arXiv subjects

Alfonso Lanuza

Publications and source records attributed to Alfonso Lanuza.

6 recordsLinked to original sources

arXiv2025

Universal spectral bounds for the quantum Rabi model: Reformulating Braak's conjecture

The quantum Rabi model is a paradigmatic example of a minimal yet nontrivial light-matter interaction, whose spectrum is transcendental yet exhibits a number of regularities. Braak observed that the eigenvalues bunch or anti-bunch following strict rules, leading to a conjecture that links integrability in quantum systems and residual order in their spectra. Understanding this structure is crucial for distinguishing deterministic quantum dynamics from chaotic behavior. Here, we present violations of Braak's conjecture and reformulate it through a set of eigenvalue inequalities that conjecturally hold across all parameter regimes. We prove these bounds in the low splitting regime, we characterize the strongly-coupled widely-split limit, and provide universal upper bounds on the entire spectrum. Our results uncover additional layers of spectral organization in the quantum Rabi model and expand the analytic toolkit for strongly coupled quantum systems.

quant-ph↗

arXiv2024

Exact solution for the collective non-Markovian decay of two fully excited quantum emitters

Waveguide quantum electrodynamics constitutes a modern paradigm for the interaction of light and matter, in which strong coupling, bath structure, and propagation delays can break the radiative conditions that quantum emitters typically encounter in free space. These characteristics intertwine the excitations of quantum emitters and guided radiation modes to form complex multiphoton dynamics. So far, combining the collective decay of the emitters with the non-Markovian effects induced by the modes has escaped a full solution and the detailed physics behind these systems remains unknown. Here we analyze such a collective non-Markovian decay in a minimal system of two excited emitters coupled to a one-dimensional single-band waveguide. We develop an exact solution for this system in terms of elementary functions that unveils hidden symmetries and predicts new forms of spontaneous decay. The collective non-Markovian dynamics, which are strongly dependent on the vacuum coupling and the detuning from the center of the band, show exotic features that can be characterized with a simple and readily available criterion. Our analytic methods shed light on the complexity of collective light-matter interactions and open up a pathway for understanding multiparticle open quantum systems.

quant-ph↗

arXiv2023

Super- and subradiant dynamics of quantum emitters mediated by atomic matter waves

The cooperative modification of spontaneous radiative decay is a paradigmatic many-emitter effect in quantum optics. So far its experimental realization has involved interactions mediated by rapidly escaping photons that do not play an active role in the emitter dynamics. Here we explore cooperative dynamics of quantum emitters in an optical lattice that interact by radiating atomic matter waves. Using the ability to prepare weakly and strongly interacting many-body phases of excitations in an array of matter-wave emitters, we demonstrate directional super- and subradiance from a superfluid phase with tunable radiative phase lags, and directly access the buildup of coherence imprinted by the emitted radiation across a Mott insulator. We investigate the onset of cooperative dynamics for slow wave propagation and observe a coupling to collective bound states with radiation trapped at and between the emitters. Our results in open-system quantum electrodynamics establish ultracold matter waves as a versatile tool for studying many-body quantum optics in spatially extended and ordered systems.

quant-ph↗

arXiv2021

Formation of Matter-Wave Polaritons in an Optical Lattice

The polariton, a quasiparticle formed by strong coupling of a photon to a matter excitation, is a fundamental ingredient of emergent photonic quantum systems ranging from semiconductor nanophotonics to circuit quantum electrodynamics. Exploiting the interaction between polaritons has led to the realization of superfluids of light as well as of strongly correlated phases in the microwave domain, with similar efforts underway for microcavity exciton-polaritons. Here, we develop an ultracold-atom analogue of an exciton-polariton system in which interacting polaritonic phases can be studied with full tunability and without dissipation. In our optical-lattice system, the exciton is replaced by an atomic excitation, while an atomic matter wave is substituted for the photon under a strong dynamical coupling. We access the band structure of the matter-wave polariton spectroscopically by coupling the upper and lower polariton branches, and explore polaritonic many-body transport in the superfluid and Mott-insulating regimes, finding quantitative agreement with our theoretical expectations. Our work opens up novel possibilities for studies of polaritonic quantum matter.

cond-mat.quant-gas↗

arXiv2021

Multiband and array effects in matter-wave-based waveguide QED

Recent experiments on spontaneous emission of atomic matter waves open a new window into the behavior of quantum emitters coupled to a waveguide. Here we develop an approach based on infinite products to study this system theoretically, without the need to approximate the band dispersion relation of the waveguide. We solve the system for a one-dimensional array of one, multiple and an infinite number of quantum emitters and compare with the experiments. This leads to a detailed characterization of the decay spectrum, with a family of in-gap bound states, new mechanisms for enhanced Markovian emission different from superradiance, and the emergence of matter-wave polaritons.

quant-ph↗

arXiv2020

Dynamics of Matter-Wave Quantum Emitters in a Structured Vacuum

The characteristics of spontaneous emission can be strongly modified by the mode structure of the vacuum. In waveguide quantum-electrodynamics based on photonic crystals, this modification is exploited to engineer atom-photon interactions near a band edge, but the physics of coupling to an entire band has not yet been explored in experiments. Using ultracold atoms in an optical lattice, we study the decay dynamics of matter-wave quantum emitters coupled to a single band of an effective photonic crystal waveguide structure with tunable characteristics. Depending on the ratio between vacuum coupling and bandwidth, we observe a transition from irreversible decay to fully oscillatory dynamics linked to the interplay of matter-wave bound states near the band edges, whose spatial structure we characterize. Our results shed light on the emergence of coherence in an open quantum system in a controllable environment, and are of relevance for the understanding of vacuum-induced decay phenomena in photonic systems.

cond-mat.quant-gas↗