Engineering the Level Structure of a Giant Artificial Atom in Waveguide Quantum Electrodynamics

  1. A. M. Vadiraj,
  2. Andreas Ask,
  3. T.G. McConkey,
  4. I. Nsanzineza,
  5. C.W. Sandbo Chang,
  6. Anton Frisk Kockum,
  7. and C. M. Wilson
Engineering light-matter interactions at the quantum level has been central to the pursuit of quantum optics for decades. Traditionally, this has been done by coupling emitters, typically
natural atoms and ions, to quantized electromagnetic fields in optical and microwave cavities. In these systems, the emitter is approximated as an idealized dipole, as its physical size is orders of magnitude smaller than the wavelength of light. Recently, artificial atoms made from superconducting circuits have enabled new frontiers in light-matter coupling, including the study of „giant“ atoms which cannot be approximated as simple dipoles. Here, we explore a new implementation of a giant artificial atom, formed from a transmon qubit coupled to propagating microwaves at multiple points along an open transmission line. The nature of this coupling allows the qubit radiation field to interfere with itself leading to some striking giant-atom effects. For instance, we observe strong frequency-dependent couplings of the qubit energy levels to the electromagnetic modes of the transmission line. Combined with the ability to in situ tune the qubit energy levels, we show that we can modify the relative coupling rates of multiple qubit transitions by more than an order of magnitude. By doing so, we engineer a metastable excited state, allowing us to operate the giant transmon as an effective lambda system where we clearly demonstrate electromagnetically induced transparency.

Cavity-free vacuum-Rabi splitting in circuit quantum acoustodynamics

  1. Andreas Ask,
  2. Maria Ekström,
  3. Per Delsing,
  4. and Göran Johansson
Artificial atoms coupled to surface acoustic waves (SAWs) have played a crucial role in the recent development of circuit quantum acoustodynamics (cQAD). In this paper, we have investigated
the interaction of an artificial atom and SAWs beyond the weak coupling regime, focusing on the role of the interdigital transducer (IDT) that enables the coupling. We find a parameter regime in which the IDT acts as a cavity for the atom, rather than an antenna. In other words, the atom forms its own cavity. Similar to an atom coupled to an explicit cavity, this regime is characterized by vacuum-Rabi splitting, as the atom hybridizes with the phononic vacuum inside the IDT. This hybridization is possible because of the interdigitated coupling, which has a large spatial extension, and the slow propagation speed of SAWs. We work out a criterion for entering this regime from a model based on standard circuit-quantization techniques, taking only material parameters as inputs. Most notably, we find this regime hard to avoid for an atom on top of a strong piezoelectric material, such as LiNbO3. The SAW-coupled atom on top of LiNbO3 can thus be regarded as an atom-cavity-bath system. On weaker piezoelectric materials, the number of IDT electrodes need to be large in order to reach this regime.