In this thesis work, after a brief introduction of some basic concepts of quantum optics, some experimental techniques useful for the manipulation of quantum states of light are presented. In particular, I focused on two transformations operated at the single-photon level, like the addition and the subtraction of a single photon from a travelling light beam. Both these methods share a probabilistic approach, that is their implementation has a non unitary success probability. Anyway, the detection of an additional photon in an ancillary mode exactly announces when the operation has been actually implemented. That's why this type of implementation is generally called heralded. Starting from these basic operations, a way to implement some of their superpositions is then analyzed. In the second part of this thesis, I describe two state-engineering experiments based on these fundamental operations. The first one realizes a so-called universal orthogonalizer, an universal strategy that, given an arbitrary input state, can be used to produce at the output an orthogonal one. The adjective universal refers to the fact that the procedure works equally well for arbitrary input fields. The other experiment uses coherent superpositions of basic quantum operations to emulate a strong Kerr nonlinearity at the few-photon level, and could be used to implement new logic gates for quantum computation. The third part of this thesis is dedicated to a different experimental approach, in some sense complementary to state engineering: mode engineering. Indeed, while in state engineering the purpose is the detection of an unknown state after its manipulation, in mode engineering experiments the perspective is completely reversed: the final state is assumed to be known and what has to be inferred are its spatio-temporal features. By following this alternative experimental approach, the interaction of broadband single photons with resonant atomic vapours is analyzed by observing the traces left on the photon temporal shape.
In this thesis work, after a brief introduction of some basic concepts of quantum optics, some experimental techniques useful for the manipulation of quantum states of light are presented. In particular, I focused on two transformations operated at the single-photon level, like the addition and the subtraction of a single photon from a travelling light beam. Both these methods share a probabilistic approach, that is their implementation has a non unitary success probability. Anyway, the detection of an additional photon in an ancillary mode exactly announces when the operation has been actually implemented. That's why this type of implementation is generally called heralded. Starting from these basic operations, a way to implement some of their superpositions is then analyzed. In the second part of this thesis, I describe two state-engineering experiments based on these fundamental operations. The first one realizes a so-called universal orthogonalizer, an universal strategy that, given an arbitrary input state, can be used to produce at the output an orthogonal one. The adjective universal refers to the fact that the procedure works equally well for arbitrary input fields. The other experiment uses coherent superpositions of basic quantum operations to emulate a strong Kerr nonlinearity at the few-photon level, and could be used to implement new logic gates for quantum computation. The third part of this thesis is dedicated to a different experimental approach, in some sense complementary to state engineering: mode engineering. Indeed, while in state engineering the purpose is the detection of an unknown state after its manipulation, in mode engineering experiments the perspective is completely reversed: the final state is assumed to be known and what has to be inferred are its spatio-temporal features. By following this alternative experimental approach, the interaction of broadband single photons with resonant atomic vapours is analyzed by observing the traces left on the photon temporal shape.
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.
Quantum optics is an extremely fruitful area of research in that it allows the probing of fundamental and basic science, as well as the opportunity to investigate potential applications that exploit these exotic phenomena. Entanglement is a uniquely quantum phenomenon that is a key feature of quantum mechanics. In this thesis we investigate the scalability of entangled states of light, as well as modest demonstrations of one-way quantum computations, based on a resource that exploits entanglement. Scalability has been a bottleneck of entanglement experiments since the first demonstrations began surfacing a few decades ago. In Part II of this thesis we investigate different methods of multiplexing quantum light modes in order to achieve a scalable architecture. 2 experiments are presented in spatial multiplexing, that lead to programmable linear optics networks that can produce up to 8 entangled modes. Building on this, and shifting domains we then demonstrate temporal multiplexing, where we entangle tens of thousands of quantum modes together in a highly entangled state called a cluster state. This is several orders of magnitude larger than any entangled state created to date, in any platform (be it atomic or electronic). The highly entangled cluster state is a universal resource for quantum computing, and we investigate a particular class of computations in Part III. We demonstrate that a cluster state consisting of only 4 highly entangled quantum modes is sufficient for universal 1-mode Gaussian quantum computing. Further, we demonstrate a tuneable entanglement gate that can be retrofitted to cluster states of arbitrary size or shape. Although the experiments in Part II and Part III are conceptually quite different, they employ very similar optical and electronic equipment. These experimental techniques as well as a survey of relevant quantum optics theory is presented in Part I.
This thesis covers several topics in optical quantum computation. A quantum computer is a computational device which is able to manipulate information by performing unitary operations on some physical system whose state can be described as a vector (or mixture of vectors) in a Hilbert space. The basic unit of information, called the qubit, is considered to be a system with two orthogonal states, which are assigned logical values of 0 and 1. Photons make excellent candidates to serve as qubits. They have little interactions with the environment. Many operations can be performed using very simple linear optical devices such as beam splitters and phase shifters. Photons can easily be processed through circuit-like networks. Operations can be performed in very short times. Photons are ideally suited for the long-distance communication of quantum information. The great difficulty in constructing an optical quantum computer is that photons naturally interact weakly with one another. This thesis first gives a brief review of two early approaches to optical quantum computation. It will describe how any discrete unitary operation can be performed using a single photon and a network of beam splitters, and how the Kerr effect can be used to construct a two photon logic gate. Second, this work provides a thorough introduction to the linear optical quantum computer developed by Knill, Laflamme, and Milburn. It then presents this author's results on the reliability of this scheme when implemented using imperfect photon detectors. This author finds that quantum computers of this sort cannot be built using current technology. Third, this dissertation describes a method for constructing a linear optical quantum computer using nearly orthogonal coherent states of light as the qubits. It shows how a universal set of logic operations can be performed, including calculations of the fidelity with which these operations may be accomplished. It discusses methods for reducing and correcting errors and recovering from failed operations. Lastly it describes an analysis of the long distance transmission of the coherent state qubits and shows how transmission errors can be corrected.
Quantum opto-mechanics is a fast moving, broad research field which aims to investigate and control the interaction of light and the quantized motion of mechanical objects and the coupling between them. This allows one to engineer the mechanical quantum state by controlling and measuring the electromagnetic field, and conversely, to control the dynamical evolution of the optical field via its interaction with a mechanical system. Mechanical resonators are accessible in many different shapes and sizes, from kilogram scales in experiments that test gravitational theories to micro- and nano-scales in resolved sideband regime, where the frequency of the mechanical resonator is larger than the bandwidth of the optical cavity. In this regime, it is possible to cool the mechanical object to the ground state which allows a high degree of control and the ability to prepare interesting non-classical states, states that could aid in investigating the quantum dynamics of macroscopic objects. The field of quantum opto-mechanics is a promising testbed for quantum control technologies, such as fundamental tests of quantum mechanics, ultra-sensitive quantum sensors for precise measurements, phonon lasing, mechanical memories with long lifetime and quantum information processing. Furthermore, experiments are progressing toward strong opto-mechanical coupling regimes with single photons. This creates a platform that can take advantage of the nonlinear optical interactions to generate highly non-classical states in such light-matter interfaces. Photonic systems working in the single photon regime will be promising for low power applications and also for implementations of quantum information and computation. Motivated by these applications, this thesis consists of theoretical studies in the context of single photonics. This includes investigating non-classical mechanical state generation and mechanically controlling the dynamical evolution of optical fields in the strong coupling regime single photon opto-mechanics and furthermore, proposing a quantum sensor using single photonics. In this regard, we employ three important formalisms to treat and measure open quantum systems with non-Gaussian input fields: the conditional and unconditional cascaded master equations, Fock-state master equations and input-output Langevin equations. In the context of opto-mechanics, the system to be investigated is composed of two optical cavities with a coupling modulated via a mechanical resonator. In one case, a sequence of single photons are irreversibly sent to one of the optical cavities. After photons go through the opto-mechanical interaction, photon counting measurements are performed on the cavity output, conditionally steering the mechanical state to a highly non-classical Fock state. Numerical calculations are performed to predict the experimental parameters needed to generate a mechanical Fock state before the mechanical object is thermalized. In another case, the same opto-mechanical scheme is employed to conversely modify the state of the optical mode by engineering the state of the mechanical degree of freedom. In this case, it is shown that the opto-mechanical system operates as an effective beam splitter for the input single photons, with a transmission that can be controlled by tuning the number of excitations in the bulk mechanical resonator. When the mechanical resonator has a small coherent amplitude it acts as a quantum control, entangling the optical and mechanical degrees of freedom. As the coherent amplitude of the resonator increases, it acts as a classical control for the input photons. Finally, we propose a novel multi-purpose sensor architecture that can be used for force, refractive index and possibly local temperature detection. In this scheme, two coupled cavities behave as an effective beam splitter. This sensor is based on fourth order interference (the Hong-Ou-Mandel effect) and requires a sequence of single photon pulses and consequently has low pulse power. Changes in the parameter to be measured induce variations in the effective beam splitter reflectivity and result in changes to the visibility of interference. We propose that this generic scheme could be implemented with coupled L3 photonic crystal cavities and find that this system, which only relies on photon coincidence detection and does not need any spectral resolution, can estimate forces as small as 10-7 Newtons and can measure one part per million change in refractive index using a very low input power of 10-10 W.
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.
The implementation of optical quantum technologies requires precise and complete characterisation tools for multi-mode nonclassical states of light. In this thesis, we propose and we implement experimentally three applications of optical quantum measurements, which combine aspects of light that are described by both discrete and continuous variables. In the rst part of this thesis, we present a new scheme for interferometric phase estimation, which uses double homodyne measurements as an alternative to photon counting. We show that, without requiring a phase reference, this scheme can achieve the optimum phase estimation precision for all path-symmetric probe states with a de ned number of photons. Furthermore, the estimation procedure is robust against state preparation imperfections, and naturally yields the same precision for all values of the estimated phase. We implement the proposed scheme experimentally, using a polarisation interferometer probed by a single-photon signal, and temporally multiplexing a single homodyne detector. We repeat the experiment with classical probe states. For both cases, we demonstrate the accuracy and precision of the proposed method, our implementation deviating by 5% from the fundamental precision bound. We demonstrate that, in our scheme, single-photon probe states can provide better precision than weak coherent ones. These results indicate that hybrid quantum resources, which combine tools from the discrete and continuous-variable paradigms, can play a signi cant role in practical sensing scenarios. In the second part of this thesis, we present an application in the eld of information thermodynamics, reporting an experimental realisation, in a photonic setup, of Maxwell's demon. We show that a measurement at the few-photons 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 leads us to the derivation of an equality relating work extraction to information acquired by measurement. We prove 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 in the eld of information thermodynamics. In the third part of this thesis, we present a new method for the characterisation of broadband parametric down-conversion, based on stimulated frequency generation. In particular, we analyse the information contained in the frequency generation produced in the same mode as a seeding beam, for a type-II down-conversion process. We derive a model for this signal and argue that its detection can provide a useful characterisation tool in the high-gain regime, allowing for a self-referenced estimation of squeezing gain. We propose a method for measuring this signal and present an experimental realisation, based on a wave-guided source, at telecommunication wavelengths. Our experimental results demonstrate that the proposed measurement is an e ective experimental handle for discerning the intricate structure of broadband parametric down-conversion.