Quantum coherence and population transfer in a driven cascade three-level artificial atom

TL;DR

This paper experimentally explores the spectral behavior of a three-level artificial atom under external driving, revealing two types of Autler-Townes splitting influenced by decay rates and field strengths, and analyzing quantum interference effects.

Contribution

It provides the first experimental observation of two distinct Autler-Townes splitting types in a driven three-level artificial atom and analyzes the underlying quantum coherence phenomena.

Findings

Identification of two AT splitting types influenced by decay and field strength

Observation of two-photon coherence due to quantum interference

Analysis of spectral characteristics in a driven transmon qubit

Abstract

We present an experimental investigation on the spectral characteristics of an artificial atom "transmon qubit" constituting a three-level cascade system ({\Xi}-system) in the presence of a pair of external driving fields. We observe two different types of Autler-Townes (AT) splitting: type I, where the phenomenon of two-photon resonance tends to diminish as the coupling field strength increases, and type II, where this phenomenon mostly stays constant. We find that the types are determined by the cooperative effect of the decay rates and the field strengths. Theoretically analyzing the density-matrix elements in the weak-field limit where the AT effect is suppressed, we single out events of pure two-photon coherence occurring owing to constructive quantum interference.