P.G.R. Smith

Boson Sampling on a Photonic Chip

Computing Power of Quantum Mechanics There is much interest in developing quantum computers in order to perform certain tasks much faster than, or that are intractable for, a classical computer. A general quantum computer, however, requires the fabrication and operation a number of quantum logic devices (see the Perspective by Franson). Broome et al. (p. 794, published online 20 December) and Spring et al. (p. 798, published online 20 December) describe experiments in which single photons and quantum interference were used to perform a calculation (the permanent of a matrix) that is very difficult on a classical computer. Similar to random walks, quantum walks on a graph describe the movement of a walker on a set of predetermined paths; instead of flipping a coin to decide which way to go at each point, a quantum walker can take several paths at once. Childs et al. (p. 791) propose an architecture for a quantum computer, based on quantum walks of multiple interacting walkers. The system is capable of performing any quantum operation using a subset of its nodes, with the size of the subset scaling favorably with the complexity of the operation. Optical circuits are used to demonstrate a quantum-enhanced calculation. [Also see Perspective by Franson] 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.

Generation of high-power blue light in periodically poled LiNbO3

We report the generation of 450-mW average blue (473-nm) power by frequency doubling of a diode-pumped 946-nm Nd:YAG laser. We achieved pulsed operation at a high repetition rate (~160kHz) by driving the relaxation oscillations of the laser. A 40% conversion efficiency to the second harmonic was obtained in a single-pass, extracavity, first-order, quasi-phase-matched process in which periodically poled lithium niobate (period 4.5microm , thickness 0.5mm , and length 15mm) at 140 degrees C was used. The resulting high-power blue beam was circular in profile and nearly diffraction limited, indicating that photorefractive effects do not appear to limit device performance.

Microstructuring of lithium niobate using differential etch-rate between inverted and non-inverted ferroelectric domains

Single crystal samples of lithium niobate have been spatially patterned with photoresist, and subsequently domain inverted using electric field poling, to produce a range of two dimensional spatial domain structures. Differential etching has subsequently been carried out using mixtures of hydrofluoric and nitric acids, at a range of temperatures between room temperature and the boiling point. The structures produced show very smooth, well defined, deep features, which have a range of applications in optical ridge waveguides, alignment structures, V-grooves, and micro-tips. Details are given of the fabrication procedures, and examples of structures are shown.

Multiphoton quantum interference in a multiport integrated photonic device

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 devices 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.

Phase-controlled integrated photonic quantum circuits

Scalable photonic quantum technologies are based on multiple nested interferometers. To realize this architecture, integrated optical structures are needed to ensure stable, controllable, and repeatable operation. Here we show a key proof-of-principle demonstration of an externallycontrolled photonic quantum circuit based upon UV-written waveguide technology. In particular, we present non-classical interference of photon pairs in a Mach-Zehnder interferometer constructed with X couplers in an integrated optical circuit with a thermo-optic phase shifter in one of the interferometer arms.

Experimental and computational study of the gas-sensor behaviour and surface chemistry of the solid-solution Cr2−xTixO3(x≤ 0.5)

The solid solution Cr2−xTixO3 is an excellent gas sensor material, with stability of performance over the short and long-term and minor influences of variations of humidity. It is the first new material to be successfully commercialised in large-volume manufacture for sensing of hydrocarbons, volatile organic compounds (VOC), hydrogen and carbon monoxide at low (ppm) concentrations in air since the introduction of SnO2 for this purpose in the 1960s. The phase limit is at x ≃ 0.3–0.4, above which a 2-phase mixture with CrTiO3 is found. Substitution of Ti strongly decreases the electrical conductivity of the porous bodies studied. Surface high-valency Cr, assumed to be CrVI, whose proportion is decreased by Ti substitution, is detected by XPS. This effect, and the surface segregation of Ti, control the gas sensor behaviour. Defect models of the (0001) and (102) surfaces have been assessed by computational modelling: in the absence of Ti, one stable defect is a CrVI–VCr‴ pair, which is surface segregated at (0001) and contributes to the relatively high p-type conductivity shown by finely porous bodies of Cr2O3 at elevated temperature; with Ti addition, a stable defect, also surface segregated, is the complex (Ti˙Cr)3VCr‴. Distortion of the arrangement of surface oxygen above the Cr vacancy creates a possible binding site; the high-valency surface cation creates another. It is suggested that the two sites act in concert to promote the dissociation of oxygen and the surface reaction needed for gas sensing.

Direct ultraviolet writing of channel waveguides in congruent lithium niobate single crystals

We report the fabrication of optical channel waveguides in congruent lithium niobate single crystals by direct writing with continuous-wave ultraviolet frequency-doubled Ar+ laser radiation (244 nm). The properties and performance of such waveguides are investigated, and first results are presented.

Phase-mapping of periodically domain-inverted LiNbO3 with coherent X-rays

A varying refractive index across a wavefront leads to a change in the direction of propagation of the wave,. This provides the basis for phase-contrast imaging of transparent or weakly absorbing materials with highly coherent X-ray beams,. Lattice distortions can also change the direction of propagation of a wave field diffracted from a crystal. Here we report the use of this principle to effect phase-contrast imaging of the domain structure of a ferroelectric material, lithium niobate. A periodically domain-inverted structure for quasi-phase-matching of second-harmonic generation is created in this material, in which the direction of spontaneous polarization is sequentially inverted. Because of complex interactions during domain-inversion processing, this is accompanied by lattice distortions across the domain walls. These distortions split the diffracted wavefront of a beam of coherent X-rays from an advanced synchrotron source, giving rise to a pattern of interference that reflects the underlying pattern of lattice distortions. These results show that this phase-contrast imaging technique with sub-micrometre spatial resolution permits the non-destructive, highly sensitive phase-mapping of various structural defects and distortions introduced into materials during processing.

Quantum teleportation on a photonic chip

Quantum teleportation is a fundamental concept in quantum physics that now finds important applications at the heart of quantum technology, including quantum relays, quantum repeaters and linear optics quantum computing. Photonic implementations have largely focused on achieving long-distance teleportation for decoherence-free quantum communication. Teleportation also plays a vital role in photonic quantum computing for which large linear optical networks will probably require an integrated architecture. Here, we report a fully integrated implementation of quantum teleportation in which all key parts of the circuit - entangled state preparation, Bell-state analysis and tomographic state measurement - are performed on a reconfigurable photonic chip. We also show that a novel element-wise characterization method is critical to the mitigation of component errors, a key technique that will become increasingly important as integrated circuits reach the higher complexities necessary for quantum enhanced operation.

Highly efficient second-harmonic and sum-frequency generation of nanosecond pulses in a cascaded erbium-doped fiber:periodically poled lithium niobate source.

By combining erbium-doped fiber sources based on a large mode-area design and periodically poled lithium niobate, we have obtained single-pass conversion efficiencies of as much as 83% (energy efficiency) for second-harmonic generation into the near IR (768 nm) and of 34% for sum-frequency generation into the green (512 nm) for nanosecond pulses, using first-order quasi-phase matching. Pulse energies in excess of 80microJ of second harmonic have been obtained from systems pumped by a single laser diode.