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Gaussian Boson Sampling (GBS) is a quantum computing concept based on drawing samples from a multimode nonclassical Gaussian state using photon-number resolving detectors. It was initially posed as a near-term approach aiming to achieve quantum advantage, but several applications have been proposed ever since, such as the calculation of graph features or molecular vibronic spectra, among others. For the first time, we use a time-bin encoded interferometer to implement GBS experimentally and extract samples to enhance the search for dense subgraphs in a graph. Our results indicate an improvement over classical methods for subgraphs of sizes three and four in a graph containing ten nodes. In addition, we numerically explore the role of imperfections in the optical circuit and on the performance of the algorithm.

Photonics integrated circuitry would benefit considerably from the ability to arbitrarily control waveguide cross-sections with high precision and low loss, in order to provide more degrees of freedom in manipulating propagating light. Here, we report on a new optical-fibres-compatible glass waveguide by femtosecond laser writing, namely spherical phase induced multi-core waveguide (SPIM-WG), which addresses this challenging task with three dimensional on-chip light control. Precise deformation of cross-sections is achievable along the waveguide, with shapes and sizes finely controllable of high resolution in both horizontal and vertical transversal directions. We observed that these waveguides have high refractive index contrast of 0.017, low propagation loss of 0.14 dB/cm, and very low coupling loss of 0.19 dB coupled from a single mode fibre. SPIM-WG devices were easily fabricated that were able to perform on-chip beam rotation through varying angles, or manipulate polarization state of propagating light for target wavelengths. We also demonstrated SPIM-WG mode converters that provide arbitrary adiabatic mode conversion with high efficiency between symmetric and asymmetric non-uniform modes; examples include circular, elliptical modes and asymmetric modes from ppKTP waveguides which are generally applied in frequency conversion and quantum light sources. Created inside optical glass, these waveguides and devices have the capability to operate across ultra-broad bands from visible to infrared wavelengths. The compatibility with optical fibre also paves the way toward packaged photonic integrated circuitry, which usually needs input and output fibre connections.

We present a new method for the spectral characterization of pulsed twin beam sources in the high gain regime, using cascaded stimulated emission. We show an implementation of this method for a ppKTP spontaneous parametric down-conversion source generating up to 60 photon pairs per pulse, and demonstrate excellent agreement between our experiments and our theory. This work enables the complete and accurate experimental characterization of high gain effects in parametric down conversion, including self and cross-phase modulation. Moreover, our theory allows the exploration of designs with the goal of improving the specifications of twin beam sources for application in quantum information, computation, sampling, and metrology.

Science, medicine and engineering demand efficient information processing. It is a long-standing goal to use quantum mechanics to significantly improve such computations. The processing routinely involves examining data as a function of complementary variables, e.g., time and frequency. This is done by the Fourier transform approximations which accurately compute inputs of $2^n$ samples in $O(n 2^n)$ steps. In the quantum domain, an analogous process exists, namely a Fourier transform of quantum amplitudes, which requires exponentially fewer $O(n \log n)$ quantum gates. Here, we report a quantum fractional Kravchuk-Fourier transform, a related process suited to finite string processing. Unlike previous demonstrations, our architecture involves only one gate, resulting in constant-time processing of quantum information. The gate exploits a generalized Hong--Ou--Mandel effect, the basis for quantum-photonic information applications. We perform a proof-of-concept experiment by creation of large photon number states, interfering them on a beam splitter and using photon-counting detection. Existing quantum technologies may scale it up towards diverse applications.

We discuss the use of quasi-phase-matched downconversion to engineer the amount and type of space-time entanglement present in two-photon states. We also discuss a quantum positioning scheme based on a source of multiple entangled photons and report the implementation of this scheme for the case of two entangled photons.

We demonstrate increased accuracy and precision in the reconstruction of the multidimensional phase of electromagnetic fields based on multiple spectral shearing interferometry measurements made with shears of an arbitrary magnitude. © 2011 Optical Society of America.

Calculations relating to two experiments that demonstrate coherent control of preformed rubidium-85 molecules in a magneto-optical trap using ultrafast laser pulses are presented. The two experiments are a step toward the stabilization of ultracold rubidium dimers using ultrafast lasers. In the first experiment, it is shown that preassociated molecules in an incoherent mixture of states can be made to oscillate coherently using a single ultrafast pulse. A mechanism that can transfer molecular population to more deeply bound vibrational levels is used in the second. Optimal parameters of the control pulse are presented for the application of the mechanism to molecules in a magneto-optical trap. The calculations make use of an experimental determination of the initial state of molecules photoassociated by the trapping lasers in the magneto-optical trap and use shaped pulses consistent with a standard ultrafast laser system. The experiment's purpose is to demonstrate and evaluate the use of ultrafast shaped pulses to manipulate ultracold rubidium dimers with a view to the eventual stabilization of the molecules. © 2009 The American Physical Society.

We report an experimental realisation of Maxwell's demon in a photonic setup. We show that a measurement at the single-photon level followed by a feed-forward operation allows the extraction of work from intense thermal light into an electric circuit. The interpretation of the experiment stimulates the derivation of a new equality relating work extraction to information acquired by measurement. We derive a bound using this relation and show that it is in agreement with the experimental results. Our work puts forward photonic systems as a platform for experiments related to information in thermodynamics., 8 pages, 3 figures

We report a comprehensive approach to analysing continuous-output photon detectors. We employ principal component analysis to maximise the information extracted, followed by a novel noise-tolerant parameterised approach to the tomography of PNRDs. We further propose a measure for rigorously quantifying a detector's photon-number-resolving capability. Our approach applies to all detectors with continuous-output signals. We illustrate our methods by applying them to experimental data obtained from a transition-edge sensor (TES) detector., 5 pages, 3 figures, also includes supplementary information

We present a technique for frequency-resolved wavefront characterization of high harmonics based on lateral shearing interferometry. Tilted replicas of the driving laser pulse are produced by a Mach-Zehnder interferometer, producing separate focii in the target. The interference of the resulting harmonics on a flat-field extreme ultraviolet spectrometer yields the spatial phase derivative. A comprehensive set of spatial profiles, resolved by harmonic order, validate the technique and reveal the interplay of single-atom and macroscopic effects. © 2011 Optical Society of America.

Quantum storage of multiple optical modes affords improved performance for quantum repeaters. We present new analytic and numerical results unifying the scaling of the multimode storage capacity for various memory protocols in artificially broadened ensembles. © 2008 Optical Society of America.

We present the first quantum tomography of a detector, using as examples an avalanche photodiode and a photon-number resolving detector. The resulting POVM set agrees well with one derived from a model of the detector. © 2008 Optical Society of America.

We demonstrate experimentally the realization of heralded pure photons generation in a birefringent optical fiber. © 2008 Optical Society of America.

We experimentally demonstrate a heralded source of high-purity two-photon NOON states derived from heralded single-photon sources. © 2008 Optical Society of America.

We propose two methods for measuring the complete phase and amplitude of attosecond X-ray pulses produced by high harmonic generation in an atomic gas. These methods are based on spectral shearing interferometry and are achieved by pumping the harmonic source with two spectrally sheared driving pulses. This can be achieved in two configurations: either by having a time delay between the two driving pulses or by separating them spatially in the generation region. We show by numerical simulation that both these techniques are capable of retrieving full field information of the attosecond pulses produced and discuss their experimental implementation.

An optical quantum memory, a device which can store and retrieve on demand an arbitrary quantum state at the single photon level, has been identified as a significant cornerstone of photonic quantum technologies. The development of such a memory would allow the creation and development of a large number of quantum technologies. These would range from the ability to synchronise processing steps in an optical quantum computer, to the construction of a quantum repeater which would allow faithful transmission of quantum states over arbitrarily long distances. In this thesis I investigate broadband light storage in a room-temperature Raman memory for purposes of quantum information processing with photons. During the course of this thesis, a novel scheme to suppress four-wave mixing noise is simulated, based on equations derived to describe the physics of the memory interaction. The experimental arrangement to test the suppression method is described and an original method using photon statistics is demonstrated. The results of these experiments prove the ability of the memory to retrieve and maintain the non-classicality of the state, given a single photon input. Following the affirmation that, using this noise suppression technique, the memory can function at the single-photon level, I go on to consider how the temporal-spectral properties of a pulsed mode of light may be manipulated using the memory. There are two primary reasons for this investigation: 1) Increasing the efficiency of the read-in process by carving the optimum mode of the memory 2) Using the memory as a device operating in a high dimensional quantum information basis. The restrictions of such applications are considered and a method to test the efficacy of transformations in this basis is devised and experimentally tested. I also perform demonstrations of light storage using conventional temporal modes using a novel method of velocity selection within warm vapour. Finally I detail further experiments that are achievable with the Raman quantum memory given the progress that has been achieved during my DPhil research. The results show that the Raman memory is a viable candidate for a practical route towards low-noise quantum storage and for manipulation of temporal modes of light.

Measurements on states of light underpins the entire fields of classical and quantum optics. Without measurement we are unable to gain any information about the light we are studying or about the processes it has undergone. The property of the density matrix of light prior to measurement that we want to measure also relies heavily on the measurement technique we employ. We can use a single-photon detector to measure the particle nature of light--the photon--the discrete quanta of the electromagnetic field. We can conversely measure the wave nature of the light field by making phase-sensitive measurements. In this thesis, we shall explore some advanced implementations and applications of these measurement schemes. First, we shall focus on a specific type of single-photon detector: the Superconducting-Nanowire Single-Photon Detector. We will first consider the integration of these detectors on optical waveguides, and discuss their optical characterisation. Then we will consider a practical application of these detectors by investigating a method to perform broadband spectroscopy with a single detector. We shall then progress to phase-sensitive measurements, and consider the intermediate regime between measuring the wave-like and particle-like nature of light. First we will perform a fundamental investigation of the transition between both measurement regimes. We will see how varying the phase reference itself allows us to tune between both regimes. Following this we will discuss an application of this intermediate regime, by considering an experimental setup to perform state tomography with a weak phase reference. The final consideration of this thesis will be a theoretical benchmarking scheme for a practical application of quantum optics. The manipulation of certain quantum states of light followed by a photon-number measurement can have practical applications in a variety of fields, and therefore validating the results is of extreme importance. On this final topic, we will discuss an experimentally friendly technique to benchmark the output of such measurements by considering smaller subsystems, and the practical requirements of this technique.

The main topic of this thesis is the development of new characterization techniques for Ultrashort pulses with particular focus on pulses with complex spectra, often obtained with light pulse synthesisers. More in detail, I have addressed the longstanding problem of retrieving the spectral phase of a pulse which exhibits a spectral gap, i.e. a spectral null larger than any of the spectral features of the pulse. The major challenge in this scenario is attaining a reliable reconstruction of the relative spectral phase between the spectral components. I discuss the different approaches that I have employed in order to achieve this objective. The first result has been the realisation of a low-power XFROG for the charac- terisation of a coherent pulse synthesis source developed by Prof. D. Reid’s group at Heriot-Watt University. We used it to characterise, compress, and synchronise the individual components of the source, however the relative phase remained elusive. In order to solve this problem, a radically new characterisation technique needed to be implemented. This consisted in the development of a new algorithm (MICE), which enabled the mutual characterisation of multiple fields, and in a new experimental implementation of the well-known SPIDER technique (SPICE). Both the algorithm and the new technique have been tested experimentally in a proof-of-principle experiment which constitutes the main result of this work. The appeal of these solutions for mastering light synthesisers will likely stimulate further refinements towards making it the standard technique for complex spectra.

Photonics has consistently played an important role in the investigation of quantum-enhanced technologies and the corresponding study of fundamental quantum phenomena. The majority of these experiments have relied on the free space propagation of light between bulk optical components. This relatively simple and flexible approach often provides the fastest route to small proof-of-principle demonstrations. Unfortunately, such experiments occupy significant space, are not inherently phase stable, and can exhibit significant scattering loss which severely limits their use. Integrated photonics offers a scalable route to building larger quantum states of light by surmounting these barriers. In the first half of this thesis, we describe the operation of on-chip heralded sources of single photons. Loss plays a critical role in determining whether many quantum technologies have any hope of outperforming their classical analogues. Minimizing loss leads us to choose Spontaneous Four-Wave Mixing (SFWM) in a silica waveguide for our source design; silica exhibits extremely low scattering loss and emission can be efficiently coupled to the silica chips and fibers that are widely used in quantum optics experiments. We show there is a straightforward route to maximizing heralded photon purity by minimizing the spectral correlations between emitted photon pairs. Fabrication of identical sources on a large scale is demonstrated by a series of high-visibility interference experiments. This architecture offers a promising route to the construction of nonclassical states of higher photon number by operating many on-chip SFWM sources in parallel. In the second half, we detail one of the first proof-of-principle demonstrations of a new intermediate model of quantum computation called boson sampling. While likely less powerful than a universal quantum computer, boson sampling machines appear significantly easier to build and may allow the first convincing demonstration of a quantum-enhanced computation in the not-distant future. Boson sampling requires a large interferometric network which are challenging to build with bulk optics, we therefore perform our experiment on-chip. We model the effect of loss on our postselected experiment and implement a circuit characterization technique that accounts for this loss. Experimental imperfections, including higher-order emission from our photon pair sources and photon distinguishability, are modeled and found to explain the sampling error observed in our experiment.

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., 7 pages, 4 figures

The generation of large multiphoton quantum states - for applications in computing, metrology, and simulation - requires a network of high-efficiency quantum memories capable of storing broadband pulses. Integrating these memories into a fibre offers a number of advantages towards realising this goal: strong light-matter coupling at low powers, simplified alignment, and compatibility with existing photonic architectures. Here, we introduce a large-core kagome-structured hollow-core fibre as a suitable platform for an integrated fibre-based quantum memory with a warm atomic vapour. We demonstrate, for the first time, efficient optical pumping in a hollow-core photonic-crystal fibre with a warm atomic vapour, where (90 $\pm$ 1)% of atoms are prepared in the ground state. We measure high optical depths (3$\times 10^{4}$) and, also, narrow homogeneous linewidths that do not exhibit significant transit-time broadening. Our results establish that kagome fibres are suitable for implementing a broadband, room-temperature quantum memory., 10 pages, 4 figures

Path entanglement is a key resource for quantum metrology. Using path-entangled states, the standard quantum limit can be beaten, and the Heisenberg limit can be achieved. However, the preparation and detection of such states scales unfavourably with the number of photons. Here we introduce sequential path entanglement, in which photons are distributed across distinct time bins with arbitrary separation, as a resource for quantum metrology. We demonstrate a scheme for converting polarization Greenberger-Horne-Zeilinger entanglement into sequential path entanglement. We observe the same enhanced phase resolution expected for conventional path entanglement, independent of the delay between consecutive photons. Sequential path entanglement can be prepared comparably easily from polarization entanglement, can be detected without using photon-number-resolving detectors, and enables novel applications.

Nonclassical states of light are necessary resources for quantum technologies such as cryptography, computation and the definition of metrological standards. Observing signatures of nonclassicality generally requires inferring either the photon number distribution or a quasi-probability distribution indirectly from a set of measurements. Here, we report an experiment in which the nonclassical character of families of quantum states is assessed by direct inspection of the outcomes from a multiplexed photon counter. This scheme does not register the actual photon number distribution; the statistics of the detector clicks alone serve as a witness of nonclassicality, as proposed by Sperling et al. in Phys. Rev. Lett. 109, 093601 (2012). Our work paves a way for the practical characterisation of increasingly sophisticated states and detectors., Comment: 5 pages, 4 figures. References added. Note that Eq. (2) has been corrected in this version from the published manuscript. We are grateful to Arturo Lezama for pointing out the error

While 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 construct 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 benchmark our QBSM with three and four photons and analyze sources of sampling inaccuracy. Our studies pave the way to larger devices that could offer the first definitive quantum-enhanced computation., Main text: 5 pages, 4 figures. Supp Info: 6 pages