Дисертації з теми "Entanglement generation"

We study the entanglement of two-qubit systems resulting from local interactions with spatially extended bosonic systems. Our results apply to the case where the initial state of the bosonic system is represented by a statistical mixture of states with fixed particle number. In particular, we derive and discuss necessary conditions to generate entanglement in the two-qubit system. We also study the scenario where the joint system is initially in its ground state and the interaction is switched on adiabatically. Using time independent perturbation theory and the adiabatic theorem, we show conditions under which the qubits become entangled as the joint system evolves into the ground state of the interacting theory

This dissertation consists of three sections. In the first section, we discuss the generation of arbitrary two-qubit entangled states and present three generation methods. The first method is based on the interaction of an atom with classical and quantized cavity fields. The second method is based on the interaction of two coupled two-level atoms with a laser field. In the last method, we use two spin-1/2 systems which interact with a tuned radio frequency pulse. Using those methods we have generated two qubit arbitrary entangled states which is widely used in quantum computing and quantum information. In the second section, we discuss a possible experimental implementation of quantum walk which is based on the passage of an atom through a high-Q cavity. The chirality is determined by the atomic states and the displacement is characterized by the photon number inside the cavity. Our scheme makes quantum walk possible in a cavity QED system and the results could be widely used on quantum computer. In the last section, we investigate the properties of teleporting an arbitrary superposition of entangled Dicke states of any number of atoms (qubits) between two distant cavities. We also studied teleporting continuous variables of an optical field. Teleportation of Dicke states relies on adiabatic passage using multiatom dark states in each cavity and a conditional detection of photons leaking out of both cavities. In the continuous variables teleportation scheme we first reformulate the protocol of quantum teleportation of arbitrary input optical field states in the density matrix form, and established the relation between the P-function of the input and output states. We then present a condition involving squeeze parameter and detection efficiency under which the P-function of the output state becomes the Q function of the input state such that any nonclassical features in the input state will be eliminated in the teleported state. Based on the research in this section we have made it possible of arbitrary atomic Dicke states teleportation from one cavity to another, and this teleortation will play an essential role in quantum communication. Since quantum properties is so important in quantum communication, the condition we give in this section to distinguish classical and quantum teleportation is also important.

Since its conception three decades ago, quantum computation has evolved from a theoretical construct into a variety of different physical implementations. In many implementations, quantum optics is a familiar tool for manipulating or transporting quantum information. Even as some individual components of quantum photonics technologies have shifted from lab-based setups into commercial products, effort is being devoted to the creation of quantum networks that would link these components together to form scalable computation devices. Here, I investigate optical phonons in bulk diamond, a previously overlooked system, as a physical resource for the construction of these devices. In this thesis, I measured the coherence properties of the diamond phonon, implemented a quantum memory write-read protocol using far-detuned Raman scattering, and entangled the phonon modes from two spatially separated pieces of diamonds in an adaptation of the seminal quantum repeater protocol proposed by Duan, Lukin, Cirac and Zoller (DLCZ). All of these experiments were conducted at room temperature with no optical pumping, using ultrafast broadband pulses (sub 100fs) - this is made possible by the unique physical properties of bulk diamond. Quantum memories and the creation of entangled states are key ingredients towards a working quantum network. By demonstrating that diamond can be used as a bulk solid in ambient conditions to implement these complex quantum interactions, I show that bulk diamond is a credible candidate for the construction of robust integrated nanophotonics chips capable of operating at THz frequencies. The quantum dynamics demonstrated here encompasses the motion of r-1016 atoms, which is several orders of magnitudes larger than the excitations created in other systems. This manifestation of quantum features at room temperature, in a regime that is traditionally described classical physics, is of fundamental interest, and highlights the need for further studies into the transition between quantum and classical physics.

Cette thèse présente de nouveaux protocoles pour les portes quantiques multi-qubits non locales et la génération d’intrication dans des systèmes où plusieurs émetteurs quantiques interagissent avec un mode bosonique partagé. Elle introduit les portes de phase géométrique et adiabatique, avec des expressions analytiques de l’infidélité dépendant du nombre de qubits et de la coopérativité. Pour deux qubits, elles forment un ensemble universel, tandis que dans les systèmes multi-qubits, elles permettent des portes déterministes pour la simulation quantique et la correction d’erreurs. Une contribution majeure est un protocole de détection optimisé par l’intrication, atteignant une haute précision de mesure grâce au contrôle optimal. La thèse explore aussi un mécanisme de blocage polaritonique en cavité pour la génération d’états W non locaux et de portes multi-qubits. Ces opérations déterministes, basées sur des excitations classiques de cavité et parfois des impulsions globales, offrent une base évolutive pour l’informatique quantique, la détection quantique et l'internet quantique de demain, en particulier pour les systèmes à atomes neutres
This thesis presents novel protocols for non-local multi-qubit quantum gates and entanglement generation in systems where multiple quantum emitters interact with a shared bosonic mode. It introduces the Geometric and Adiabatic Phase Gates, with closed-form infidelity expressions scaling with qubit number and cooperativity. For two qubits, these form a universal gate set, while in multi-qubit systems, they enable deterministic gates for quantum simulation and quantum error correction. A key contribution is an entanglement-enhanced sensing protocol that achieves high measurement precision via optimal control. The thesis also examines a cavity polariton blockade mechanism for non-local W-state generation and multi-qubit gates. These deterministic multi-qubit operations rely only on classical cavity drives and, in some cases, global qubit pulses, providing a scalable foundation for quantum computing, sensing, and the future quantum internet, especially for neutral atom systems

In this thesis, I report on how to use Single Photon Entanglement for generating certified quantum random numbers. Single Photon Entanglement is a particular type of entanglement which involves non-contextual correlations between two degrees of freedom of a single photon. In particular, here I consider momentum and polarization. The presence of the entanglement was validated using different attenuated coherent and incoherent sources of light by evaluating the Bell inequality, a well-known entanglement witness. Different non-idealities in the calculation of the inequality are discussed addressing them both theoretically and experimentally. Then, I discuss how to use the Single Photon Entanglement for generating certified quantum random numbers using a semi-device independent protocol. The protocol is based on a partial characterization of the experimental setup and the violation of the Bell's inequality. An analysis of the non-idealities of the devices employed in the experimental setup is also presented In the last part of the thesis, the integrated photonic version of the previously introduced experiments is discussed: first, it is presented how to generate single photon entangled states exploiting different degrees of freedom with respect to the bulk experiment. Second, I discuss how to perform an integrated test of the Bell's inequality.

Spin degrees of freedom have been extensively explored in the context of quantum information processing. Many proposals of quantum computation architectures use spins as carriers of quantum of information. A central problem is to efficiently generate quantum entanglement between spin qubits which proves to be a crucial resource for quantum information tasks. On the other hand, uncontrollable spin degrees of freedom in the environment of spin qubits are the major causes of errors at low temperature, for example, the lattice nuclear spins hyperfine coupled to single electron spin qubit localized in semiconductor nano-structures. An outstanding problem for scalable quantum computation is to suppress the collective fluctuations from such spin baths so that the coherence time of the spin qubit can be improved. With these two motivations, the problems of suppressing collective spin fluctuations and generating entanglement in various spin ensembles are addressed in this thesis. In the first half of the thesis, two approaches are introduced for suppressing the collective fluctuations in the nuclear spin bath so that the quantum coherence time of electron spin qubit in semiconductor quantum dots can be improved. The first approach works for a coupled double dot system. A theory for the interaction with the nuclear spins is developed when the two-electron singlet state is in resonance with one of the triplet state in moderate external magnetic field. At this resonance condition, the nuclear-electron flip-flop process caused by the hyperfine interaction can lead to a feedback mechanism, which can be used to suppress the nuclear hyperfine field. The second approach works for a single dot system. It is shown that strong pumping of the nuclear spins in dynamic nuclear polarization processes can saturate the nuclear spin bath towards the collective “dark states”. In such dark states, the transverse nuclear field fluctuation can be substantially suppressed compared to the value at thermal equilibrium. Two physical schemes are proposed to realize the nuclear dark states for suppression of the nuclear field fluctuations. In the second half of the thesis, schemes are presented for generating large scale quantum entanglement in two types of spin qubit systems. For atomic spin qubits in optical lattices, schemes are proposed on how to prepare pure spin coherent state (SCS) with low collective spin by incoherent pumping with collective spin raising and lowering operations. Such SCS realize networks of mutually entangled spins which can be idea resources for the quantum telecloning algorithm. For donor nuclear spin qubits in silicon architecture, proposals are shown on how to deterministic prepare Dicke states which constitute an important class of multipartite entangled states. Our scheme is capable of preparing both symmetric and asymmetric Dicke states which form a complete basis set of the spin Hilbert space. The required controls are in situ to the prototype Kane’s quantum computer. The preparation is robust because each desired Dicke state is the steady state under designed pumping process. The schemes presented here also make possible the construction of decoherence free subspaces where quantum information is protected from collective noises.
published_or_final_version
Physics
Doctoral
Doctor of Philosophy

I punti quantici (quantum dots, QDs) epitassiali possono generare fotoni in uno stato di polarizzazione entangled tramite la cascata radiativa bieccitone-eccitone. Le potenzialità di funzionamento deterministico e di scalabilità dei dispositivi sono vantaggi unici per le applicazioni in reti quantistiche. Tuttavia, una alta simmetria strutturale e una scelta ponderata dei materiali sono cruciali per affrontare le principali cause di degradazione dell'entanglement, ossia la separazione in energia (fine structure splitting, FSS) tra i due stati eccitonici ed i campi magnetici nucleari oscillanti legati all'interazione iperfine. Questa tesi si concentra su QDs di GaAs/AlGaAs cresciuti su un substrato di GaAs (111)A con un nuovo approccio alla epitassia da goccia, in cui la fase fondamentale di cristallizzazione è eseguita ad una temperatura significativamente più alta rispetto ai precedenti tentativi. La scelta specifica di orientazione del substrato, denotata da un basso coefficiente di adsorbimento per l'As, favorisce l'incorporazione di As nella goccia piuttosto che con adatomi di Ga sulla superficie. Diversamente dall'epitassia da goccia convenzionale, limitata a temperature del substrato sotto i 250°C, la formazione di nanostrutture è osservata fino a 520°C. Ciò porta ad una maggiore qualità cristallina dei QDs e della barriera circostante e ad un ridotto impatto dell'interdiffusione, evidenziato dal confronto tra macro-fotoluminescenza e simulazioni dei livelli energetici basate sulle geometrie misurate tramite microscopia a forza atomica su campioni senza copertura. Il controllo sulla dinamica di crescita porta alla fabbricazione di QDs con diversi rapporti di forma e, quindi, alla progettazione riproducibile della lunghezza d'onda d'emissione. Così è dimostrato il funzionamento attorno a 780 nm, che permette l'integrazione con memorie ottiche al Rb, un importante obiettivo per la realizzazione di ripetitori quantistici. La forma di piramide troncata con base ad esagono regolare soddisfa anche i requisiti di elevata simmetria nel piano per azzerare il FSS. L'impatto dei parametri di crescita sulle proprietà ottiche è investigato nel dettaglio tramite fotoluminescenza da singolo QD. La migliore qualità cristallina come effetto dell'alta temperatura di processo è quantificata in termini di larghezza della linea eccitonica e confermato da valori ridotti fino a 9 μeV in condizioni ottimali. In linea con la simmetria di forma, una media di FSS molto bassa pari a 4,5 μeV è riportata nella regione spettrale di interesse, mentre misure risolte in tempo in eccitazione risonante svelano un tempo di vita dell’eccitone inferiore a 240 ps. Date queste figure di merito, la quasi totalità (95%) degli emettitori soddisfa i requisiti base per generare coppie di fotoni con fedeltà di entanglement sopra 0,5. Misure di correlazione mutua sono eseguite su un QD rappresentativo in eccitazione risonante a due fotoni e restituiscono un valore di fedeltà di 0.77, che è ben al di sopra del limite classico ed è in accordo con le previsioni del modello di evoluzione di fase dell’eccitone per QDs di GaAs. Questa tesi esplora anche la possibilità di integrare queste nanostrutture su un substrato piezoelettrico per calibrare la lunghezza d'onda d'emissione ed il FSS tramite deformazione elastica controllata. Una procedura di micro-fabbricazione per rimozione chimica selettiva e collegamento su wafer tramite termoadesivazione è adottata con successo per trasferire una membrana di semiconduttore contenente QDs. L'introduzione di un'inclinazione di 2° nell'orientazione del substrato è impiegata per depositare AlGaAs ad alto contenuto di Al come strato sacrificale, con un impatto moderato sulle proprietà ottiche dei QDs. La presenza di gradini atomici permette l'aumento della velocità di deposizione, un importante passo avanti verso la fabbricazione di micro-cavità ottiche per ottimizzare l'estrazione di luce.
Epitaxial semiconductor quantum dots (QDs) can generate polarization-entangled photons through the biexciton-exciton radiative cascade. The potential for on demand operation and device scalability are unique assets for future applications in quantum networking. However, a high QD structural symmetry and a proper choice of materials are crucial to tackle the main sources of entanglement degradation, namely the presence of a fine structure energy splitting (FSS) between the two bright exciton states and fluctuating nuclear magnetic fields due to the hyperfine interaction. This thesis focuses on GaAs/AlGaAs QDs grown on a (111)A GaAs substrate by a novel approach based on droplet epitaxy, where the fundamental crystallization step is performed at a temperature which is significantly higher than in previous reports. The specific choice of substrate orientation, characterized by a very low As sticking coefficient, favors As incorporation inside the droplet rather than with Ga adatoms on the surface. In contrast to standard droplet epitaxy, which is restricted to substrate temperatures below 250°C, quantum dot formation is observed up to 520°C. The increased growth temperature improves the crystalline quality of the QDs and their surrounding barrier and strongly reduces the impact of interdiffusion. This is confirmed by comparing ensemble photoluminescence with energy level simulations based on the geometrical features probed by atomic force microscopy on uncapped samples. The control over the growth dynamics leads to the fabrication of QDs with different aspect ratios and, therefore, to the reproducible design of the emission wavelength. Thus, operation in the 780 nm range is demonstrated, which allows the frequency-matching of these QDs with Rb-based optical quantum memories, an important target for the realization of quantum repeaters. At the same time, a truncated pyramid shape with regular hexagonal base is achieved, also fulfilling the requirements on high in-plane symmetry for vanishing FSS. The impact of growth parameters on optical properties is thoroughly investigated by means of polarization-resolved single dot photoluminescence. The improvement of the crystalline quality as an effect of the high temperature of crystallization is evaluated in terms of neutral exciton linewidth and ultimately confirmed, as spectral wandering in optimal conditions is reduced down to 9 μeV. Consistently with the considerations on shape symmetry, a very low average FSS of 4.5 μeV is reported in the spectral region of interest, whereas time-resolved measurements under resonant excitation unveil a short exciton lifetime below 240 ps. Given these figures of merit, a remarkably high fraction - 95% - of the emitters satisfy the basic requirements for generating photon pairs with fidelity above 0.5. Cross correlation measurements were performed on a representative dot under resonant two-photon excitation and yielded a fidelity value of 0.77, which is well above the classical limit and it is consistent with the predictions of the exciton phase evolution model for GaAs QDs. This thesis also explores the possibility of integrating these nanostructures on a piezoelectric substrate in order to precisely control the emission wavelength and the fine structure splitting by strain tuning. A sample processing by chemical back-etching and adhesive wafer bonding is successfully adopted to transfer a semiconductor membrane containing the QDs. The introduction of a 2° miscut in the orientation of the substrate is employed to deposit defect-free high Al content AlGaAs to act as a sacrificial layer, with moderate impact on the optical properties of the QDs. The transition to a step flow regime increases the growth rate, an important progress towards the fabrication of thick optical microcavities for enhancing light extraction.

The processing of information based on the generation of common quantum optical states (e.g. coherent states) and the measurement of the quadrature components of the light field (e.g. homodyne detection) is often referred to as continuous-variable quantum information processing. It is a very fertile field of investigation, at a crossroads between quantum optics and information theory, with notable successes such as unconditional continuous-variable quantum teleportation or Gaussian quantum key distribution. In quantum optics, the states of the light field are conveniently characterized using a phase-space representation (e.g. Wigner function), and the common optical components effect simple affine transformations in phase space (e.g. rotations). In quantum information theory, one often needs to determine entropic characteristics of quantum states and operations, since the von Neuman entropy is the quantity at the heart of entanglement measures or channel capacities. Computing entropies of quantum optical states requires instead turning to a state-space representation of the light field, which formally is the Fock space of a bosonic mode.

This interplay between phase-space and state-space representations does not represent a particular problem as long as Gaussian states (e.g. coherent, squeezed, or thermal states) and Gaussian operations (e.g. beam splitters or squeezers) are concerned. Indeed, Gaussian states are fully characterized by the first- and second-order moments of mode operators, while Gaussian operations are defined via their actions on these moments. The so-called symplectic formalism can be used to treat all Gaussian transformations on Gaussian states, including mixed states of an arbitrary number of modes, and the entropies of Gaussian states are directly linked to their symplectic eigenvalues.

This thesis is concerned with the Gaussian transformations applied onto arbitrary states of light, in which case the symplectic formalism is unapplicable and this phase-to-state space interplay becomes highly non trivial. A first motivation to consider arbitrary (non-Gaussian) states of light results from various Gaussian no-go theorems in continuous-variable quantum information theory. For instance, universal quantum computing, quantum entanglement concentration, or quantum error correction are known to be impossible when restricted to the Gaussian realm. A second motivation comes from the fact that several fundamental quantities, such as the entanglement of formation of a Gaussian state or the communication capacity of a Gaussian channel, rely on an optimization over all states, including non-Gaussian states even though the considered state or channel is Gaussian. This thesis is therefore devoted to developing new tools in order to compute state-space properties (e.g. entropies) of transformations defined in phase-space or conversely to computing phase-space properties (e.g. mean-field amplitudes) of transformations defined in state space. Remarkably, even some basic questions such as the entanglement generation of optical squeezers or beam splitters were unsolved, which gave us a nice work-bench to investigate this interplay.

In the first part of this thesis (Chapter 3), we considered a recently discovered Gaussian probabilistic transformation called the noiseless optical amplifier. More specifically, this is a process enabling the amplification of a quantum state without introducing noise. As it has long been known, when amplifing a quantum signal, the arising of noise is inevitable due to the unitary evolution that governs quantum mechanics. It was recently realized, however, that one can drop the unitarity of the amplification procedure and trade it for a noiseless, albeit probabilistic (heralded) transformation. The fact that the transformation is probabilistic is mathematically reflected in the fact that it is non trace-preserving. This quantum device has gained much interest during the last years because it can be used to compensate losses in a quantum channel, for entanglement distillation, probabilistic quantum cloning, or quantum error correction. Several experimental demonstrations of this device have already been carried out. Our contribution to this topic has been to derive the action of this device on squeezed states and to prove that it acts quite surprisingly as a universal (phase-insensitive) optical squeezer, conserving the signal-to-noise ratio just as a phase-sensitive optical amplifier but for all quadratures at the same time. This also brought into surface a paradoxical effect, namely that such a device could seemingly lead to instantaneous signaling by circumventing the quantum no-cloning theorem. This paradox was discussed and resolved in our work.

In a second step, the action of the noiseless optical amplifier and it dual operation (i.e. heralded noiseless attenuator) on non-Gaussian states has been examined. We have observed that the mean-field amplitude may decrease in the process of noiseless amplification (or may increase in the process of noiseless attenuation), a very counterintuitive effect that Gaussian states cannot exhibit. This work illustrates the above-mentioned phase-to-state space interplay since these devices are defined as simple filtering operations in state space but inferring their action on phase-space quantities such as the mean-field amplitude is not straightforward. It also illustrates the difficulty of dealing with non-Gaussian states in Gaussian transformations (these noiseless devices are probabilistic but Gaussian). Furthermore, we have exhibited an experimental proposal that could be used to test this counterintuitive feature. The proposed set-up is feasible with current technology and robust against usual inefficiencies that occur in optical experiment.

Noiseless amplification and attenuation represent new important tools, which may offer interesting perspectives in quantum optical communications. Therefore, further understanding of these transformations is both of fundamental interest and important for the development and analysis of protocols exploiting these tools. Our work provides a better understanding of these transformations and reveals that the intuition based on ordinary (deterministic phase-insensitive) amplifiers and losses is not always applicable to the noiseless amplifiers and attenuators.

In the last part of this thesis, we have considered the entropic characterization of some of the most fundamental Gaussian transformations in quantum optics, namely a beam splitter and two-mode squeezer. A beam splitter effects a simple rotation in phase space, while a two-mode squeezer produces a Bogoliubov transformation. Thus, there is a well-known phase-space characterization in terms of symplectic transformations, but the difficulty originates from that one must return to state space in order to access quantum entropies or entanglement. This is again a hard problem, linked to the above-mentioned interplay in the reverse direction this time. As soon as non-Gaussian states are concerned, there is no way of calculating the entropy produced by such Gaussian transformations. We have investigated two novel tools in order to treat non-Gaussian states under Gaussian transformations, namely majorization theory and the replica method.

In Chapter 4, we have started by analyzing the entanglement generated by a beam splitter that is fed with a photon-number state, and have shown that the entanglement monotones can be neatly combined with majorization theory in this context. Majorization theory provides a preorder relation between bipartite pure quantum states, and gives a necessary and sufficient condition for the existence of a deterministic LOCC (local operations and classical communication) transformation from one state to another. We have shown that the state resulting from n photons impinging on a beam splitter majorizes the corresponding state with any larger photon number n’ > n, implying that the entanglement monotonically grows with n, as expected. In contrast, we have proven that such a seemingly simple optical component may have a rather surprising behavior when it comes to majorization theory: it does not necessarily lead to states that obey a majorization relation if one varies the transmittance (moving towards a balanced beam splitter). These results are significant for entanglement manipulation, giving rise in particular to a catalysis effect.

Moving forward, in Chapter 5, we took the step of introducing the replica method in quantum optics, with the goal of achieving an entropic characterization of general Gaussian operations on a bosonic quantum field. The replica method, a tool borrowed from statistical physics, can also be used to calculate the von Neumann entropy and is the last line of defense when the usual definition is not practical, which is often the case in quantum optics since the definition involves calculating the eigenvalues of some (infinite-dimensional) density matrix. With this method, the entropy produced by a two-mode squeezer (or parametric optical amplifier) with non-trivial input states has been studied. As an application, we have determined the entropy generated by amplifying a binary superposition of the vacuum and an arbitrary Fock state, which yields a surprisingly simple, yet unknown analytical expression. Finally, we have turned to the replica method in the context of field theory, and have examined the behavior of a bosonic field with finite temperature when the temperature decreases. To this end, information theoretical tools were used, such as the geometric entropy and the mutual information, and interesting connection between phase transitions and informational quantities were found. More specifically, dividing the field in two spatial regions and calculating the mutual information between these two regions, it turns out that the mutual information is non-differentiable exactly at the critical temperature for the formation of the Bose-Einstein condensate.

The replica method provides a new angle of attack to access quantum entropies in fundamental Gaussian bosonic transformations, that is quadratic interactions between bosonic mode operators such as Bogoliubov transformations. The difficulty of accessing entropies produced when transforming non-Gaussian states is also linked to several currently unproven entropic conjectures on Gaussian optimality in the context of bosonic channels. Notably, determining the capacity of a multiple-access or broadcast Gaussian bosonic channel is pending on being able to access entropies. We anticipate that the replica method may become an invaluable tool in order to reach a complete entropic characterization of Gaussian bosonic transformations, or perhaps even solve some of these pending conjectures on Gaussian bosonic channels.

Doctorat en Sciences de l'ingénieur
info:eu-repo/semantics/nonPublished

This work concerns the generation of optical entangled states by means of parametric down-conversion and their applications in the fields of quantum metrology and quantum information. Particularly we studied the manipulation of multiparameter entanglement by means of adaptive optics, demonstrating a way to improve entanglement visibility with type-II downconversion in the case of large detection apertures and a spatial counterpart to quantum dispersion-cancellation. Moreover, we worked on satellite quantum information, demonstrating the feasibility of single photon exchange between a LEO satellite and a ground station.

Quantum entanglement is employed as a resource throughout quantum information science. However, before entanglement can be put to intelligent use, the issues of its production and detection must be considered. This thesis proposes four schemes for producing the bound entangled Smolin state. Three of these schemes produce the Smolin state by means of general quantum gates acting on different initial states - an all-zero state, a GHZ-state and two combined Bell states. The fourth scheme is based on one-qubit operations acting on two-photon states produced by SPDC. Furthermore, a maximum overlap entanglement witness detecting entanglement in the Smolin state is derived. This witness is measurable in three measurement settings with the maximal noise tolerance p=2/3. Lastly, simplified entanglement witnesses for the 4-, 6- and 8-qubit unitary invariant states are derived. These witnesses are measurable in three measurement settings with noise tolerances p=0.1802..., p=0.1502... and p=0.0751..., respectively.

Light, which is composed of discrete quanta, or photons, is one of the most fundamental concepts in physics. Being an elementary entity, the behaviour of photons is governed by the rules of quantum mechanics. The ability to create, manipulate and measure quantum states of light is not only useful in foundational tests of quantum theory, but also in a wide range of quantum technologies – which aim to utilize non-classical properties of quantum systems to perform tasks not possible with classical resources. Only recently has it been possible to control the properties of number states of light, which have a fixed photon-number. Two-photon states are central to testing fundamental physical theories (such as locality and reality) and the implementation of quantum information technologies. The versatility of photon-pair states is en- abled by the potential entanglement properties it can posses. Thus controlling the correlations between photons is crucial to both pure and applied physics. To produce a single photon, a photon-pair state can be used. Detection of one photon indicates its twin’s existence. Many applications, such as optical quantum computation, require pure indistinguishable single photons. Heralding single pho- tons from a photon-pair will, in general, produce single photons in a mixed quantum state due to correlations within the pair. A common approach to creating photon-pairs is through the nonlinear sponta- neous four-wave mixing interaction in optical fibres. This thesis presents a theoreti- cal and experimental implementation of a scheme to tailor the spectral correlations within the pairs. Emphasis is placed on engineering the two-photon state such that they are completely uncorrelated. Spatial entanglement is naturally avoided due to the discrete nature of the optical fibre modes. Spectral correlations are eliminated by careful choice of dispersion characteristics and conditions. The purity of the photons generated by this scheme is demonstrated by means of two-photon inter- ference from independent sources. We measure a purity of (85.9 ± 1.6)% with no spectral filtering, exhibiting the usefulness of this source for quantum technologies and applications.

Les sources de paires de photons sont un composant essentiel des technologies émergentes en information quantique. De nombreux travaux ont permis des avancées importantes utilisant des processus non linéaires d'ordre 2 dans les cristaux et les guides d'ondes, et d'ordre 3 dans les fibres. Les limitations viennent dans le premier cas, des pertes et en particulier des pertes de couplage avec les fibres optiques et dans le second cas, du bruit dû à l'effet Raman dont le spectre est très large dans les fibres de silice. Ce projet propose une nouvelle architecture basée sur des fibres à cristal photonique à coeur creux (FCPCC) que l'on peut remplir de liquide ou de gaz non linéaire. Cette configuration permet la génération paramétrique de paires de photons corrélés par mélange à quatre ondes sans l'inconvénient de la diffusion Raman. Cette technologie offre une large gamme de paramètres à explorer en s'appuyant sur les propriétés physiques et linéaires contrôlables des FCPCC et la possibilité de remplissage de ces fibres avec des fluides aux propriétés non-linéaires variées. En effet, par une conception judicieuse de la FCPCC et un choix approprié du liquide ou du gaz, il est possible de (i) contrôler la dispersion et la transmission pour générer des photons corrélés sur une large gamme spectrale avec la condition d'accord de phase la plus favorable, (ii) d'ajuster la taille de coeur de la fibre et/ou sa forme pour augmenter sa non-linéarité ou son efficacité de couplage avec d'autres fibres et (iii) de s'affranchir totalement de l'effet Raman si on utilise par exemple un gaz monoatomique, ou d'obtenir des raies Raman fines, aisément discriminables des raies paramétriques dans le cas d'un liquide
Photon pair sources are an essential component of the emerging quantum information technology. Despite ingenious proposals being explored in the recent years based on either second order nonlinear processes in crystals and waveguides or on third order processes in fibers, limitations remain, due to losses and specifically coupling losses in the former case and due to Raman generation in silica, giving rise to a broad spectrum noise in the latter. These limitations have been challenging to lift because of the limited alternative nonlinear materials that fulfil the conditions for the generation of bright and high fidelity photon pairs in integrable photonic structures. In the present project, we develop a new and versatile type of photonic architecture for quantum information applications that offers access to a variety of nonlinear optical materials that are micro-structured in optical fiber forms to generate photon pairs, without the drawback of Raman scattering and with a large design parameter-space. Indeed, with a careful design of the HCPCF along with the appropriate choice of fluid, one can (i) control the dispersion and the transmission to generate photons with the most favourable phase-matching condition over a large spectral range, (ii) adjust the fibre core size and/or shape to enhance nonlinearity or the coupling efficiency with other fibres, (iii) totally suppress the Raman effect in monoatomic gases for instance or have only narrow and separated Raman lines that can thus be easily separated from the useful parametric lines in liquids

Les sources de paires de photons sont un composant essentiel des technologies émergentes en information quantique. De nombreux travaux ont permis des avancées importantes utilisant des processus non linéaires d'ordre 2 dans les cristaux et les guides d'ondes, et d'ordre 3 dans les fibres. Les limitations viennent dans le premier cas, des pertes et en particulier des pertes de couplage avec les fibres optiques et dans le second cas, du bruit dû à l'effet Raman dont le spectre est très large dans les fibres de silice. Ce projet propose une nouvelle architecture basée sur des fibres à cristal photonique à coeur creux (FCPCC) que l'on peut remplir de liquide ou de gaz non linéaire. Cette configuration permet la génération paramétrique de paires de photons corrélés par mélange à quatre ondes sans l'inconvénient de la diffusion Raman. Cette technologie offre une large gamme de paramètres à explorer en s'appuyant sur les propriétés physiques et linéaires contrôlables des FCPCC et la possibilité de remplissage de ces fibres avec des fluides aux propriétés non-linéaires variées. En effet, par une conception judicieuse de la FCPCC et un choix approprié du liquide ou du gaz, il est possible de (i) contrôler la dispersion et la transmission pour générer des photons corrélés sur une large gamme spectrale avec la condition d'accord de phase la plus favorable, (ii) d'ajuster la taille de coeur de la fibre et/ou sa forme pour augmenter sa non-linéarité ou son efficacité de couplage avec d'autres fibres et (iii) de s'affranchir totalement de l'effet Raman si on utilise par exemple un gaz monoatomique, ou d'obtenir des raies Raman fines, aisément discriminables des raies paramétriques dans le cas d'un liquide
Photon pair sources are an essential component of the emerging quantum information technology. Despite ingenious proposals being explored in the recent years based on either second order nonlinear processes in crystals and waveguides or on third order processes in fibers, limitations remain, due to losses and specifically coupling losses in the former case and due to Raman generation in silica, giving rise to a broad spectrum noise in the latter. These limitations have been challenging to lift because of the limited alternative nonlinear materials that fulfil the conditions for the generation of bright and high fidelity photon pairs in integrable photonic structures. In the present project, we develop a new and versatile type of photonic architecture for quantum information applications that offers access to a variety of nonlinear optical materials that are micro-structured in optical fiber forms to generate photon pairs, without the drawback of Raman scattering and with a large design parameter-space. Indeed, with a careful design of the HCPCF along with the appropriate choice of fluid, one can (i) control the dispersion and the transmission to generate photons with the most favourable phase-matching condition over a large spectral range, (ii) adjust the fibre core size and/or shape to enhance nonlinearity or the coupling efficiency with other fibres, (iii) totally suppress the Raman effect in monoatomic gases for instance or have only narrow and separated Raman lines that can thus be easily separated from the useful parametric lines in liquids

Cette thèse est consacrée au développement de nouveaux dispositifs semi-conducteurs intégrés et de méthodes pour la génération et la manipulation d'états lumineux de haute dimension. Nous présentons l'étude d'un guide d'onde AlGaAs utilisant un processus de conversion paramétrique spontanée de type II en régime de pompage monochromatique, s’intéressant en particulier à l'amplitude spectrale jointe de l'état émis. La source fonctionne à température ambiante, émet des paires de photons dans le domaine des télécommunications et est compatible avec l'injection électrique. La génération d'états biphotoniques à large bande est démontrée expérimentalement par la reconstruction de l'intensité spectrale jointe et par une expérience de Hong-Ou-Mandel indiquant que les photons signal et complémentaire sont émis sur une large bande spectrale (170 nm) et avec un haut degré d'indiscernabilité (V=0,86). De plus, nous montrons que l'effet de cavité dû à la réflectivité des facettes des guides d'onde conduit à la production de peignes de fréquence à deux photons. Cette plateforme est utilisée pour démontrer une méthode originale de génération et de contrôle de la symétrie des états peignes exploitant les effets de cavité et un retard imposé entre les deux photons de chaque paire. Plus spécifiquement, nous montrons qu'un réglage fin de la fréquence de la pompe permet de générer des états peignes résonnants et anti-résonants permettant de manipuler la symétrie de la fonction d'onde. La méthode peut être adaptée et appliquée à une grande variété de systèmes, massifs ou intégrés, augmentant ainsi leur flexibilité et la richesse des états générés en vue de la mise en œuvre de nouveaux protocoles d'information quantique. En outre, nous démontrons la réalisation d'un guide d'onde AlGaAs pour la génération de faisceaux lumineux portant un moment angulaire de spin et présentons la conception d'un dispositif pour la génération d'un faisceau lumineux portant un moment orbital angulaire de premier ordre
This thesis is devoted to the development of novel integrated semiconductor devices and methods for the generation and manipulation of high-dimensional states of light. We report on the study of an AlGaAs waveguide implementing type-II spontaneous parametric down conversion process in a monochromatic pump regime, with a focus on the joint spectral amplitude of the emitted biphoton state. The source works at room temperature, emits photon pairs in the telecom range and is compliant with electrical injection. The generation of broadband biphoton states is experimentally demonstrated via the reconstruction of the joint spectral intensity and via a Hong-Ou-Mandel experiment indicating that signal and idler photons are emitted over a large bandwidth (170nm) and with a high degree of indistinguishability (V=0.86). Moreover, we show that the cavity effect due to waveguide facets reflectivity leads to the production of biphoton frequency-comb states. This platform is used to demonstrate an original method to generate and control the symmetry of biphoton frequency combs exploiting cavity effects and a delay between the two photons of each pair. More specifically, we show that a fine tuning of the pump frequency enables the generation of resonant and anti-resonant comb states allowing to manipulate the wavefunction symmetry. The method can be adapted and applied to a large variety of systems, either bulk or integrated, thus increasing their flexibility and the richness of the generated states in view of implementation of new quantum information protocols.In addition, we demonstrate the realization of an AlGaAs ridge waveguide for the generation of light beams with tailored phase and polarization distributions, carrying spin angular momentum, and present the design of a device for the generation of a twisted light beam, carrying first order orbital angular momentum

Depuis l’avènement de la mécanique quantique, l’étude des interactions lumière-matière à l’échelle quantique s’est énormément développée en tant que domaine de recherche. Par exemple, grâce à des prédictions théoriques surprenantes, des interactions d’une force sans précédant ont été démontrées entre de la matière et des radiations terahertz et microonde. Ces résultats correspondent à un régime dit de couplage ultrafort, atteint lorsque l’énergie d’interaction devient comparable aux énergies propres de la lumière et de la matière lorsque celles-ci n’interagissent pas. Dans ce régime, des propriétés intrigantes peuvent subsister telles que la présence de photons même lors qu’aucune énergie n’est fournie au système. Cependant, ces photons ne peuvent, a priori, être émis du système vers l’extérieur de manière à pouvoir être mesurés et par conséquent démontrer ces propriétés.Dans cette thèse, nous avons étudié ces propriétés intrigantes et proposé plusieurs moyens permettant d’y accéder expérimentalement. Nous nous sommes appuyés sur plusieurs plate-formes physiques qui sont de bon candidats pour ces études, et pour chacun de ces systèmes nous avons mis au point un modèle mettant en évidence ces propriétés d’une manière ou d’une autre. De cette façon, nous avons exploré le lien entre le régime de couplage ultrafort et la génération d’états non-classiques de la lumière. En outre, dans une étude plus ouverte nous avons montré que les interactions lumière- matière dans l’une de ces plate-formes peuvent être utilisés pour concevoir des protocols de communication quantique. En plus de montrer un intérêt fondamental, nos résultats s’inscrivent dans une optique de développement d’applications pour les technologies quantiques en utilisant différents systèmes expérimentaux disponibles actuellement
Since the advent of quantum mechanics, the study of light-matter interactions at thequantum level has been greatly developed as a research field. For instance, surprisingtheoretical predictions gave rise to experiments with unprecedented interactionstrengths between matter, and terahertz and microwave radiations. These results correspondto the so-called ultrastrong coupling regime, that is reached when the interactionenergy becomes comparable to the typical energies of the light and matter when they arenot interacting. In this regime, intriguing properties can be found such as the presenceof photons even when no energy is given to the system. However, these photons cannot,a priori, be emitted from the system to the outside world in order to be measured andtherefore demonstrate these properties. In this thesis, we studied these intriguing properties and proposed several means toaccess them experimentally. We relied on several physical platforms which are goodcandidates for such studies, and for each one of these systems we devised a model thatcan evidence these properties one way or another. By doing so, we explored the linkbetween the ultrastrong coupling regime and the generation of nonclassical states oflight. Additionally, as an outlook we showed that the light-matter interactions in oneof these platforms could be used to design quantum communication protocols. On topof showing fundamental interest, our results fit in the line of developing applications forquantum technologies using different experimentally available systems

Un circuit intégré photonique (CI) idéal devrait avoir des composants capables d'effectuer l'amplification, le traitement du signal, les opérations de porte logique et plusieurs fonctions équivalentes à celles d'un CI électronique, le tout intégré dans un espace de quelques millimètres. Pour réaliser une telle circuit, l'utilisation de différents matériaux adaptés à des fonctions différentes est inévitable. Parmi eux, les matériaux optiques non linéaires sont cruciaux en tant que source potentielle de photons uniques ou doubles. Cependant, lorsqu'on réduit la taille d'un matériau non linéaire à l'échelle nanométrique, il devient nécessaire d'amplifier l'excitation et l'émission par le biais d'interactions résonantes pour compenser la perte d'efficacité non linéaire due à la réduction du volume. Dans cette étude, en combinant la nanofabrication de structures plasmoniques et non linéaires (hybrides), un dispositif expérimental polyvalent et une simulation numérique quantitative des processus de génération du second harmonique (SHG) et de fluorescence paramétrique (SPDC) du second ordre, il est possible d'obtenir une compréhension complète de ces interactions non linéaires et de leur efficacité dans nos systèmes. L'un des principaux objectifs de ce travail consiste donc à étudier l'origine du SHG à partir de nanostructures en or afin d'identifier la source non linéaire dominante. Cela permet d'éclaircir et de distinguer entre les conclusions incompatibles présentées dans la littérature. Notre enquête suggère que, parmi les trois principales sources non linéaires invoquées dans la littérature, la source en surface parallèle et normale et la source en volume non local, la source en volume non local et en surface parallèle domine la réponse, tandis que la source en surface normale s'est révélée négligeable contrairement à la plupart de la littérature. Un deuxième objectif de ce travail de thèse était d'atteindre un taux d'émission expérimentalement observable de paires de photons SPDC en utilisant des structures hybrides, ce qui n'avait pas été possible jusqu'à présent. En optimisant par calcul numérique les tailles des nanoantennes plasmoniques, on améliore le taux de paires de photons grâce à l'interaction résonante, et en combinant cela avec un matériau présentant une meilleure non-linéarité, tel que le phosphure de gallium (GaP), on augmente le taux d'un ordre de grandeur par rapport à avant. La forme de nanofils de GaP et les variations structurelles de ce matériau nous ont permis de développer un protocole expérimental de fabrication de structures hybrides basé sur leurs réponses SHG distinctives. Ainsi, cela ouvre de nouvelles possibilités pour des sources de paires de photons intégrées à l'échelle nanométrique
An ideal all photonic integrated circuit (IC) requires components that perform amplification, signal processing, logic gate operations and several equivalent functions of an electronic IC packed in to a space of few millimeters. To achieve such a feat, use of different materials adapted to different functions is inevitable. Among them, nonlinear optical materials are crucial as a potential source of single or twin-photons. However, when reducing size of a nonlinear material to nanoscale, enhancing the excitation and emission through resonant interactions becomes a prerequisite to balance the drop in nonlinear efficiency due to volume reduction. In this study, by combining nanofabrication of plasmonic and nonlinear (hybrid) structures, a versatile experimental setup and quantitative numerical simulation of both second harmonic generation (SHG) and spontaneous parametric down conversion (SPDC) second order processes, a comprehensive understanding of these nonlinear interactions and their efficiency in our systems is possible. One of the primary objectives of this work, therefore, consists in studying the origin of SHG from gold nanostructures in order to identify the dominant nonlinear source. It allows to shed some light on and discriminate between incompatible conclusions presented in the literature. Our investigation suggests that, of the three main nonlinear sources invoked in the literature, namely, parallel and normal surface source and non-local bulk source, non-local bulk and parallel surface source dominates the response while normal surface was found to be negligible contrary to most literature. A second objective of this thesis work was to achieve experimentally observable SPDC photon pair emission rate using hybrid structures which has not been possible to date. While optimising the plasmonic nanoantennas theoretically improves the photon pair rate due to resonant interaction, combining it with a material of better nonlinearity such as gallium phosphide (GaP) increases the rate to an order of magnitude higher than before. The nanowire form of GaP and the structural variations of this material, as a result, leads us to develop an experimental protocol of hybrid structure fabrication based on their distinctive SHG responses. Thus, it opens up novel possibilities for integrated nanoscale photon pair sources

High quality entangled photon sources are a key requirement for many promising quantum optical technologies. However, the production of multi-photon entangled states with good fidelity is challenging. Current sources of multi-photon entanglement require the use of post-selection, which limits their usefulness for some applications. It has been an open challenge to create a source capable of directly producing three-photon entanglement. An important step in this direction was achieved with the demonstration of photon triplets produced by a new process called cascaded downconversion, but these previous measurements were not sufficient to show whether these photons were in an entangled state and only had detection rates of five triplets per hour. In this thesis, we show the first demonstration of a direct source of three-photon entanglement. Our source is based on cascaded downconversion, and we verify that it produces genuine tripartite entanglement in two degrees of freedom: energy-time and polarization. The energy-time entanglement is similar to a three-particle generalization of an Einstein-Podolski-Rosen state; the three photons are created simultaneously, yet the sum of their energies is well defined, which is an indication of energy-time entanglement. To prove it, we use time-bandwidth inequalities which check for genuine tripartite entanglement. Our measurements show that the state violates the inequalities with what constitute, to the best of our knowledge, the strongest violation of time-bandwidth inequalities in a tripartite continuous-variable system to date. We create polarization entanglement by modifying our experimental setup so that two downconversion processes producing orthogonally polarized triplets interfere to create Greenberger-Horne-Zeilinger states. By using highly efficient superconducting nanowire single photon detectors, we improve the detected triplet rate by 2 orders of magnitude to 660 triplets per hour. We characterize the state using quantum state tomography, and find a fidelity of 86\% with the ideal state, beating the previous best value for a three-photon entangled state fidelity measured by tomography. We also use the state to perform two tests of local realism. We violate the Mermin and Svetlichny inequalities by 10 and 5 standard deviations respectively, the latter being the strongest violation to date. Finally, we show that, unlike previous sources of tree-photon entanglement, our source can be used as a source of heralded Bell pairs. We demonstrate this by measuring a CHSH inequality with the heralded Bell pairs, and by reconstructing their state using quantum state tomography.

The setup in which two quantum systems, Alice and Bob, communicate using bosonic field quanta can be viewed as a prototype for wireless quantum communication. In this thesis we focus on the most basic case, where Alice and Bob are modeled as Unruh-DeWitt detectors, i.e., as two-level quantum systems that interact locally through a scalar quantum field. Our aim is to study how information propagation and entanglement generation between the two detectors are impacted by both relativity and by the unavoidable noise that is due to the quantum fluctuations of the field. We start by studying information propagation between the two detectors. Concretely, we construct and study the information-theoretic quantum channel, ξ, i.e., the completely positive trace preserving map between the input density matrix ϱ, in which Alice prepares her detector for the emission, and the output density matrix ϱ '=ξ(ϱ) of Bob's detector at a later time. We confirm that the classical as well as the quantum channel capacity are strictly zero to all orders in perturbation theory for spacelike separations. We then study entanglement generation between the two detectors. Specifically, we discuss how two Unruh-DeWitt detectors can extract entanglement from the vacuum. We show that the detectors can naturally and instantaneously become entangled through a Casimir-Polder effect. We then analyze the impact of various additions to this setup, such as the presence of a weak gravitational field, the presence of boundary conditions in the field, the presence of a weak classical potential, etc.

This thesis investigates an entangled light source with an in-built quantum memory based on the protocol of rephased amplified spontaneous emission (RASE). RASE has promising applications as a building-block of a quantum repeater: a device essential for extending the range of current quantum communication links. To be useful RASE must be able to produce high fidelity non-classical light with high efficiency, and be able to store multimode entanglement for long times. This thesis characterises the RASE source and determines to what degree these requirements can be met. The experimental RASE demonstration was conducted in a rare-earth ion doped crystal. Rare-earth ions provide a particularly promising platform for developing quantum technologies as they possess long coherence times on both the optical and hyperfine transitions. In the RASE protocol an inverted ensemble of two-level atoms amplifies the vacuum fluctuations resulting in amplified spontaneous emission (ASE). This results in entanglement between the output optical field and the collective modes of the amplifying ensemble. The collective atomic state dephases due to the inhomogeneous broadening of the ensemble but this can be rephased using photon echo techniques. When the ensemble rephases, a second optical field, the rephased amplified spontaneous emission (RASE), is emitted and is entangled with the ASE. In this thesis, a modified four-level rephasing scheme is used that allows the single photon signals to be spectrally resolved from any coherent background emission associated with the bright driving fields. In addition, four-level RASE incorporates storage on the long-lived hyperfine ground states. Two experiments are described in this thesis. First, a free-space four-level RASE demonstration using continuous-variable detection. In this experiment the different sources of noise were characterised and low noise operation was shown to be possible. Entanglement of the ASE and RASE was confirmed by violating the inseparability criterion with 98.6% confidence. In addition, entanglement was demonstrated after storage of the collective atomic state on the spin states and RASE was shown to be temporally multimode, with almost perfect distinguishability between two temporal modes demonstrated. The degree of entanglement between the ASE and RASE was limited by the rephasing efficiency, which saturated at 3%. It was determined that distortion of the rephasing pulses as they propagate through the optically thick ensemble was the probable cause of the low efficiency. The second experiment was a preliminary cavity-enhanced RASE demonstration. Theoretically perfect rephasing efficiency can be obtained by placing the crystal in an impedance-matched optical cavity. The initial cavity design showed encouraging evidence of an enhancement in the rephasing efficiency, with a 4-fold improvement over the free-space experiment. Improvements to the cavity design were proposed to allow a further increase in the rephasing efficiency of RASE. In summary, this thesis provides an extensive characterisation of an entangled light source with an in-built quantum memory based on rephasing spontaneous emission from an ensemble of ions. Importantly, the RASE scheme allows generation and storage of entanglement in a single protocol, which holds great promise for the development of integrated quantum networks.

Electro-optic phase modulation can be used to generate high repetition rate optical frequency combs. The optical frequency comb (OFC) has garnered much attention upon its inception, acting as a crucial component in applications ranging from metrology and spectroscopy, to optical communications. Electro-optic frequency combs (EO combs) can be generated by concatenating an intensity modulator and phase modulator together. The first part of this work focuses on broadening the modest bandwidth inherent to the EO combs. This is achieved by propagation in a nonlinear medium, specifically propagation in a nonlinear optical loop mirror (NOLM). This allows for broadening the EO frequency comb spectrum to a bandwidth of 40 nm with a spectral power variation of < 10 dB. This spectrally broadened EO comb is then used in dual comb interferometry measurements to characterize the single soliton generated in an anomalous dispersion silicone-nitride microresonator. This measurement allows for rapid characterization with low average power. Finally, electro-optic phase modulation is used in a technique to prove frequency-bin entanglement. A quantum network based on optical fiber will require the ability to perform phase modulation independent of photon polarization due to propagation in optical fiber scrambling the polarization of input light. Commercially available phase modulators are inherently dependent on the polarization state of input light making them unsuited to be used in such a depolarized environment. This limitation is overcome by implementing a polarization diversity scheme to measure frequency-bin entanglement for arbitrary orientations of co- and cross- polarized frequency-bin entangled photon pairs.

The quantum resources of entanglement and single photons are key to a range of quantum applications. These resources are challenging to produce cleanly and efficiently. This thesis investigates and improves methods of producing entanglement and single photons using measurement and control. To robustly produce entanglement, this thesis extends work by Carvalho et al., who developed a method for creating an entangled state of two atoms. This is done by coupling the atoms to a damped cavity, and using measurement of the output of the cavity to trigger a feedback pulse on the atoms. The robustness of this scheme against imperfect localisation of the two atoms is tested in this thesis, and limits are placed on the size of a trap that will still allow an entangled state to be produced. Additionally, using three-level (Lambda configuration) atoms slows the rate at which the system evolves, allowing more time for measurement and feedback to be applied, though it does not counteract the influence of spontaneous emission. In extending this work to multiple atoms we found that the system rapidly becomes more complex, and we developed a strategy for choosing a feedback pulse that stabilises a specific, highly entangled steady state of multiple particles. In addition to this general strategy, we developed a local and separable feedback for generating entanglement in a four-partite system. This thesis also investigates the production of single photons through the rephased amplified spontaneous emission (RASE) protocol. This protocol offers the promise of a stream of precisely shaped single photons, though it is plagued by efficiency issues. We develop a model of RASE and show that it overcomes the trade-off between efficiency and photon spacing by using different optical depths for the amplified spontaneous emission (ASE) preparation of the ensemble, and the RASE emission of single photons. By tailoring the density profile of the ensemble to mode-match the RASE emission of a single photon, we also able to eliminate reabsorption of the emitted photon, which would otherwise have limited the efficiency to 70%.