Phase-dependent exciton transport and energy harvesting from thermal environments

Non-Markovian effects in the evolution of open quantum systems have recently attracted widespread interest, particularly in the context of assessing the efficiency of energy and charge transfer in nanoscale biomolecular networks and quantum technologies. With the aid of many-body simulation methods, we uncover and analyze an ultrafast environmental process that causes energy relaxation in the reduced system to depend explicitly on the phase relation of the initial-state preparation. Remarkably, for particular phases and system parameters, the net energy flow is uphill, transiently violating the principle of detailed balance, and implying that energy is spontaneously taken up from the environment. A theoretical analysis reveals that nonsecular contributions, significant only within the environmental correlation time, underlie this effect. This suggests that environmental energy harvesting will be observable across a wide range of coupled quantum systems.

Figure 1

Schematic representation of a dimeric model system subject to local dephasing ${\gamma }_d$. Coherent intersite electronic coupling $J$ yields the formation of excitonic eigenstates (states $|1\rangle$ and $|2\rangle$). For $J=52{\text{cm}}^{-1}$ and optical frequencies ${\omega }_a$ and ${\omega }_b$, the excitonic splitting ${\omega }_0$ falls in the far infrared. Exciton dynamics is dictated by transverse (dephasing) and longitudinal (relaxation, marked with a red double arrow) environmental processes, whose relative strength is governed by the exciton delocalization over sites. In the ultrafast regime, these two types of fluctuation may interact, which leads to long-lasting nonsecular effects.

Figure 2

(Exact) time evolution of excitonic populations ($|1\rangle$ in black, $|2\rangle$ in red) as calculated by TEDOPA for a dimer system with ${\omega }_a-{\omega }_b=135{\text{cm}}^{-1}$ and $J=52{\text{cm}}^{-1}$ for representative $\xi =0,\pi /2,\pi ,3\pi /2$ phases at $T=277$ K in (a) and for temperatures 0–300 K following the initial-state preparation with $\xi =\pi$ in (b). In both cases the system is subject to the super-Ohmic dephasing background characterized by the AR spectral density [41].

Figure 3

(Exact) time evolution of excitonic populations ($|1\rangle$ in black, $|2\rangle$ in red) as calculated by TEDOPA for a dimer system with ${\omega }_a-{\omega }_b=135{\text{cm}}^{-1}$ and $J=52{\text{cm}}^{-1}$ in the super-Ohmic background characterized by the AR spectral density with two discrete vibrational modes of frequencies 180 and 37 ${\text{cm}}^{-1}$ at $T=277$ K for representative $\xi =0,\pi /2,\pi ,3\pi /2$ phases.