Although universal quantum computers ideally solve problems such as factoring integers exponentially more efficiently than classical machines, the formidable challenges in building such devices motivate the demonstration of simpler, problem-specific algorithms that still promise a quantum speedup. We constructed a quantum boson-sampling machine (QBSM) to sample the output distribution resulting from the nonclassical interference of photons in an integrated photonic circuit, a problem thought to be exponentially hard to solve classically. Unlike universal quantum computation, boson sampling merely requires indistinguishable photons, linear state evolution, and detectors. We benchmarked our QBSM with three and four photons and analyzed sources of sampling inaccuracy. Scaling up to larger devices could offer the first definitive quantum-enhanced computation.

Quantum teleportation is a fundamental concept in quantum physics [1] which now finds important applications at the heart of quantum technology including quantum relays [2,3], quantum repeaters [4] and linear optics quantum computing (LOQC) [5,6]. Photonic implementations have largely focussed on achieving long distance teleportation due to its suitability for decoherence-free communication [7][8][9]. Teleportation also plays a vital role in the scalability of photonic quantum computing [5,6], for which large linear optical networks will likely require an integrated architecture. Here we report the first demonstration of quantum teleportation in which all key parts-entanglement preparation, Bellstate analysis and quantum state tomographyare performed on a reconfigurable integrated photonic chip. We also show that a novel elementwise characterisation method is critical to mitigate component errors, a key technique which will become increasingly important as integrated circuits reach higher complexities necessary for quantum enhanced operation.Quantum teleportation is essential to many schemes for universal fault-tolerant quantum computation, making it an important protocol for any physical implementation of a quantum information processor [10,11]. In their seminal work, Knill, Laflamme, and Milburn showed that such a quantum processor could be constructed using only linear optical elements, at the expense of rendering each quantum logic gate probabilistic [5]. Adapting the teleportation scheme of Gottesman and Chuang [6], they then showed that this protocol could be efficiently scaled to a large number of concatenated gates, motivating a renewed interest in building more complex linear optical circuits for quantum information processing [11]. Realizing such a scheme requires building large, sophisticated networks of nested optical interferometers. This motivates the use of waveguides integrated onto compact and inherently stable photonic chips, and pioneering work has shown the viability of this approach for two- [12][13][14] and three-photon interference experiments [15][16][17]. These latter works highlighted the problems caused by photon loss, low data rates, and fabrication imperfections which make the extension to even higher photon numbers far from straightforward.Whilst photonic experiments were the first to realize quantum teleportation [18,19], demonstrations of this protocol in a waveguide architecture have been limited to fiber-based experiments [9,20]. Although there has been recent progress [21], no integrated photonic experiments have yet been able to demonstrate actual teleportation, due to the difficulty in realizing three photonic qubits on a sufficiently complex circuit [15]. In particular, integrated components require careful attention to fabricated deviations from design and the effects of increased and potentially unbalanced propagation loss. Experimental verification that integrated photonic circuits continue to perform well as their complexity increases is therefore of considerable interes...

Increasing the complexity of quantum photonic devices is essential for many optical information processing applications to reach a regime beyond what can be classically simulated, and integrated photonics has emerged as a leading platform for achieving this. Here we demonstrate three-photon quantum operation of an integrated device containing three coupled interferometers, eight spatial modes and many classical and nonclassical interferences. This represents a critical advance over previous complexities and the first on-chip nonclassical interference with more than two photonic inputs. We introduce a new scheme to verify quantum behaviour, using classically characterised device elements and hierarchies of photon correlation functions. We accurately predict the device's quantum behaviour and show operation inconsistent with both classical and bi-separable quantum models. Such methods for verifying multiphoton quantum behaviour are vital for achieving increased circuit complexity. Our experiment paves the way for the next generation of integrated photonic quantum simulation and computing devices.

Interference between independent single photons is perhaps the most fundamental interaction in quantum optics. It has become increasingly important as a tool for optical quantum information science, as one of the rudimentary quantum operations, together with photon detection, for generating entanglement between non-interacting particles. Despite this, demonstrations of large-scale photonic networks involving more than two independent sources of quantum light have been limited due to the difficulty in constructing large arrays of high-quality single photon sources. Here, we solve the key challenge, reporting a novel array of more than eighteen near-identical, low-loss, highpurity, heralded single photon sources achieved using spontaneous four-wave mixing (SFWM) on a silica chip. We verify source quality through a series of heralded Hong-Ou-Mandel experiments, and further report the experimental three-photon extension of the entire Hong-Ou-Mandel interference curves, which map out the interference landscape between three independent single photon sources for the first time.Recently, integration of photon pair sources on-chip has been recognized as one of the most promising approaches to scaling due to their small size, direct compatibility with integrated photonic architectures, reduction in required pump power, and potentially exquisite control of the populated optical modes [10,[21][22][23]. Unfortunately, fabrication imperfections or material limitations frequently spoil this dream. Optical loss is a key parameter for any quantum light source and on-chip sources frequently suffer from large losses due to high scattering and outcoupling mode mismatch [23][24][25]. In addition, the phase-matching conditions for the spontaneous scattering process are highly sensitive to optical dispersion. arXiv:1603.06984v1 [quant-ph]

Abstract:The integrated optical circuit is a promising architecture for the realization of complex quantum optical states and information networks. One element that is required for many of these applications is a highefficiency photon detector capable of photon-number discrimination. We present an integrated photonic system in the telecom band at 1550 nm based on UV-written silica-on-silicon waveguides and modified transition-edge sensors capable of number resolution and over 40 % efficiency. Exploiting the mode transmission failure of these devices, we multiplex three detectors in series to demonstrate a combined 79 % ± 2 % detection efficiency with a single pass, and 88 % ± 3 % at the operating wavelength of an on-chip terminal reflection grating. Furthermore, our optical measurements clearly demonstrate no significant unexplained loss in this system due to scattering or reflections. This waveguide and detector design therefore allows the placement of number-resolving single-photon detectors of predictable efficiency at arbitrary locations within a photonic circuit -a capability that offers great potential for many quantum optical applications. *Contribution of NIST, an agency of the U.S. government, not subject to copyright

Integration is currently the only feasible route toward scalable photonic quantum processing devices that are sufficiently complex to be genuinely useful in computing, metrology, and simulation. Embedded on-chip detection will be critical to such devices. We demonstrate an integrated photon-number-resolving detector, operating in the telecom band at 1550 nm, employing an evanescently coupled design that allows it to be placed at arbitrary locations within a planar circuit. Up to five photons are resolved in the guided optical mode via absorption from the evanescent field into a tungsten transition-edge sensor. The detection efficiency is 7.2 ± 0.5 %. The polarization sensitivity of the detector is also demonstrated. Detailed modeling of device designs shows a clear and feasible route to reaching high detection efficiencies.Photonics provides a promising path for building and using complex quantum systems for both exploring fundamental physics and delivering quantum-enhanced technologies in information processing, metrology, and communications. Currently, the only feasible route toward sufficient complexity is integration, due to the high density of optical modes that can be contained within a single device and the extraordinary level of control that can be exercised over them. Although much research has gone into developing integrated elements at telecom wavelengths for classical applications, their use in the quantum regime has been limited, in large part because of intrinsic inefficiencies in input coupling, detector coupling, and propagation. The effect of these inefficiencies is to reduce or remove any quantum advantage attainable with a given device [1][2][3][4][5][6][7].Current single-photon-sensitive detectors for telecom wavelengths include avalanche photodiodes (APDs) [8], superconducting nanowires [9], and transition-edge sensors (TESs) [10,11]. In x Ga 1-x As APDs, the only commercially available telecom-band, single-photon-sensitive detectors, suffer from high dark-count rates, whereas nanowire detectors have much lower dark-count rates, are extremely fast, and can have high quantum efficiencies comparable to those of In x Ga 1-x As APDs [12]. In order to achieve high efficiencies with these normal incidence detectors, care must be taken to impedance match the incident field to the detector in order to avoid reflections of the optical signal. Moreover, normal incidencedetection schemes are intrinsically limited to monitoring the modes that emerge from the end facet of the device. As a result, inferring information about a quantum state or circuit element inside a device will only become more problematic as circuits move toward the complexities required to study effects beyond the scope of classical computational power [7,13,14]. Developing high-efficiency detectors that are compatible with these complex, high-density systems is therefore a critical enabling step for quantum photonics.In this paper, we demonstrate the operation of a new concept for broadband, efficient, single-photon detection, evanesc...

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A direct UV grating writing technique based on phase-controlled interferometry is proposed and demonstrated in a silica-on-silicon platform, with a wider wavelength detuning range than any previously reported UV writing technology. Electro-optic phase modulation of one beam in the interferometer is used to manipulate the fringe pattern and thus control the parameters of the Bragg gratings and waveguides. Various grating structures with refractive index apodization, phase shifts and index contrasts of up to 0.8 × 10 −3 have been demonstrated. The method offers significant time/energy efficiency as well as simplified optical layout and fabrication process. We have shown Bragg gratings can be made from 1200nm to 1900nm exclusively under software control and the maximum peak grating reflectivity only decreases by 3dBover a 250 nm (~32THz) bandwidth.