Squeezed states of light addressed in this Thesis have proved to be the most readily accessible optical fields with demonstrably quantum mechanical behaviour in the Continuous Variable regime and they are considered at the heart of Quantum Mechanics. They may be concretely applied in several research fields such as metrology and quantum information science, as well as enabling to investigate the properties of the quantum world. Since their first observation, considerable progresses have been made in their generation and detection techniques. The research activity presented in this Thesis is devoted to the development of an experimental setup based thereon. The heart of generation process is a sub-threshold Optical Parametric Oscillator whereas the detection system is based on Homodyne Detector. On this structural background, we implemented an electronic/optical apparatus whereby it is possible to generate different kinds of squeezed states on demand. This Thesis presents how these engineered generation/acquisition processes work. Besides, it features a miniaturized Homodyne detection system based on a waveguide beam splitter inscribed in a glass substrate by femtosecond laser writing technology. We demonstrate for the first time the possibility to use such a device to detect genuine nonclassical features of light.
Squeezed states of light addressed in this Thesis have proved to be the most readily accessible optical fields with demonstrably quantum mechanical behaviour in the Continuous Variable regime and they are considered at the heart of Quantum Mechanics. They may be concretely applied in several research fields such as metrology and quantum information science, as well as enabling to investigate the properties of the quantum world. Since their first observation, considerable progresses have been made in their generation and detection techniques. The research activity presented in this Thesis is devoted to the development of an experimental setup based thereon. The heart of generation process is a sub-threshold Optical Parametric Oscillator whereas the detection system is based on Homodyne Detector. On this structural background, we implemented an electronic/optical apparatus whereby it is possible to generate different kinds of squeezed states on demand. This Thesis presents how these engineered generation/acquisition processes work. Besides, it features a miniaturized Homodyne detection system based on a waveguide beam splitter inscribed in a glass substrate by femtosecond laser writing technology. We demonstrate for the first time the possibility to use such a device to detect genuine nonclassical features of light.
We design and demonstrate a homodyne detection scheme based on a glass-integrated optical device (GID) operating in the quantum regime, that is, able to detect genuine nonclassical features. Our device is entirely fabricated by femtosecond laser micromachining. The GID incorporates on the same chip a balanced waveguide beamsplitter and a thermo-optic phase shifter, allowing us to record homodyne traces at different phases and to perform reliable quantum state tomography. In particular, we show that the GID allows for the detection of nonclassical features of continuous-variable quantum states, such as squeezed states.
This thesis is concerned with the generation of non-classical quantum states of light, the photon-level manipulation of quantum states and the accurate tomography of both quantum states and quantum processes. In optics, quantum information can be encoded and processed in both discrete and continuous variables. Hybrid approaches combining for example homodyne detection with conditional state preparation and manipulation are gaining increasing prominence. The development and characterization of a time-domain balanced homodyne detector (BHD) is presented. The detector has a bandwidth of 80 MHz, a signal-to-noise ratio of 14.5 dB and an efficiency of 86% making it well-suited to pulse-to-pulse measurement of quantum optical states. The BHD is employed to perform quantum state tomography (QST) of non-classical multi-photon Fock states generated by spontaneous parametric down-conversion. A detailed investigation of the mode-matching between the local oscillator used for homodyne detection and the generated Fock states is presented. The one-, two- and three-photon Fock states are reconstructed with a combined preparation and detection efficiency exceeding 50%. Fock states have a number of applications in quantum state engineering, where non-classical ancilla states and conditional measurements enable photon-level manipulation of quantum states. Fock state filtration (FSF) is investigated - an example of a post-selected beam splitter which is a basic building block for many quantum state engineering protocols. A model is developed incorporating the effect of experimental imperfections. An experimental implementation of a Fock state filter is fully characterized by means of coherent-state quantum process tomography (QPT). The reconstructed process is found to be consistent with the model. The filter preferentially removes the single-photon component from an arbitrary input quantum state. Calibration of optical detectors in the quantum regime is discussed. Quantum detector tomography (QDT) is reviewed and contrasted with a new technique for performing QST with a calibrated detector known as the fitting of data patterns (FDP). The first experimental characterization of a BHD is performed by probing the detector with phase-averaged coherent states. The FDP method is shown to be applicable to the estimation of quantum processes, where a detector response is not assumed - thus demonstrating the versatility of the FDP approach as a new method in the quantum tomography toolbox.
Optical quantum information science plays a central role in the development of advanced photonic technologies, with applications ranging from quantum imaging and communication to continuous-variable (CV) quantum computing. The efficient realization of these technologies depends not only on the generation of nonclassical states of light but also on their proper detection. Optical parametric amplification (OPA) has long been known as both a fundamental nonlinear process for generating nonclassical light and as a method to boost detection efficiency through pre-amplification. Recently, OPA has been revisited as a tool for all-optical quadrature detection, offering a powerful alternative to conventional homodyne detection by enabling broadband, loss-tolerant squeezing measurements across various optical platforms. In this thesis, I establish optical parametric amplification as a versatile tool for the detection of CV quantum states through all-optical quadrature detection. I extend squeezing detection to high-dimensional scenarios by exploiting the intrinsic multimode nature of OPA, thereby enabling, for the first time to the best of my knowledge, the real-time monitoring of multimode squeezed light, a long-standing challenge in quantum optics. This approach further permits the simultaneous monitoring of multiple CV Einstein–Podolsky–Rosen (EPR) mode pairs, providing a framework for scalable detection of multimode entanglement, namely optical cluster states, which constitute essential resources for CV quantum computing. Furthermore, I demonstrate OPA as a platform for loss-tolerant and broadband quantum state tomography, applying it to both Gaussian and non-Gaussian states. This paves the way for the scalable tomography of multimode quantum states. Finally, I show that OPA enables the certification of the non-Gaussian and nonclassical character of quantum states without the need for photon-number-resolving detectors, relying instead on post-amplification intensity statistics. In particular, I amplify a single photon sufficiently strongly that its non-Gaussian character becomes accessible through conventional intensity detectors. Altogether, this work establishes optical parametric amplification as a powerful and scalable platform for high-dimensional continuous-variable optical quantum technologies.
Quantum correlations play a fundamental role in quantum information science. The variety of their manifestations has become increasingly apparent following the development of novel light sources, protocols and photodetectors. One broad classification identifies two instances of non-classical correlations: particle and mode entanglement. These categories mirror two coexisting descriptions of quantum systems in terms of discrete and continuous variables of the electromagnetic field. The past decades have generated a number of promising results based on schemes which encompass elements from both frameworks, rather than viewing the two descriptions as mutually exclusive. In this context, it is possible to conceive and realise experiments where either the quantum resource or the detection system is 'hybrid'. Optical weak-field homodyne detectors bring together phase sensitivity and photon counting; as such, they represent a detection scheme which works across continuous and discrete variables of the radiation field. In this thesis we present a two-mode weak-field homodyne detection layout with added photon-number resolution and apply it to the study of a split single-photon state and a squeezed vacuum state. As a first test of the capabilities of this system, we investigate the reconstruction of relevant features of a given quantum resource - such as its photon statistics - with our detection scheme. Further, we experimentally demonstrate the observation of an instance of non-classical optical coherence which combines the continuous- and discrete-variable descriptions explicitly. The ability to probe phenomena at the interface of wave and particle regimes opens the way to novel, improved schemes for quantum information processing. From a more fundamental perspective, such hybrid approaches may shed light on the very roots of quantum enhancement.
Quantum optics is central to emerging quantum technologies such as quantum metrology and quantum information. The continuous variable (CV) approach to encoding information in a physical system, for example the electromagnetic field quadratures has significant advantages over discrete variables (DV) encoding. CV offers higher density encoding in a single optical state and on-demand state preparation for example. However, techniques for CV preparation, manipulation and measurement are not as well developed as the corresponding DV approach. In this thesis we present methods to characterize quantum detectors that operate in the CV regime, and discuss their application to CV state preparation. A quantum detector is described by its positive operator valued measure (POVM) set, with one operator for each measurement outcome. We present an experimental scheme for detector tomography, that is reconstruction of the POVM set of an unknown detector by means of measurement outcomes for a set of tomographically complete input states. We use this scheme to experimentally determine the POVM set of an avalanche photodiode (APD) and a time-multiplexed-detector (TMD) and compare the results to models of the detectors. We introduce a weak-homodyne photon-number-resolving detector (PNRD) , which uses a weak local oscillator beam to set a phase reference for the detection process. We develop a theoretical description of the sensitivity regimes for this detector using the Wigner phase space picture. We also present experimental detector tomography for a weak-homodyne detector based on two APDs. In many cases it is sufficient to calibrate the efficiencies of previously characterized PNRDs. We experimentally demonstrate a scheme using correlated photons produced by spontaneous parametric downconversion (SPDC) to determine the efficiencies of PNRDs, based on the photon number statistics of the source. The same SPDC source is also used to demonstrate preparation of photonic states in the photon number basis and for a first proof-of-principle experiment with a weak-homodyne PNRD based on a TMD.
This thesis presents the experimental demonstration of an atomic source of narrowband nonclassical states of light. Employing four-wave mixing in hot atomic Rubidium vapour, the optical states produced are naturally compatible with atomic transitions and may be thus employed in atom-based quantum communication protocols. We first demonstrate the production of two-mode intensity-squeezed light and analyze the correlations between the two produced modes. Using homodyne detection in each mode, we verify the production of two-mode quadrature-squeezed light, achieving a reduction in quadrature variance of 3 dB below the standard quantum limit. Employing conditional detection on one of the channels, we then demonstrate the generation of single-photon Fock states as well as controllable superpositions of vacuum and 1-photon states. We fully characterize the produced light by means of optical homodyne tomography and maximum likelihood estimation. The narrowband nature of the produced light yields a resolvable temporal wave-function, and we develop a method to infer this wave function from the continuous photocurrent provided by the homodyne detector. The nature of the atomic process opens the door to a new direction of research: generation of arbitrary superpositions of collective atomic states. We perform the first proof-of-principle experiment towards this new field and discuss a proposal for extending this work to obtain full control over the collective-atomic Hilbert space.