# Subradiant states of quantum bits coupled to a one-dimensional waveguide

## TL;DR

This paper investigates subradiant states in arrays of quantum emitters coupled to a 1D waveguide, revealing universal decay suppression, fermionic multi-excitation states, and proposing methods for their creation and measurement.

## Contribution

It introduces a comprehensive analysis of subradiant modes, their universal decay scaling, fermionic multi-excitation states, and practical measurement protocols with superconducting qubits.

## Key findings

- Subradiant decay rates scale cubically with the number of emitters.
- Multi-excitation states exhibit fermionic spatial correlations.
- Proposed methods enable efficient creation and measurement of subradiant states.

## Abstract

The properties of coupled emitters can differ dramatically from those of their individual constituents. Canonical examples include sub- and super-radiance, wherein the decay rate of a collective excitation is reduced or enhanced due to correlated interactions with the environment. Here, we systematically study the properties of collective excitations for regularly spaced arrays of quantum emitters coupled to a one-dimensional (1D) waveguide. We find that, for low excitation numbers, the modal properties are well-characterized by spin waves with a definite wavevector. Moreover, the decay rate of the most subradiant modes obeys a universal scaling with a cubic suppression in the number of emitters. Multi-excitation subradiant eigenstates can be built from fermionic combinations of single excitation eigenstates; such "fermionization" results in multiple excitations that spatially repel one another. We put forward a method to efficiently create and measure such subradiant states, which can be realized with superconducting qubits. These measurement protocols probe both real-space correlations (using on-site dispersive readout) and temporal correlations in the emitted field (using photon correlation techniques).