O'Dea Nicholas, Burnell Fiona, Chandran Anushya, Khemani Vedika
Department of Physics, Stanford University, Stanford, California 94305, USA.
Department of Physics, University of Minnesota Twin Cities, Minnesota 55455, USA.
Phys Rev Lett. 2024 Mar 8;132(10):100401. doi: 10.1103/PhysRevLett.132.100401.
Systems subject to high-frequency driving exhibit Floquet prethermalization, that is, they heat exponentially slowly on a timescale that is large in the drive frequency, τ_{h}∼exp(ω). Nonetheless, local observables can decay much faster via energy conserving processes, which are expected to cause a rapid decay in the fidelity of an initial state. Here we show instead that the fidelities of eigenstates of the time-averaged Hamiltonian, H_{0}, display an exponentially long lifetime over a wide range of frequencies-even as generic initial states decay rapidly. When H_{0} has quantum scars, or highly excited eigenstates of low entanglement, this leads to long-lived nonthermal behavior of local observables in certain initial states. We present a two-channel theory describing the fidelity decay time τ_{f}: the interzone channel causes fidelity decay through energy absorption, i.e., coupling across Floquet zones, and ties τ_{f} to the slow heating timescale, while the intrazone channel causes hybridization between states in the same Floquet zone. Our work informs the robustness of experimental approaches for using Floquet engineering to generate interesting many-body Hamiltonians, with and without scars.
受到高频驱动的系统会呈现弗洛凯预热现象，也就是说，它们在一个相对于驱动频率而言很大的时间尺度上呈指数缓慢升温，τₕ ∼ exp(ω)。然而，局部可观测量可以通过能量守恒过程更快地衰减，预计这会导致初始态保真度的快速下降。相反，我们在此表明，时间平均哈密顿量H₀的本征态保真度在很宽的频率范围内呈现指数长寿命，即便一般的初始态会快速衰减。当H₀存在量子疤痕或低纠缠的高激发本征态时，这会导致某些初始态下局部可观测量的长寿命非热行为。我们提出了一个双通道理论来描述保真度衰减时间τ_f：区际通道通过能量吸收导致保真度衰减，即跨越弗洛凯区的耦合，并将τ_f与缓慢升温时间尺度联系起来，而区内通道导致同一弗洛凯区内态之间的杂化。我们的工作为利用弗洛凯工程生成有趣的多体哈密顿量（有无疤痕）的实验方法的稳健性提供了参考。
