Heralded multiphoton states with coherent spin interactions in waveguide QED

WaveguideQEDoffers the possibility of generating strong coherent atomic interactions either through appropriate atomic configurations in the dissipative regime or in the bandgap regime. In this work, we show how to harness these interactions in order to herald the generation of highly entangled atomic states, which afterwards can be mapped to generate single mode multi-photonic states with high fidelities.Weintroduce two protocols for the preparation of the atomic states, we discuss their performance and compare them to previous proposals. In particular, we show that one of them reaches high probability of success for systems with many atoms but low Purcell factors

Deterministic generation of arbitrary photonic states assisted by dissipation

A scheme to utilize atom-like emitters coupled to nanophotonic waveguides is proposed for the generation of many-body entangled states and for the reversible mapping of these states of matter to photonic states of an optical pulse in the waveguide. Our protocol makes use of decoherence-free subspaces (DFS) for the atomic emitters with coherent evolution within the DFS enforced by strong dissipative coupling to the waveguide. By switching from subradiant to superradiant states, entangled atomic states are mapped to photonic states with high fidelity. An implementation using ultracold atoms coupled to a photonic crystal waveguide is discussed.Comment: 15 pages, 4 figure

Waveguide quantum electrodynamics

Waveguide quantum electrodynamics (waveguide QED) describes the interaction between an electromagnetic field confined to a one-dimensional waveguide with atom-like quantum emitters close by. The characteristics of these kinds of systems are the possibility for strong and even ultrastrong interactions between the photonic and atom-like systems, and practically infinite range interactions between the emitters. These properties are valuable for a wide range of quantum optical applications, like quantum communication, quantum networks and quantum metrology. In this thesis we focus on two challenges in quantum optics, namely the generation and scattering of multiphoton states. In the first part of this thesis we demonstrate how waveguide QED systems can be exploited for the generation of multiphoton states, in particular of single-mode Fock states and superpositions thereof as well as multi-mode photonic states with metrological applications. The basic setup for this goal is an ensemble of quantum emitters coupled to a waveguide. In the so-called atomic mirror configuration symmetric Dicke states (or a superposition thereof) decay superradiantly to the ground state and emit the desired multiphoton state, which can be efficiently collected at the ends of the waveguide. We propose various protocols for the preparation of these Dicke states in waveguide QED systems and characterize the emitted photonic state. It turns out that in the low excitation regime, that is, if the number of photons is much lower than the ensemble size, the photonic state is emitted into a single mode. This single-mode structure is fundamental to current proposals for applications in quantum metrology with optical interferometers. Outside the low excitation regime, a multi-mode photonic state is generated, for which the metrological capabilities were unknown. We were able to show that these states still lead to quantum-enhanced optical interferometry. In the second part of this thesis we demonstrate how the scattering of multiphoton states on a single quantum emitter coupled to the waveguide can be used for testing the limits of the light-matter interaction strength. In particular, we investigate systems in the so-called ultrastrong coupling regime, where the coupling strength between the photonic and atom-like system is of the order of the emitter's transition energy. In this regime, many methods and approximations used in quantum optics, especially the Rotating Wave Approximation, break down and one needs to develop new analytical and numerical methods to study these systems. One approach is the so-called polaron Transformation. We show how this transformation can be applied for predicting the scattering amplitude of few photons scattering on the emitter. The comparison of these results with numerical simulations using matrix product states shows a good agreement for moderate coupling strengths. Both parts together show that waveguide QED systems are promising candidates for the generation of multiphoton states as well as for the investigation of fundamental limits probed through the scattering of multiphoton states. The latter also finds application in the implementation of photon-photon nonlinearities induced by the interaction with the quantum emitters. Apart from these and many other theoretical predictions, waveguide QED systems are also undergoing rapid progress in experiments. Therefore, we expect these kinds of systems to bring forth several advances in the field of quantum optics.Die Wellenleiter-Quantenelektrodynamik (Wellenleiter-QED) beschreibt die Wechselwirkung zwischen dem elektromagnetischen Feld eines Wellenleiters und nahe liegenden atom-ähnlichen Quatenemittern. Charakteristisch für diese Art von Systemen ist die Möglichkeit starker und sogar ultrastarker Wechselwirkungen zwischen den photonischen und atom-ähnlichen Systemen, sowie Wechselwirkungen zwischen den Emittern mit praktisch unendlicher Reichweite. Diese Eigenschaften sind wertvoll für eine Vielzahl von quantenoptischen Anwendungen, wie die Quantenkommunikation, Quantennetzwerke und Quantenmetrologie. In dieser Arbeit befassen wir uns mit zwei Herausforderungen in der Quantenoptik, nämlich der Erzeugung und der Streuung von Multiphoton-Zuständen. Im ersten Teil dieser Arbeit zeigen wir, wie Wellenleiter-QED-Systeme für die Erzeugung von Multiphoton-Zuständen, insbesondere von monomodalen Fock-Zuständen und deren Superposition, sowie multimodalen photonischen Zuständen mit Nutzen für die Quantenmetrologie, verwendet werden können. Der grundlegende Aufbau für dieses Ziel ist ein Ensemble von Quantenemittern, die an einen Wellenleiter gekoppelt sind. In der sogenannten Atomspiegelkonfiguration zerfallen symmetrische Dicke-Zustände (oder eine Superposition davon) superradiant in den Grundzustand und emittieren den gewünschten Multiphoton-Zustand, der effizient an den Enden des Wellenleiters entnommen werden kann. Wir schlagen verschiedene Protokolle zur Bereitstellung dieser Dicke-Zustände in Wellenleiter-QED-Systemen vor und charakterisieren den emittierten photonischen Zustand. Es stellt sich heraus, dass im niedrigen Anregungsregime, d.h. wenn die Anzahl der Photonen viel geringer ist als die Größe des Ensembles, der photonische Zustand in eine einzige Mode emittiert wird. Diese monomodale Struktur ist grundlegend für derzeitige Methoden und Ansätze in der Quantenmetrologie mit optischen Interferometern. Außerhalb des niederen Anregungsregimes wird ein multimodaler photonischer Zustand erzeugt, für den die metrologischen Eigenschaften bislang unbekannt waren. Wir konnten zeigen, dass diese Zustände immer noch zu einer durch Quantenmechanik verbesserten optischen Interferometrie führen. Im zweiten Teil dieser Arbeit zeigen wir, wie die Streuung von Multiphoton-Zuständen an einem einzelnen Quantenemitter, der an den Wellenleiter gekoppelt ist, zum Testen der Grenzen der Licht-Materie-Wechselwirkung genutzt werden kann. Insbesondere untersuchen wir Systeme im Bereich der so genannten ultrastarken Wechselwirkung, bei dem die Kopplungsstärke zwischen dem photonischen und dem atom-ähnlichen System in der Größenordnung der Übergangsenergie des Emitters liegt. In diesem Regime, können viele Methoden und Näherungen, vor allem die Rotating Wave Approximation, nicht mehr angewendet werden und neue analytische und numerische Methoden müssen entwickeln werden, um diese Systeme zu untersuchen. Ein Ansatz ist die so genannte Polaron-Transformation. Wir zeigen, wie diese Transformation angewendet werden kann, um die Streuamplitude von Photonen, die an dem Emitter streuen, zu berechnen. Der Vergleich dieser Ergebnisse mit numerischen Simulationen zeigt eine gute Übereinstimmung bei moderaten Kopplungsstärken. Beide Teile zusammen zeigen, dass Wellenleiter-QED-Systeme vielversprechende Kandidaten, einerseits für die Erzeugung von Multiphoton-Zuständen und andererseits für die Untersuchung fundamentaler Grenzen, die durch die Streuung von Multiphoton-Zuständen erforscht werden können, sind. Letzteres findet auch Anwendung in der Erzeugung von Photon-Photon-Interaktionen, die durch die Wechselwirkung mit den Quantenemittern induziert werden. Abgesehen von diesen und vielen anderen theoretischen Vorhersagen, werden Wellenleiter-QED-Systeme auch in Experimenten rapide weiterentwickelt. Daher erwarten wir, dass diese Art von Systemen zahlreiche Fortschritte auf dem Gebiet der Quantenoptik hervorbringen wird

Universal quantum computation in waveguide QED using decoherence free subspaces

The interaction of quantum emitters with one-dimensional photon-like reservoirs induces strong and long-range dissipative couplings that give rise to the emergence of the so-called decoherence free subspaces (DFSs) which are decoupled from dissipation. When introducing weak perturbations on the emitters, e.g., driving, the strong collective dissipation enforces an effective coherent evolution within the DFS. In this work, we show explicitly how by introducing single-site resolved drivings, we can use the effective dynamics within the DFS to design a universal set of one and two-qubit gates within the DFS of an ensemble of two-level atom-like systems. Using Liouvillian perturbation theory we calculate the scaling with the relevant figures of merit of the systems, such as the Purcell factor and imperfect control of the drivings. Finally, we compare our results with previous proposals using atomic Λ systems in leaky cavities

Quantum metrology with one-dimensional superradiant photonic states

Photonic states with large and fixed photon numbers, such as Fock states, enable quantum-enhanced metrology but remain an experimentally elusive resource. A potentially simple, deterministic and scalable way to generate these states consists of fully exciting $N$ quantum emitters equally coupled to a common photonic reservoir, which leads to a collective decay known as Dicke superradiance. The emitted $N$-photon state turns out to be a highly entangled multimode state, and to characterise its metrological properties in this work we: (i) develop theoretical tools to compute the Quantum Fisher Information of general multimode photonic states; (ii) use it to show that Dicke superradiant photons in 1D waveguides achieve Heisenberg scaling, which can be saturated by a parity measurement; (iii) and study the robustness of these states to experimental limitations in state-of-art atom-waveguide QED setups.Comment: 17 pages, 3 figures. v2: substantially improved version with new result

Generation of single and two-mode multiphoton states in waveguide QED

Single and two-mode multiphoton states are the cornerstone of many quantum technologies, e.g., metrology. In the optical regime these states are generally obtained combining heralded single-photons with linear optics tools and post-selection, leading to inherent low success probabilities. In a recent paper, we design several protocols that harness the long-range atomic interactions induced in waveguide QED to improve fidelities and protocols of single-mode multiphoton emission. Here, we give full details of these protocols, revisit them to simplify some of their requirements and also extend them to generate two-mode multiphoton states, such as Yurke or NOON states.Comment: 16 pages, 8 figure