Abstract
With the introduction of superconducting circuits into the field of quantum optics, many experimental demonstrations of the quantum physics of an artificial atom coupled to a single-mode light field have been realized. Engineering such quantum systems offers the opportunity to explore extreme regimes of light-matter interaction that are inaccessible with natural systems. For instance the coupling strength g can be increased until it is comparable with the atomic or mode frequency ωa,m and the atom can be coupled to multiple modes which has always challenged our understanding of light-matter interaction. Here, we experimentally realize a transmon qubit in the ultra-strong coupling regime, reaching coupling ratios of g/ωm = 0.19 and we measure multi-mode interactions through a hybridization of the qubit up to the fifth mode of the resonator. This is enabled by a qubit with 88% of its capacitance formed by a vacuum-gap capacitance with the center conductor of a coplanar waveguide resonator. In addition to potential applications in quantum information technologies due to its small size, this architecture offers the potential to further explore the regime of multi-mode ultra-strong coupling.
Highlights
Superconducting circuits such as microwave cavities and Josephson junction based artificial atoms[1] have opened up a wealth of new experimental possibilities by enabling much stronger lightmatter coupling than in analog experiments with natural atoms[2] and by taking advantage of the versatility of engineered circuits
where the coupling is larger than the dissipation rates
beautifully displayed the quantum physics of a single-atom coupled to the electromagnetic field of a single mode
Summary
Superconducting circuits such as microwave cavities and Josephson junction based artificial atoms[1] have opened up a wealth of new experimental possibilities by enabling much stronger lightmatter coupling than in analog experiments with natural atoms[2] and by taking advantage of the versatility of engineered circuits. With strong coupling,[3] where the coupling is larger than the dissipation rates γ and κ of the atom and mode respectively, experiments such as photon-number resolution[4] or Schrödingercat revivals[5] have beautifully displayed the quantum physics of a single-atom coupled to the electromagnetic field of a single mode. One example is the interaction between an (artificial) atom and an electromagnetic mode where the coupling rate becomes a considerable fraction of the atomic or mode eigen-frequency. This ultra-strong coupling (USC) regime, described by the quantum. Shows the breakdown of excitation number as conserved quantity, resulting in a significant theoretical challenge.[9,10] In the regime of g=ωa;m ’ 1, known as deep-strong coupling (DSC), a symmetry breaking of the vacuum is predicted[11]
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