Bath engineering of a fluorescing artificial atom with a photonic crystal. (arXiv:1812.04205v2 [quant-ph] UPDATED)

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 decays due to
the dissipative interaction 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.

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