Simonet, Juliette
Date: Friday, December 9, 2016
Time: 11:00
Place: ETH Zurich, Hönggerberg, HPF G 6
Host: Tilman Esslinger / Rémi Desbuquois
Floquet engineering in periodically driven optical lattices and beyond
Juliette Simonet
Institut für Laserphysik, Universität Hamburg, Germany
The realization of artificial gauge potentials for neutral atoms constitutes a major achievement for the emulation of condensed matter models with quantum gases. Diverse static gauge potentials that effectively emerge in the atomic dynamics can now be experimentally engineered: Abelian gauge potentials, giving rise to synthetic electric and magnetic fields as well as non-Abelian gauge potentials. A well-known example of the later is the spin–orbit coupling, which links a particle’s velocity to its quantum-mechanical spin.
Periodic driving of quantum systems allows for the targeted engineering of such gauge potentials. In a 1D optical lattice, time-periodic forcing can lead to complex valued tunnel matrix elements if the driving force breaks specific symmetries, resulting in a gauge-dependent shift of the dispersion relation [1].
A 1D spin-orbit coupling can be realized by forcing two magnetic spin states trapped in a spin-dependent lattice in a spin dependent manner with a time-varying magnetic field gradient [2]. The effective dispersion relations of the two spin states are thus shifted in opposite direction due to the inverted drive for both states. An additional radio-frequency coupling between the spin states leads to a mixing of the spin dispersion relations and a spin-orbit gap in the band structure. In contrast to previous experimental realizations in ultracold gases, this scheme does not involve near-resonant laser fields, avoiding the heating processes induced by the spontaneous emission of photons.
The underlying principle of all driving schemes implemented so far is that the properties of the periodically driven system are determined by a time-independent effective Hamiltonian. This so-called Floquet engineering typically assumes that excited Bloch bands can be neglected. However, periodic inertial forcing of the atomic ensemble, similar to an oscillating light field, results in interband excitations due to the absorption of low-energy driving photons. Thus, a deeper understanding of such excitations processes is essential for tailoring appropriate driving schemes. We report here on the first systematic study of multiphoton excitations of ultracold bosonic quantum gases in driven optical lattices, which reveals close analogies with laser-irradiated solid state materials where nonlinear processes play crucial role at large field strengths [3].
References:
[1] J. Struck et al., Phys. Rev. Lett. 108, 225304 (2012)
[2] J. Struck et al., Phys. Rev. A 90, 031301(R) (2014)
[3] M. Weinberg et al., Phys. Rev. A 92, 043621 (2015)