Dynamics of many-body photon bound states in chiral waveguide QED

We theoretically study the few- and many-body dynamics of photons in chiral waveguides. In particular, we examine pulse propagation through a system of $N$ two-level systems chirally coupled to a waveguide. We show that the system supports correlated multi-photon bound states, which have a well-defined photon number $n$ and propagate through the system with a group delay scaling as $1/n^2$. This has the interesting consequence that, during propagation, an incident coherent state pulse breaks up into different bound state components that can become spatially separated at the output in a sufficiently long system. For sufficiently many photons and sufficiently short systems, we show that linear combinations of $n$-body bound states recover the well-known phenomenon of mean-field solitons in self-induced transparency. For longer systems, however, the solitons break apart through quantum correlated dynamics. Our work thus covers the entire spectrum from few-photon quantum propagation, to genuine quantum many-body (atom and photon) phenomena, and ultimately the quantum-to-classical transition. Finally, we demonstrate that the bound states can undergo elastic scattering with additional photons. Together, our results demonstrate that photon bound states are truly distinct physical objects emerging from the most elementary light-matter interaction between photons and two-level emitters. Our work opens the door to studying quantum many-body physics and soliton physics with photons in chiral waveguide QED.Comment: Updated with new results. 14 pages plus supplementary materia

Dynamics of Many-Body Photon Bound States in Chiral Waveguide QED

We theoretically study the few- and many-body dynamics of photons in chiral waveguides. In particular, we examine pulse propagation through an ensemble of N two-level systems chirally coupled to a waveguide. We show that the system supports correlated multiphoton bound states, which have a well-defined photon number n and propagate through the system with a group delay scaling as 1/n2. This has the interesting consequence that, during propagation, an incident coherent-state pulse breaks up into different bound-state components that can become spatially separated at the output in a sufficiently long system. For sufficiently many photons and sufficiently short systems, we show that linear combinations of n-body bound states recover the well-known phenomenon of mean-field solitons in self-induced transparency. Our work thus covers the entire spectrum from few-photon quantum propagation, to genuine quantum many-body (atom and photon) phenomena, and ultimately the quantum-to-classical transition. Finally, we demonstrate that the bound states can undergo elastic scattering with additional photons. Together, our results demonstrate that photon bound states are truly distinct physical objects emerging from the most elementary light-matter interaction between photons and two-level emitters. Our work opens the door to studying quantum many-body physics and soliton physics with photons in chiral waveguide QED. © 2020 authors

Optical Nanofibers: a new platform for quantum optics

The development of optical nanofibers (ONF) and the study and control of their optical properties when coupling atoms to their electromagnetic modes has opened new possibilities for their use in quantum optics and quantum information science. These ONFs offer tight optical mode confinement (less than the wavelength of light) and diffraction-free propagation. The small cross section of the transverse field allows probing of linear and non-linear spectroscopic features of atoms with exquisitely low power. The cooperativity -- the figure of merit in many quantum optics and quantum information systems -- tends to be large even for a single atom in the mode of an ONF, as it is proportional to the ratio of the atomic cross section to the electromagnetic mode cross section. ONFs offer a natural bus for information and for inter-atomic coupling through the tightly-confined modes, which opens the possibility of one-dimensional many-body physics and interesting quantum interconnection applications. The presence of the ONF modifies the vacuum field, affecting the spontaneous emission rates of atoms in its vicinity. The high gradients in the radial intensity naturally provide the potential for trapping atoms around the ONF, allowing the creation of one-dimensional arrays of atoms. The same radial gradient in the transverse direction of the field is responsible for the existence of a large longitudinal component that introduces the possibility of spin-orbit coupling of the light and the atom, enabling the exploration of chiral quantum optics.Comment: 65 pages, to appear in Advances in Atomic, Molecular and Optical Physic

Atom-photon Interactions in slow-light waveguide QED

Das Gebiet der Wellenleiterquantenelektrodynamik (QED) befasst sich mit der Kopplung von Atomen oder Emittern in Festkörpern an das Lichtfeld in einem eindimensionalen optischen Leiter. Durch den transversalen Einschluss der emittierten Photonen können diese langreichweitige Wechselwirkungen zwischen den einzelnen Quantensystemen vermitteln, was diese Architektur für die Untersuchung quantenoptischer Phänomene und für die Realisierung zukünftiger Quantennetzwerke besonders interessant macht. In dieser Doktorarbeit werden verschiedene neue Aspekte der Wechselwirkung zwischen Licht und Atomen in nanophotonischen Wellenleitern theoretisch untersucht. Diese Arbeiten adressieren dabei vor allem ein neues Regime der "Wellenleiter-QED mit langsamen Photonen", in dem die maximale Gruppengeschwindigkeit im Inneren des Wellenleiters, im Vergleich zum freien Raum, erheblich reduziert ist. Solche Bedingungen ergebenen sich, z.B., in der Nähe von Bandkanten in photonischen Kristallen und führen zu einer extremen Verstärkung der Atom-Licht-Kopplung. In dieser Dissertation werden zunächst die Eigenschaften gebundener Zustände zwischen Atomen und Photonen, die die neuen Elementaranregungen dieses Systems darstellen, untersucht. Dabei werden zum ersten Mal auch die Bindung von mehreren Photonen an ein einzelnes Atom analysiert und die sich daraus ergebenden linearen und nichtlinearen spektralen Charakteristika dieser "multiphoton dressed states" beschrieben. Des Weiteren wird der interessante Fall betrachtet, in dem sich die Atome mit einer Geschwindigkeit bewegen, die mit der reduzierten Lichtgeschwindigkeit der Photonen vergleichbar ist. Unter diesen Vorraussetzungen beobachtet man eine von der Bewegung induzierten Richtungsabhängigkeit der emittierten Photonen und das Auftreten von nicht-perturbativen Effekten in der Atom-Licht-Kopplung. Diese Anomalien ergeben sich aus der lang anhaltenden Wechselwirkung mit den emittierten Cherenkov-Photonen, welche sich mit gleicher Geschwindigkeit wie die Atom entlang des Wellenleiters bewegen. Als Gegenstück dazu werden in einem weiterem Projekt dann die Auswirkungen von starken akustischen Wellen auf die Emissionseigenschaften von statischen Atomen analysiert. Dabei findet man, dass, im Regime des langsamen Lichts, diese akustische Wellen die Photon im Wellenleiter "mitziehen" können und damit sowohl die Richtung als auch die Form der emittierten Lichtpakete beeinflussen. Diese Effekte können direkt für die Übertragung von Quantenzuständen in photonischen Netzwerken ausgenützte werden.Waveguide quantum electrodynamics (QED) refers to a scenario where single or multiple atoms or solid-state emitters are coupled to a one dimensional optical channel. The efficient interaction between individual quantum systems with photons that are confined along a single direction makes this setting particularly interesting for investigating quantum optical phenomena and for future quantum networking applications. In this thesis, we go beyond the standard scenario and address the new regime of \qq Such bound states are formed by an atom and a localized photonic excitation and represent the continuum analog of the familiar dressed states in single-mode cavity QED. In this thesis we analyze the linear and nonlinear spectral features associated with singleand multi-photon dressed states and we describe how the formation of bound states affects the waveguide-mediated dipole-dipole interactions between separated atoms. We then consider a narrow-bandwidth waveguide coupled to atoms that are moving with velocities comparable to the reduced speed of light. Under these conditions, we observe a velocity-induced directionality and the emergence of effective divergencies in the photonic density of states. This anomalous interaction between atoms and co-propagating Cherenkov photons gives rise to a range of novel phenomena and non-perturbative effects in the emission of photons and the resulting photon-mediated interactions between moving atoms. Finally, we consider the coupling of multiple emitters to a slow-light waveguide in the presence of propagating acoustic waves. In this case, the strong index modulations induced by such waves can substantially modify the effective photonic density of states and thereby influence the strength, the directionality, as well as the overall characteristic of photon emission and absorption processes. The generalization of these control techniques to two dimensional photonic lattices creates a new scenario for chiral quantum optics, where nonreciprocal light-matter interactions are established along a single direction and with an extremely slow radial decay. These effect provide a versatile tool for implementing various quantum communication protocol in large-scale photonic networks.19