2026 Theses Doctoral

Spontaneous emergence of order via dissipation in one-dimensional reservoirs

Cardenas Lopez, Silvia Fernanda

When atoms dissipate into a common reservoir, their decay becomes collective: the emission of each atom depends on the state of all others. This collective dissipation is a ubiquitous feature of quantum platforms, including atomic arrays, superconducting qubits, and quantum dots. In this regime, dissipation is not merely a limitation to coherent quantum dynamics, but can itself generate spontaneous coherence and self-organization. A prime example is Dicke superradiance, where initially excited atoms in a single-mode cavity synchronize their phases and emit a superradiant burst with enhanced intensity and metrologically advantageous properties. With the addition of incoherent pumping, this mechanism gives rise to superradiant lasing, characterized by continuous emission with ultranarrow linewidths relevant for precision metrology.

A central open question is how these phenomena extend beyond the highly symmetric setting of single-mode cavities. While such systems are exactly solvable when all atoms couple identically to the field, more general configurations quickly become computationally challenging due to the exponential growth of the Hilbert space with atom number. At the same time, recent experimental advances in atomic arrays and superconducting circuits enable the exploration of collective dissipation in settings that go beyond the cavity paradigm.

In this thesis, I investigate collective light emission in waveguide quantum electrodynamics (wQED), where atoms couple to one-dimensional reservoirs. This setting provides a minimal yet nontrivial platform: it breaks the full symmetry of the Dicke model while remaining more tractable than three-dimensional free space. First, I identify the minimal conditions under which atoms in a waveguide exhibit a superradiant burst, thereby contributing to demonstrate that superradiance can arise beyond the Dicke limit. I further analyze the directional properties of the emitted radiation and show that competition between left- and right-propagating modes leads to mirror-symmetry breaking at the level of individual quantum trajectories.


I then study steady-state superradiance in both ring cavities and waveguides, and elucidate the effect of dipole-dipole interactions and competition between different collective decay channels. Mode competition gives rise to bistable steady states in which all atoms synchronize to radiate predominantly in one direction. When dipole-dipole interactions are included, this global synchronization transforms into a phase-separated state with a domain wall between the two regions. While line narrowing with increasing atom number persists in the presence of mode competition, numerical results indicate that dipole-dipole interactions ultimately limit the achievable minimal linewidth.

Finally, I analyze the propagation of weak broadband pulses through a wQED setup. In particular, I discuss the emergence of precursors, transient signals that appear to propagate faster than the main pulse. In large ensembles with weak coupling, the system behaves as an effective Lorentz medium. This description captures experimental observations of pulse transmission through nanofibers coupled to cold Cs atoms. In contrast, in the regime of few emitters and strong coupling, relevant to superconducting qubits, transmission becomes highly sensitive to atom number and spatial configuration, providing a route toward dispersion engineering at the few-emitter level.

Overall, this work elucidates the roles of mode competition and dipole-dipole interactions in collective dissipation, laying the groundwork for future investigations of self-organization beyond cavity QED and identifying signatures accessible in current experimental platforms.