Atom-atom interactions around the band edge of a photonic crystal waveguide (1603.02771v2)

Abstract: Tailoring the interactions between quantum emitters and single photons constitutes one of the cornerstones of quantum optics. Coupling a quantum emitter to the band edge of a photonic crystal waveguide (PCW) provides a unique platform for tuning these interactions. In particular, the crossover from propagating fields $E(x) \propto e^{\pm ik_x x}$ outside the bandgap to localized fields $E(x) \propto e^{-\kappa_x |x|}$ within the bandgap should be accompanied by a transition from largely dissipative atom-atom interactions to a regime where dispersive atom-atom interactions are dominant. Here, we experimentally observe this transition for the first time by shifting the band edge frequency of the PCW relative to the $\rm D_1$ line of atomic cesium for $\bar{N}=3.0\pm 0.5$ atoms trapped along the PCW. Our results are the initial demonstration of this new paradigm for coherent atom-atom interactions with low dissipation into the guided mode.

Atom-Atom Interactions Around Photonic Crystal Band Edges

The paper "Atom-atom interactions around the band edge of a photonic crystal waveguide" explores the transition between dissipative and dispersive atom-atom interactions facilitated by photonic crystal waveguides (PCWs). This paper explores the manipulation of atom-light interactions by coupling quantum emitters to the band edge of PCWs, thereby presenting a novel regime of quantum optical phenomena that allows for enhanced coherent atom-atom interactions with minimized dissipation.

Key Findings

1. Transition from Dissipation to Dispersion: The paper experimentally observes the crossover from dissipative interactions to dispersive interactions as atoms are coupled to the band edge of a photonic crystal waveguide. The transition is marked by a shift in the behavior of the photonic modes from propagating to evanescent—field dynamics that are critical in modifying the nature of atom-atom interactions.
2. Experimental Observations: Using cesium atoms, the interaction transition is measured by aligning the band edge frequency of the PCW relative to the $\rm D_1$ line of atomic cesium ($\bar{N}=3.0\pm 0.5$ atoms). This alignment exhibits a paradigm shift in interactions, showcasing the suppression of guided mode decay rates within the bandgap.
3. Quantitative Analysis via Green's Function: The authors employ the electromagnetic Green's function formalism to model the atom-atom interactions, allowing them to infer peak single-atom frequency shifts, decay rates, and the ratio $\mathcal{R}$ of dissipative versus coherent guided mode rates. This analytical framework is pivotal for connecting experimental observations to theoretical predictions.
4. New Atom-Photon Bound States: Within the bandgap, the atomic transition frequency is not capable of emitting propagating waves, leading to the formation of atom-photon bound states that contribute to dispersive interactions with minimal decay into guided modes. These bound states can have a tunable range of interaction length, offering new degrees of freedom for experimental manipulation.

Implications for Future Research

The findings open new theoretical and experimental pathways for harnessing coherent atom-atom interactions in photonic systems. The reduction in dissipative decay and enhancement of dispersive interactions in PCWs implies potential applications in quantum information processing, quantum simulations of many-body physics, and development of quantum networks requiring effective matter-light interfaces.

Technological Advancements: Future developments could lead to more complex configurations, such as employing 2D PCWs or other nanophotonic structures to engineer specific atom-light interaction regimes. This could span advancements in designing photonic devices optimized for these interactions with low dissipation and enabling robust quantum state control across larger atom arrays.

Quantum Many-Body Models: Extending this work to paper various atomic configurations may lead to insights into quantum phase transitions and collective quantum dynamics within engineered photonic environments. The foundation set by this paper for atom-PCW interactions can catalyze novel implementations of many-body quantum systems with tailored interaction strengths.

In conclusion, this research presents a pivotal exploration into the dynamics of atom-atom interactions via photonic crystal waveguides. The authors substantiate the capability of manipulating quantum light-matter interactions profoundly, delineating a course for further enhancements in the control of atomic systems embedded in complex photonic architectures. The results signify the potential for significant advances in achieving coherent control in quantum technologies using nanophotonic designs.