News from the Lab 2021

  • Jan. 21 at 10:15 online (Zoom): Group seminar
    Speaker: Walter Hahn (IQOQI Innsbruck) | Title: Long-lived coherence in driven spin systems: from two to infinite spatial dimensions | Abstract: Long-lived coherences, emerging under periodic pulse driving in the disordered ensembles of strongly interacting spins, offer immense advantages for future quantum technologies, but the physical origin and the key properties of this phenomenon remain poorly understood. In this talk, I will first give a general introduction to long-lived coherences in driven spin system and then present the results of our theoretical investigation of this effect in ensembles of different dimensionality. Our results imply the existence of the long-lived coherences in two-dimensional and infinite-dimensional (where every spin is coupled to all others) systems, which are of particular importance for quantum sensing and quantum information processing. We explore the transition from two to infinite dimensions, and show that the long-time coherence dynamics in all dimensionalities is qualitatively similar, although the short-time behavior is drastically different, exhibiting dimensionality-dependent singularity. Our study establishes the common physical origin of the long-lived coherences in different dimensionalities, and suggests that this effect is a generic feature of the strongly coupled spin systems with positional disorder. Our results lay out foundation for utilizing the long-lived coherences in a range of application, from quantum sensing with two-dimensional spin ensembles, to quantum information processing with the infinitely-dimensional spin systems in the cavity-QED settings | See you online!
  • Jan. 14 at 10:15 online (Zoom): Group seminar
    Speaker: Artem Rakcheev | Title: Estimating Heating Times in Periodically Driven Quantum Many-Body Systems via Avoided Crossing Spectroscopy | Abstract: Periodically driven, also known as Floquet, quantum many-body systems have attracted a lot of attention in recent years, due to the possibility of realize Hamiltonians with otherwise inaccessible interactions and phases. For instance topological models such as the Harper-Hofstadter and Haldane models have been realized in ultracold atoms in optical lattices with periodic driving. Furthermore, genuine non-equilibrium phases (so called time crystals) can emerge in this setting. However, these systems can only be engineered approximately and persist only up to certain time scales, after which the system heats up to featureless states. Although the problem of heating is well appreciated, there are very few approaches to compute heating times for a concrete system. In this talk I will present a numerical method to estimate such heating times based on exact computations with small systems (https://arxiv.org/abs/2011.06017). The method relies on a quantitative avoided crossing spectroscopy - roughly speaking an identification of processes which can exchange energy efficiently at a given driving frequency. We relate this method to the Fermi Golden Rule approach introduced recently and study a driven ergodic spin chain, which was shown to feature a heating "threshold" behavior. Furthermore, we discuss how our methods is applicable in important regimes beyond the Fermi Golden Rule, for instance in settings with frequency depending couplings and close to a discrete time crystal. | See you online!

News from the Department

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