Quantum systems engineering

Abstract:

We show that strong electron-electron interactions in quantum materials can give rise to electronic transitions that couple strongly to cavity fields, and collective enhancement of these interactions can result in ultrastrong effective coupling strengths. As a paradigmatic example we consider a Fermi-Hubbard model coupled to a single-mode cavity and find that resonant electron-cavity interactions result in the formation of a quasi-continuum of polariton branches. The vacuum Rabi splitting of the two outermost branches is collectively enhanced and scales with USD g_{\text{eff}}\propto\sqrt{2L} USD, where USD L USD is the number of electronic sites, and the maximal achievable value for USD g_{\text{eff}} USD is determined by the volume of the unit cell of the crystal. We find that USD g_{\text{eff}} USD for existing quantum materials can by far exceed the width of the first excited Hubbard band. This effect can be experimentally observed via measurements of the optical conductivity and does not require ultrastrong coupling on the single-electron level. Quantum correlations in the electronic ground state as well as the microscopic nature of the light-matter interaction enhance the collective light-matter interaction compared to an ensemble of independent two-level atoms interacting with a cavity mode.

Abstract:

The assumption that physical systems relax to a stationary state in the long-time limit underpins statistical physics and much of our intuitive understanding of scientific phenomena. For isolated systems this follows from the eigenstate thermalization hypothesis. When an environment is present the expectation is that all of phase space is explored, eventually leading to stationarity. Notable exceptions are decoherence-free subspaces that have important implications for quantum technologies. These have been studied for systems with a few degrees of freedom only. Here we identify simple and generic conditions for dissipation to prevent a quantum many-body system from ever reaching a stationary state. We go beyond dissipative quantum state engineering approaches towards controllable long-time non-stationary dynamics typically associated with macroscopic complex systems. This coherent and oscillatory evolution constitutes a dissipative version of a quantum time-crystal. We discuss the possibility of engineering such complex dynamics with fermionic ultracold atoms in optical lattices.

Abstract:

We investigate electron paring in a two-dimensional electron system mediated by vacuum fluctuations inside a nanoplasmonic terahertz cavity. We show that the structured cavity vacuum can induce long-range attractive interactions between current fluctuations which lead to pairing in generic materials with critical temperatures in the low-kelvin regime for realistic parameters. The induced state is a pair-density wave superconductor which can show a transition from a fully gapped to a partially gapped phase—akin to the pseudogap phase in high-Tc superconductors. Our findings provide a promising tool for engineering intrinsic electron interactions in two-dimensional materials.

Abstract:

© 2019 American Physical Society. The interaction Hamiltonian (Formula Presented) Eq. (14) describing the interaction between the cavity and the electronic system was obtained by expanding the Peierls Hamiltonian in Eq. (A4) up to first order in the small parameter (Formula Presented) All results presented in the paper are consistent with this appro imate interaction Hamiltonian, leading to an effective Hamiltonian that depends quadratically on. However, it turns out that a straightforward improvement of the parameters entering the effective Hamiltonian in Eq. (26) can be obtained by including the second-order term in the Peierls Hamiltonian in Eq. (A4). This term gives rise to modifications of our results that are also of order through a renormalization of the nearest-neighbor hopping amplitude (Formula Presented) The authors would like to thank M. A. Sentef for bringing the importance of the second-order term in Eq. (A4) to our attention.