Sal J. Bosman

Multi-mode ultra-strong coupling in circuit quantum electrodynamics

  1. Sal J. Bosman,
  2. Mario F. Gely,
  3. Vibhor Singh,
  4. Alessandro Bruno,
  5. Daniel Bothner,
  6. and Gary A. Steele
With the introduction of superconducting circuits into the field of quantum optics, many novel 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 the first 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 and localization of electric fields in vacuum, this new architecture offers the potential to further explore the novel regime of multi-mode ultra-strong coupling.
arxiv pdf

Approaching ultra-strong coupling in Transmon circuit-QED using a high-impedance resonator

  1. Sal J. Bosman,
  2. Mario F. Gely,
  3. Vibhor Singh,
  4. Daniel Bothner,
  5. Andres Castellanos-Gomez,
  6. and Gary A. Steele
In this experiment, we couple a superconducting Transmon qubit to a high-impedance 645 Ω microwave resonator. Doing so leads to a large qubit-resonator coupling rate g, measured through
a large vacuum Rabi splitting of 2g≃910 MHz. The coupling is a significant fraction of the qubit and resonator oscillation frequencies ω, placing our system close to the ultra-strong coupling regime (g¯=g/ω=0.071 on resonance). Combining this setup with a vacuum-gap Transmon architecture shows the potential of reaching deep into the ultra-strong coupling g¯∼0.45 with Transmon qubits.
arxiv pdf

Divergence-free multi-mode circuit quantum electrodynamics

  1. Mario F. Gely,
  2. Adrian Parra-Rodriguez,
  3. Daniel Bothner,
  4. Ya. M. Blanter,
  5. Sal J. Bosman,
  6. Enrique Solano,
  7. and Gary A. Steele
Circuit quantum electrodynamics studies the interaction of artificial atoms and electromagnetic modes constructed from superconducting circuitry. While the theory of an atom coupled
to one mode of a resonator is well studied, considering multiple modes leads to divergences which are not well understood. Here, we introduce a full quantum model of a multi-mode resonator coupled to a Josephson junction atom. Using circuit quantization, we find a Hamiltonian in which parameters of the atom are naturally renormalized as additional modes are considered. In our model, we circumvent the divergence problem, and its formulation reveals a physical understanding of the mechanisms of convergence in ubiquitous models in circuit quantum electrodynamics.
arxiv pdf