Quantum walks of correlated particles offer the possibility to study large-scale quantum interference, simulate biological, chemical and physical systems, and a route to universal quantum computation. Here we demonstrate quantum walks of two identical photons in an array of 21 continuously evanescently-coupled waveguides in a SiOxNy chip. We observe quantum correlations, violating a classical limit by 76 standard deviations, and find that they depend critically on the input state of the quantum walk. These results open the way to a powerful approach to quantum walks using correlated particles to encode information in an exponentially larger state space.With origins dating back to observations by Lucretius in 60BC and Brown in the 1800's, random walks are a powerful tool used in a broad range of fields from genetics to economics [1]. The quantum mechanical analoguequantum walks [2, 3]-corresponds to the tunnelling of quantum particles into several possible sites, generating large coherent superposition states and allowing massive parallelism in exploring multiple trajectories through a given connected graph (eg. Fig. 1). This quantum state evolution is a reversible (unitary) process and so requires low noise (decoherence) systems for observation. In contrast to the diffusive behaviour of (classical) random walks, which tend towards a steady state, the wave function in a quantum walk propagates ballistically (Fig. 2(c)). These features are at the heart of new algorithms for database-search [4], random graph navigation, models for quantum communication using spin chains [5], universal quantum computation [6] and quantum simulation [7].Quantum walks have been demonstrated using nuclear magnetic resonance [8,9], phase [10,11] and position [12] space of trapped ions, the frequency space of an optical resonator [13], single photons in bulk [14] and fibre [15] optics and the scattering of light in coupled waveguide arrays [16]. However, to date, all realisations have been limited to single particle quantum walks, which have an exact mapping to classical wave phenomena [17], and therefore cannot provide any advantage from quantum effects (note that the quantum walk with two trapped ions [11] encodes in the centre of mass mode and is therefore effectively a single particle quantum walk on a line). Indeed single particle quantum walks have been observed using classical light [16,18]. In contrast, for quantum walks of more than one indistinguishable particle, classical theory no longer provides a sufficient description-quantum theory predicts that probability amplitudes interfere leading to distinctly non-classical correlations [19,20]. This quantum behaviour gives rise to a computational advantage in quantum walks of two identical particles, which can be used to solve the graph isomorphism problem for example [21]. The major challenge associated with realising quantum walks of correlated particles is the need for a low decoherence system that preserves their non-classical features.The intrinsically low decoherence properti...

Linear optics underpins tests of fundamental quantum mechanics and computer science, as well as quantum technologies. Here we experimentally demonstrate the longstanding goal of a single reprogrammable optical circuit that is sufficient to implement all possible linear optical protocols up to the size of that circuit. Our six-mode universal system consists of a cascade of 15 MachZehnder interferometers with 30 thermo-optic phase shifters integrated into a single photonic chip that is electrically and optically interfaced for arbitrary setting of all phase shifters, input of up to six photons and their measurement with a 12 single-photon detector system. We programmed this system to implement heralded quantum logic and entangling gates, boson sampling with verification tests, and six-dimensional complex Hadamards. We implemented 100 Haar random unitaries with average fidelity 0.999 ± 0.001. Our system is capable of switching between these and any other linear optical protocol in seconds. These results point the way to applications across fundamental science and quantum technologies.Photonics has been crucial in establishing the foundations of quantum mechanics [1], and more recently has pushed the vanguard of efforts in understanding new non-classical computational possibilities. Typical protocols involve nonlinear operations, such as the generation of quantum states of light through optical frequency conversion [2,3], or measurement-induced nonlinearities for quantum logic gates [4], together with linear operations between optical modes to implement core processing functions [5]. Encoding qubits in the polarisation of photons has been particularly appealing for the ability to implement arbitrary linear operations on the two polarisation modes using a series of wave plates [6]. For path encoding the same operations can be mapped to a sequence of beamsplitters and phase shifters. In fact, since any linear optical (LO) circuit is described by a unitary operator, and a specific array of basic two-mode operations is mathematically sufficient to implement any unitary operator on optical modes [7], it is theoretically possible to construct a single device with sufficient versatility to implement any possible LO operation up to the specified number of modes.Here we report the realisation of this longstanding goal with a six-mode device that is completely reprogrammable and universal for LO. We demonstrate the versatility of this universal LO processor (LPU) by applying it to several quantum information protocols, including tasks that were previously not possible. We im- * anthony.laing@bristol.ac.uk plement heralded quantum logic gates at the heart of the circuit model of LO quantum computing [4] and new heralded entangling gates that underpin the measurementbased model of LO quantum computing [8][9][10], both of which are the first of their kind in integrated photonics. We perform 100 different boson sampling [11][12][13][14][15] experiments and simultaneously realise new verification protocols. Finally, we use multi-p...

Advances in control techniques for vibrational quantum states in molecules present new challenges for modelling such systems, which could be amenable to quantum simulation methods. Here, by exploiting a natural mapping between vibrations in molecules and photons in waveguides, we demonstrate a reprogrammable photonic chip as a versatile simulation platform for a range of quantum dynamic behaviour in different molecules. We begin by simulating the time evolution of vibrational excitations in the harmonic approximation for several four-atom molecules, including HCS, SO, HNCO, HFHF, N and P. We then simulate coherent and dephased energy transport in the simplest model of the peptide bond in proteins-N-methylacetamide-and simulate thermal relaxation and the effect of anharmonicities in HO. Finally, we use multi-photon statistics with a feedback control algorithm to iteratively identify quantum states that increase a particular dissociation pathway of NH. These methods point to powerful new simulation tools for molecular quantum dynamics and the field of femtochemistry.

Integrated photonic circuits are one of the most promising platforms for large-scale photonic quantum information systems due to their small physical size and stable interferometers with near-perfect lateral-mode overlaps. Since many quantum information protocols are based on qubits defined by the polarization of photons, we must develop integrated building blocks to generate, manipulate, and measure the polarization-encoded quantum state on a chip. The generation unit is particularly important. Here we show the first integrated polarization-entangled photon pair source on a chip. We have implemented the source as a simple and stable silicon-on-insulator photonic circuit that generates an entangled state with 91 ± 2% fidelity. The source is equipped with versatile interfaces for silica-on-silicon or other types of waveguide platforms that accommodate the polarization manipulation and projection devices as well as pump light sources. Therefore, we are ready for the full-scale implementation of photonic quantum information systems on a chip.

Abstract:We report the distribution of time-bin entangled photon pairs over 300 km of optical fiber. We realized this by using a high-speed and high signal-to-noise ratio entanglement generation/evaluation setup that consists of periodically poled lithium niobate waveguides and superconducting single photon detectors. The observed two-photon interference fringes exhibited a visibility of 84 %. We confirmed the violation of Bell's inequality by 2.9 standard deviations.

2Optical fibers have been enabling numerous distinguished applications involving the operation and generation of light, such as soliton transmission 1 , light amplification 2 , alloptical switching 3 and supercontinuum generation 4 . The active function of optical fibers in the quantum regime is expected to be applicable to ultralow-power all-optical signal processing 5 and quantum information processing 6 . Here we demonstrate the first experimental observation of optical nonlinearity at the single-photon level in an optical fiber. Taking advantage of large nonlinearity and managed dispersion of a photonic crystal fiber 7,8 , we have successfully measured very small (10 -7 ~ 10 -8 ) conditional phase shifts induced by weak coherent pulses that contain one or less than one photon per pulse on average. In spite of its tininess, the phase shift was measurable using much (~10 6 times) stronger coherent probe pulses than the pump pulses. We discuss the feasibility of quantum information processing using optical fibers, taking into account the observed Kerr nonlinearity accompanied by ultrafast response time and low induced loss.A photon, the light quantum having much less interaction with its environment than do other quanta (e.g., electron spin, superconducting current), is an outstanding carrier of information for quantum communication and thus is called a 'flying qubit.' This also means that photons may not be suited for computations that require strong unitary interaction between qubits. Hence, fabrication of optical nonlinear media that intermediate sufficiently strong interaction between photons has been under intense study. Cavity quantum electrodynamics-based devices have performed nonlinear Kerr phase shifts of a few ten degrees at the single-photon level 9,10 . Another approach to quantum-optical information processing (QOIP) is to apply weak nonlinearity inherent in currently existing media. Recent proposals 11,12 indicated that such moderately weak nonlinearity can mediate the interaction between photons or other qubits through a strong coherent light (known as a qubus).Exploring the availability of single-photon-level nonlinearity in various media is thus an important challenge that provides a test bed for nonlinear optical phenomena that may emerge in the quantum regime of light 13 .In the present experiment, we used a photonic crystal fiber (PCF) as a Kerr medium. PCF has a high capacity for confining light in its silica core by a large core-cladding index 3 contrast 7,8 . Taking advantage of this feature incorporated with its controlled dispersion property, PCF is widely applied in various applications such as supercontinuum generation 4 , entangled photon generation 14 , squeezing light 15 and a test of the event horizon 16 . To measure the expected ultrasmall phase shift at the single-photon level, we adopted a polarizationdivision Sagnac interferometer (SI) 17 . The SI has the advantage of inherent stability; two interfering beams counter-propagate through the same path in the interferometer so tha...

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We demonstrate the generation of quantum-correlated photon pairs from a Si photonic-crystal coupled-resonator optical waveguide. A slow-light supermode realized by the collective resonance of high-Q and small-mode-volume photonic-crystal cavities successfully enhanced the efficiency of the spontaneous four-wave mixing process. The generation rate of photon pairs was improved by two orders of magnitude compared with that of a photonic-crystal line defect waveguide without a slow-light effect.