Non-Markovian spontaneous emission and photon localization in a atom-waveguide system

Chang-qi Cao, Jinrong Tian, Hui W. Cao

Stefano Longhi

Spontaneous emission of a quantum emitter, such as an excited atom, is a fundamental process in quantum electrodynamics (QED), typically associated with exponential decay to the ground state accompanied by irreversible photon emission. This simple Markovian picture, however, is profoundly modified in the presence of time-delayed feedback, structured continua, or cooperative emission, as occurs when an emitter radiates in front of a mirror, when several emitters radiate collectively, or in the case of a giant atom. In such regimes, strong non-Markovian dynamics arise from photon reabsorption and interference effects, leading to pronounced deviations from exponential decay. Here we demonstrate the emergence of exceptional points (EPs) in these highly non-Markovian waveguide-QED environments, i.e., non-Markovian EPs. These EPs appear directly in the relaxation dynamics as sharp transitions to oscillatory behavior, manifested by the appearance of real zeros in the excited-state amplitude. We analyze in detail the spontaneous emission of giant atoms with two or more coupling points, highlighting the mechanisms leading to non-Markovian EPs, and show that similar phenomena arise in other waveguide-QED settings, such as the collective spontaneous emission of spatially separated point-like emitters. Our results reveal waveguide-QED systems as experimentally accessible platforms for realizing and exploring non-Markovian EP physics.

Ronggang Liu

Roberto Passante, Lucia Rizzuto, Giuseppe Calajò

Stefano Longhi

Spontaneous emission of an excited atom in a featureless continuum of electromagnetic modes is a fundamental process in quantum electrodynamics associated with an exponential decay of the quantum emitter to its ground state accompanied by an irreversible emission of a photon. However, such a simple scenario is deeply modified when considering a "giant" atom, i.e., an atom whose dimension is larger than the wavelength of the emitted photon. In such an unconventional regime, non-Markovian effects and strong deviations from an exponential decay are observed owing to interference effects arising from nonlocal light-atom coupling. Here we suggest a photonic simulation of non-Markovian giant atom decay, based on light escape dynamics in an optical waveguide nonlocally coupled to a waveguide lattice. Major effects, such as nonexponential decay, enhancement, or slowing down of the decay, and formation of atom-field dark states can be emulated in this system.

Silvia Casulleras

In this work we study the spontaneous emission of a two-level atom placed inside a hollow cylindrical waveguide of infinite length and with perfectly electric conducting walls. The atom, whose electric dipole moment is assumed to be oriented along the symmetry axis of the waveguide, couples to a photonic band structure with a non-linear dispersion relation. We address two different regimes of coupling between the atom and the photonic band structure. In particular, we refer to the regime where the internal frequency of the a.tom is close to a band edge as the strong coupling regime and to the regime where the internal frequency is far from a band edge as the weak coupling regime. In the weak coupling regime, we find that the a.tom decays exponentially with a modified decayed rate due to the presence of the waveguide, and that the line-shape of the emitted photons within every band takes the form of a Lorentzian. In the strong coupling regime, we find a non-exponential decay of the atom, which is due to non-Markovian dynamics of the system. Moreover, the population of the atom presents oscillations in time and it converges to a non-zero positive value, indicating that the a.tom never fully decays. In this regime the spectrum of the emitted photons becomes more involved.