Experimental protocol for observing single quantum many-body scars with transmon qubits

  1. Peter Græns Larsen,
  2. Anne E. B. Nielsen,
  3. André Eckardt,
  4. and Francesco Petiziol
Quantum many-body scars are energy eigenstates which fail to reproduce thermal expectation values of local observables in systems, where the rest of the many-body spectrum fulfils eigenstate
thermalization. Experimental observation of quantum many-body scars has so far been limited to models with multiple scar states. Here we propose protocols to observe single scars in architectures of fixed-frequency, fixed-coupling superconducting qubits. We first adapt known models possessing the desired features into a form particularly suited for the experimental platform. We develop protocols for the implementation of these models, through trotterized sequences of two-qubit cross-resonance interactions, and verify the existence of the approximate scar state in the stroboscopic effective Hamiltonian. Since a single scar cannot be detected from coherent revivals in the dynamics, differently from towers of scar states, we propose and numerically investigate alternative and experimentally-accessible signatures. These include the dynamical response of the scar to local state deformations, to controlled noise, and to the resolution of the Lie-Suzuki-Trotter digitization.

Cavity-based reservoir engineering for Floquet engineered superconducting circuits

  1. Francesco Petiziol,
  2. and André Eckardt
By periodically driving a quantum system at a high frequency, it can acquire novel properties that are captured by an effective time-independent Hamiltonian. An important application
of such Floquet engineering is, e.g., the realization of effective gauge fields for charge-neutral particles. Here we consider driven Bose-Hubbard systems, as they can be realized as arrays of artificial atoms in superconducting circuits, and show that the ground state of the effective Hamiltonian can be prepared with high fidelity using reservoir engineering. For this purpose, some artificial atoms are coupled to driven leaky cavities. We derive an effective description of the open system by employing degenerate perturbation theory in the extended Floquet space with respect to both the periodic drive and the system-cavity coupling. Applying this theory to different Floquet-engineered flux ladders, we find both that it allows to cool the systems and that it shows excellent agreement with the full driven-dissipative evolution of system and cavities.