This dissertation explores the role of optimized resources in facilitating efficient optical quantum information processing and sensing. Three key applications are considered: quantum interferometry, detector characterization, and multi-photon processing. For each application, we examine the underlying quantum resource states and quantum technology resources. Careful attention is paid to how these resources can be optimized to maximize the efficiency of the overarching application in terms of resources consumed, whether photonic, temporal, or infrastructural. Such optimization is an important step in fully realizing the potential of quantum science as it emerges from the laboratory and into the mainstream. For quantum interferometry, we investigate the traditional trade-offs between two-photon quantum interference and single-photon classical interference in terms of measurement resolution and resilience against optical loss and background. We show how we can overcome these trade-offs by replacing conventional single photons with highly non-degenerate energy-entangled photons, which are more optimized for achieving high information content on a per-photon basis. In combination with a specialized dual-wavelength interferometer, our high per-photon information efficiency allows us to achieve nanometer-scale resolution in a matter of seconds with only O(10^4) photon pairs. We then show how this entanglement-enhanced quantum sensing modality can allow for new studies in challenging regimes, such as characterizing lossy thin films or detecting distant vibrations. Then, for detector characterization, we consider the instrumental role of photon detectors in various optical quantum science protocols. As these protocols expand into spectral bands with growing technological applications, such as the mid-infrared, there is a developing need to characterize suitable detectors, with their efficiencies being one of the most important metrics. Typical methods for efficiency characterization, whether classical or quantum, can face the challenge of being infrastructure-intensive, with some requiring resources such as independent reference standards or large bulk-optics apparatuses. To help realize a resource-efficient characterization system for mid-infrared detectors, we have developed a compact and portable solution involving highly non-degenerate photon pairs produced by a fiber-coupled waveguide device. We evaluate the performance of early prototypes and identify next steps for improved device engineering. Lastly, for multi-photon processing, we focus on the single-photon source, given its key role in preparing the required photonic states. From a scalability perspective, efficient single-photon generation is essential for performing many large-scale multi-photon protocols within useful timescales. To meet this requirement, we explore how multiplexing techniques allow us to pair heralded single photons with photon routing, storage, and counting technologies to realize an efficient single-photon source. We then discuss our efforts to implement and integrate optimized versions of these technologies to address limitations faced by earlier implementations with regard to preparing pure single photons at a high rate. This discussion includes an in-depth analysis of our work related to realizing an efficient photon-number-resolving detector.
This dissertation explores the role of optimized resources in facilitating efficient optical quantum information processing and sensing. Three key applications are considered: quantum interferometry, detector characterization, and multi-photon processing. For each application, we examine the underlying quantum resource states and quantum technology resources. Careful attention is paid to how these resources can be optimized to maximize the efficiency of the overarching application in terms of resources consumed, whether photonic, temporal, or infrastructural. Such optimization is an important step in fully realizing the potential of quantum science as it emerges from the laboratory and into the mainstream. For quantum interferometry, we investigate the traditional trade-offs between two-photon quantum interference and single-photon classical interference in terms of measurement resolution and resilience against optical loss and background. We show how we can overcome these trade-offs by replacing conventional single photons with highly non-degenerate energy-entangled photons, which are more optimized for achieving high information content on a per-photon basis. In combination with a specialized dual-wavelength interferometer, our high per-photon information efficiency allows us to achieve nanometer-scale resolution in a matter of seconds with only O(10^4) photon pairs. We then show how this entanglement-enhanced quantum sensing modality can allow for new studies in challenging regimes, such as characterizing lossy thin films or detecting distant vibrations. Then, for detector characterization, we consider the instrumental role of photon detectors in various optical quantum science protocols. As these protocols expand into spectral bands with growing technological applications, such as the mid-infrared, there is a developing need to characterize suitable detectors, with their efficiencies being one of the most important metrics. Typical methods for efficiency characterization, whether classical or quantum, can face the challenge of being infrastructure-intensive, with some requiring resources such as independent reference standards or large bulk-optics apparatuses. To help realize a resource-efficient characterization system for mid-infrared detectors, we have developed a compact and portable solution involving highly non-degenerate photon pairs produced by a fiber-coupled waveguide device. We evaluate the performance of early prototypes and identify next steps for improved device engineering. Lastly, for multi-photon processing, we focus on the single-photon source, given its key role in preparing the required photonic states. From a scalability perspective, efficient single-photon generation is essential for performing many large-scale multi-photon protocols within useful timescales. To meet this requirement, we explore how multiplexing techniques allow us to pair heralded single photons with photon routing, storage, and counting technologies to realize an efficient single-photon source. We then discuss our efforts to implement and integrate optimized versions of these technologies to address limitations faced by earlier implementations with regard to preparing pure single photons at a high rate. This discussion includes an in-depth analysis of our work related to realizing an efficient photon-number-resolving detector.
Over the last sixty years, classical information theory has revolutionized the understanding of the nature of information, and how it can be quantified and manipulated. Quantum information processing extends these lessons to quantum systems, where the properties of intrinsic uncertainty and entanglement fundamentally defy classical explanation. This growing field has many potential applications, including computing, cryptography, communication, and metrology. As inherently mobile quantum particles, photons are likely to play an important role in any mature large-scale quantum information processing system. However, the available methods for producing and detecting complex multi-photon states place practical limits on the feasibility of sophisticated optical quantum information processing experiments. In a typical quantum information protocol, a source first produces an interesting or useful quantum state (or set of states), perhaps involving superposition or entanglement. Then, some manipulations are performed on this state, perhaps involving quantum logic gates which further manipulate or entangle the intial state. Finally, the state must be detected, obtaining some desired measurement result, e.g., for secure communication or computationally efficient factoring. The work presented here concerns the first and last stages of this process as they relate to photons: sources and detectors. Our work on sources is based on the need for optimized non-classical states of light delivered at high rates, particularly of single photons in a pure quantum state. We seek to better understand the properties of spontaneous parameteric downconversion (SPDC) sources of photon pairs, and in doing so, produce such an optimized source. We report an SPDC source which produces pure heralded single photons with little or no spectral filtering, allowing a significant rate enhancement. Our work on detectors is based on the need to reliably measure single-photon states. We have focused on optimizing the detection efficiency of visible light photon counters (VLPCs), a single-photon detection technology that is also capable of resolving photon number states. We report a record-breaking quantum efficiency of 91 +/- 3% observed with our detection system. Both sources and detectors are independently interesting physical systems worthy of study, but together they promise to enable entire new classes and applications of information based on quantum mechanics.
The quantum description of light offers a unique set of optical effects that has led to promising applications beyond those described by classical physics. Although well-defined quantum states of light do not persist in typical classical environments, phenomena such as entanglement often enhance optical approaches to communication, measurement, and sensing. With the emergence of new tasks in classical and quantum optical technology, new tools are required that must be specifically engineered including the generation of quantum states. This thesis is concerned with three principle tasks in engineering and implementing entangled photonic states. First, the use of frequency anti-correlated and polarization entangled two-photon states generated during spontaneous parametric down conversion (SPDC) to precisely evaluate optical delays with quantum interferometry is demonstrated in a realistic commercially available optical telecommunication device. Second, the study of correlated orbital angular momentum (OAM) states for efficient object identification is presented. Finally, experimental efforts towards the development of sources for entangled weak coherent states are discussed. The generation of broadband entangled states leading to well-defined second order interference patterns is a necessary step for the application of low coherence quantum interferometry as a metrological device. The flexibility of non-uniformly chirped periodically poled nonlinear crystals offers a rich set of tools for precise state engineering. The experimental evaluation of a broadband source of polarization entanglement is presented. In addition, design considerations for applications that require optimized quantum interference features are discussed along with a numerical investigation of the limits of quantum interferometry with even order dispersion cancellation. We present an experimental demonstration of correlated OAM sensing exploiting the two-dimensional and correlated nature of states produced during SPDC projected onto the OAM basis. Efficient object recognition through the identification of azimuthal symmetries of arbitrary objects is achieved by observing the full two-photon joint OAM spectrum and focusing on non-conserved OAM components not found in the natural OAM spectrum of SPDC. Finally, quantum key distribution (QKD) is currently the most successful quantum optical application; however, a limiting trade off between the achievable rates and distances confines the approach to niche applications. The generation of entangled coherent states has been proposed to transition QKD into a new regime that would set aside single photons and two-photon entangled states for higher intensity coherent pulses. The key technical limitation that has prohibited the demonstration of such states is a reliable source of single-photon cross phase modulation. The plausibility and experimental efforts towards creating such an environment in a solid state device is presented.
Photons have proven to be an effective test-bed for the fundamental concepts and elements of quantum-enhanced technologies. As systems become increasingly complex, however, practical considerations make the traditional approach of bulk optics and free-space propagation progressively more difficult. The major obstacles are the physical space necessary to realise and operate such a complex system, its stability, and maintaining low losses. In order to address these issues, quantum optical technologies can take a cue from their classical counterparts and look towards an integrated architecture to provide miniaturisation, greatly enhanced stability, less alignment, and low loss interfaces between different system components. In this thesis the feasibility of chip-based waveguides as a platform for metrology and information processing will be explored. In Part I, the necessary criteria for a metrology system to out-perform its classical counterpart will be investigated. It will be found that loss is a major barrier to this aim and, critically, that it is unlikely to have been achieved to date by any experiment which consumes resources of a fixed photon number. The issue of loss will be addressed by developing a scalable heralded source of a class of entangled photonic states which are both robust to losses and practically feasible to prepare. A novel tomographic technique will be developed to characterize these states and it will be explicitly demonstrated how it is possible to beat some bounds on classical performance without being able to out-perform a comparable classical system. Finally, a proof of principle demonstration of a waveguide-based interferometer with an integrated phase-shifter will be undertaken. It will be shown that the device preserves quantum interference, making it suitable for use in quantum-enhanced metrology applications. In Part II, integrated optics in the context of information processing will be discussed. First, a novel characterization technique will be developed which enables the behaviour of complex circuits to be predicted. The technique is independent of loss in the device being characterized. A method of simulating these circuits will be outlined that takes advantage of the computational speed-up available from parallelisation and sparse matrix operations. A key increase in complexity for integrated photonic systems will be demonstrated by showing quantum interference of three photons from two separate sources in eight spatial modes. The resulting interference has a visibility which beats all possible classical interference visibilities for similar circuits. Finally, a fully integrated waveguide-coupled photon-number-resolving detector will be developed and demonstrated. This proof of concept demonstration will show good resolution of different photon number events. The device will be modelled and routes to high efficiency operation will be explored.
Two important challenges in quantum photonics are to generate useful states with high fidelity, and to detect them and verify their properties. Particularly valuable states are single photons and entangled photon pairs in well-defined optical modes, as they can be used in many quantum information protocols or used to build up more complex states. For sources, we employ integrated nonlinear optics (waveguides in lithium niobite and potassium titanyl phosphate) to maximize brightness and go beyond what is possible in bulk optics, showing simultaneously high state fidelity, heralding efficiency, and spectral purity across three experiments: first we show record heralding efficiency in a fully-fibered heralded single-photon source, and use it to probe the tradeoff between spectral purity and heralding efficiency in non-engineered sources. With an engineered source, we then herald up to 50 photons in a nonclassical state. The last source is for polarization-entangled photon pairs, with brightness of 3.5 million pairs/s·mW, fidelity to a Bell state of 96%, heralding efficiency of 43%, and HOM interference visibility of 82%. Once a complex state is constructed, it must also be verified. For this we employ a time-multiplexed detector consisting of a fibre loop and a single-photon detector. Surprisingly, we are able to extract information even in the saturation regime of the detector. We use the click statistics of the time-multiplexed detector to verify the non-classicality of quantum light, and we use its extremely high dynamic range (123 dB) to measure a macroscopic power level with a single-photon detector. Eliminating calibrated attenuators with this approach will allow direct standardization of quantum and classical optical power levels.
A growing number of technologies employ quantum properties in order to produce solutions that surpass the performance of conventional devices, or to execute operations that are fundamentally impossible with classical systems alone. In the field of optical quantum information science, photons are utilized to encode, communicate and manipulate information, making them vitally important. While photon production always constitutes the first step in any optical experiment, in the field of quantum information science, the recording of data through the process of photon detection is an equally crucial final step. This thesis deals with both the single photons generation (based on diamond color defects) and their detection, utilizing a novel type of superconducting detectors. In particular, part one of this thesis is devoted to the construction of custom designed microscope setup, and the development of laboratory experiments, to enable the generation of single photons as well as the investigation of the optical and spin properties of diamond color centers. Confocal microscopy is used for this purpose, as it allows for the identification and addressing of individual color centers that emit only single photons. This microscope also feature an integrated self-built microwave and magnetic hardware setup, which allows for a wide range of spin environment spectroscopy studies. Single photon emission is demonstrated through both photon anti-bunching and Rabi oscillations at room temperature. The second part of the thesis offers an exploration of superconducting single photon detectors through experiment. Since electronics are an essential part of these detectors, the possibility of using a novel alternative scheme based on capacitive readout combined with fast gating to enable simplified readout is demonstrated. This scheme overcomes the limitations of conventional readout schemes, which require large bandwidth amplification and complex counting electronics. Besides photon detection, the capabilities of these detectors are also expanded to include high-energy particles in the MeV energy range, and the detectors are demonstrated to not only detect single α- and β-particles, but to do so with near unity efficiency. Finally, a multipurpose testing station for superconducting detectors is demonstrated with a central objective of optimizing the coupling efficiency of light to the active area of the detector, as well as to allow for a fast exchange of the optical fiber, thereby facilitating an efficient characterization of the detector. The optimization of this coupling efficiency was demonstrated through proof-of-principle experiments.
This thesis examines the integration of two disparate technologies in order to perform experiments in single photon quantum optics with low loss. Many technologies and experiments in quantum optics, communication, or computing require a certain fraction of the photons involved to be received at the end of the experiment, and in many cases the required efficiency has not yet been reached. This includes the famous Einstein, Podolsky, and Rosen gedanken-experiment, now implemented in the laboratory, demonstrating the existence of entanglement to unconvinced observers. Also included is the nonlocality test of John Bell, as well as technological problems such as device-independent quantum key distribution. In this thesis I perform these experiments in the high-efficiency regime. This programme requires the integration of two lines of research: improving sources of single photons, and improving detectors thereof. Until recently detector research was focussed on development, with improvements being sought for their own sake, working towards the ultimate goal of perfect photon detection. Recent years have seen these devices move into quantum photonics laboratories, allowing for previously impossible experiments to be undertaken. In this thesis, I combine high-efficiency, number-resolving, detectors with a high-efficiency entangled photon pair source, based on another line of research going back decades: the use of spontaneous parametric down-conversion to create single-photon-like modes of light, and the entanglement of the output modes in a useful way. For small demonstrations, such as the experiments mentioned above, this can emulate a single photon with enough fidelity, and low enough loss, to successfully perform the procedure. Here I push both of these technologies, and the problems of combining them, as far as I can, and solve the problems inherent with any marriage of disparate devices. I also examine the relative performance of two different steering inequalities, one linear and one quadratic, in the presence of noise, analysing my experimental data with each of them. I perform a detection- loophole free demonstration of EPR steering, violating the local bound by 200s, setting a then world record for efficiency at 62.5%.