Nonreciprocal Atomic Scattering: A saturable, quantum Yagi-Uda antenna

TL;DR

This paper demonstrates that a pair of atoms in a 1D waveguide acts as a saturable, nonreciprocal quantum antenna due to low-power saturation effects, leading to asymmetric scattering and entanglement.

Contribution

It reveals that the asymmetry in atomic scattering arises from a low-power saturable dark state, challenging reciprocity and modeling the system as a quantum Yagi-Uda antenna.

Findings

Asymmetry results from a quasi-dark-state saturating at low power.

Output field statistics can be explained by a stochastic mirror model.

System reaches an entangled tripartite |W⟩ state at steady state.

Abstract

Recent theoretical studies of a pair of atoms in a 1D waveguide find that the system responds asymmetrically to incident fields from opposing directions at low powers. Since there is no explicit time-reversal symmetry breaking elements in the device, this has caused some debate. Here we show that the asymmetry arises from the formation of a quasi-dark-state of the two atoms, which saturates at extremely low power. In this case the nonlinear saturability explicitly breaks the assumptions of the Lorentz reciprocity theorem. Moreover, we show that the statistics of the output field from the driven system can be explained by a very simple stochastic mirror model and that at steady state, the two atoms and the local field are driven to an entangled, tripartite $\left| W \right\rangle$ state. Because of this, we argue that the device is better understood as a saturable Yagi-Uda antenna, a distributed system of differentially-tuned dipoles that couples asymmetrically to external fields.