Abstract for Talk 1:

The excitation of quantized lattice vibrations (phonons) with ultrashort optical pulses has enabled the study of novel non-equilibrium states of matter, such as metastable polar order and light-enhanced superconductivity [1,2,3]. Despite the enormous degree of experimental control, a comprehensive understanding of these phenomena is complicated by the fact that often it is hard to link experimental data to theoretical predictions. We introduce a snapshots-based theoretical tool dubbed phonon state tomography (PST) that allows to establish a connection between the dynamics of phonon excitations in momentum space, which is experimentally accessible via thermal diffuse x-ray and electron scattering [4,5,6], and the theoretical predictions of the dynamics of electronic correlations. PST decomposes the electron-phonon wave function into pure electronic states that belong to typical phonon configurations, enabling a tomographic reconstruction of the relevant electronic dynamics generated from exciting the phonon system. We apply this tool to study the post-quench dynamics of a minimal model of an optically pumped metal. We show how a spontaneous breaking of the lattice translational symmetry, which manifests itself experimentally in a signal of phonon excita- tions at a certain momentum, can be related to the formation of electronic correlations, and study possible pulse modifications to enhance these correlations.

[1] M. Mitrano et al., Nature 530, 461 (2016).
[2] A. Cantaluppi et al., Nature Physics 14, 837 (2018).
[3] M. Buddenel al., Nature Physics 17, 611 (2021).
[4] M. Trigo et al., Physical Review B 82, 514 (2010).
[5] M. Trigo et al., Nature Physics 9, 790 (2013).
[6] T. Konstantinova et al., Science Advances 4, 7427 (2018).

Abstract for Talk 2:

Increasing the fault tolerance of qubits is a major challenge towards the practical application of quantum algorithms. With the possibility to cheaply fabricate and couple superconducting (SC) qubits on NISQ devices, new possibilities emerge to compose logical qubits out of several SC elements. We propose a setup for a logical qubit built from SC qubits flux-coupled to a microwave cavity mode. Our design is based on a recently discovered stabilizing mechanism in the Bose-Hubbard wheel in the strongly interacting regime, and exploits the extreme robustness of a Z2 symmetry-protected Bose-Einstein condensate phase. We investigate the impact of practically unavoidable perturbations in form of disorder potentially perturbing the flux-coupling between SC qubits and the cavity. We show that even in the presence of typical fabrication uncertainties, the gap of the logical qubits is extreme robust. Upon a further introduction of an additional SC qubit serving as a probe site, we demonstrate that the proposed logical qubit can be read out reliably.