# Bath engineering of a fluorescing artificial atom with a photonic crystal

## TL;DR

This paper demonstrates how a microwave photonic crystal can be engineered to control the dissipation of a superconducting qubit, enabling deterministic preparation of quantum states through tailored environment interactions.

## Contribution

It introduces a method to engineer the quantum environment of a superconducting qubit using a photonic crystal, enabling new quantum control techniques.

## Key findings

- Controlled dissipation influences qubit decay rates.
- Deterministic superposition states achieved via environment engineering.
- Quantum state tomography confirms steady-state quantum control.

## Abstract

We demonstrate how the dissipative interaction between a superconducting qubit and a microwave photonic crystal can be used for quantum bath engineering. The photonic crystal is created with a step-impedance transmission line which suppresses and enhances the quantum spectral density of states, influencing decay transitions of a transmon circuit. The qubit interacts with the transmission line indirectly via dispersive coupling to a cavity. We characterize the photonic crystal density of states from both the unitary and dissipative dynamics of the qubit. When the qubit is driven, it dissipates into the frequency dependent density of states of the photonic crystal. Our result is the deterministic preparation of qubit superposition states as the steady-state of coherent driving and dissipation near by the photonic crystal band edge, which we characterize with quantum state tomography. Our results highlight how the multimode environment from the photonic crystal forms a resource for quantum control.