have been realized for quantum metrology, lithography, quantum logic gates, and other entangling circuits. However, these demonstrations have relied on large-scale (bulk) optical elements bolted to large optical tables, thereby making them inherently non-scalable and confining them to the research laboratory. Recently we report the implementation of quantum optic integrated circuits, which not only dramatically reduces the footprint of quantum circuits, but allows unprecedented stability and control of the optical path length; this reveals the possibility for realizing previously unfeasible large scale quantum circuits [2]. We demonstrated silica on silicon circuits that implement key components for quantum information, including CNOT gates [3] and the circuit at the basis of any single-qubit operation [4]. These components show promising progresses toward fault tolerance operation [5]. We also used integrated waveguides to implement a circuit that performs a compiled version of Shor's quantum algorithm [6] to factorize 15. Here we report the demonstration of circuits that extend the capabilities of the components already demonstrated, to take full advantage of the integrated optics architecture.Different techniques can be used to realize beam splitting operations in integrated optics. Multi-mode interference (MMI) splitters 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 guide [7]. MMI devices allow the design of NxM splitters with superior performances, excellent tolerance to polarization and wavelength variations and relaxed fabrication requirements compared to the other main beam splitting technology, the directional couplers. MMI splitters were designed and fabricated in Silica on Silicon chips. The devices are composed by 2 or 4 single mode waveguides that serve as input and output for the multi-mode section and terminate with a...
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