Noise Unexpectedly Reverses Material Localisation, Reviving Wave Packet Drift

Scientists Wuping Yang and H. Huang at Peking University have uncovered a new mechanism for controlling transport in open quantum systems. The addition of noise restores the dynamical skin effect in quantum systems, even when suppressed by strong quasiperiodic potentials. Introducing Ornstein-Uhlenbeck noise revives this effect, according to their perturbative analysis, which reveals noise alters the system’s dynamics to enable delocalization and directional transport. The findings demonstrate a nuanced relationship between noise strength, delocalization, and decoherence, offering a key set of tools for manipulating quantum transport.

Restoration of dynamical skin effect via amplified Ornstein-Uhlenbeck noise

A noise strength of 10 reversed the typical localization induced by quasiperiodic potentials, restoring the dynamical skin effect (DSE) in regimes where it was previously suppressed, representing a greater than five-fold improvement over prior methods unable to revive the DSE beyond a quasiperiodic strength of W = 2. The non-Hermitian skin effect (NHSE) describes the accumulation of a substantial number of eigenstates at the boundaries of a system under open boundary conditions (OBCs). This accumulation leads to the dynamical skin effect (DSE), where wave packets within the system exhibit directional motion and ultimately accumulate at a boundary. While strong quasiperiodic potentials are generally known to suppress the DSE through induced localization of the wave function, the Peking University scientists demonstrated that Ornstein-Uhlenbeck noise effectively counteracts this suppression. Their research reveals that this noise maps the non-Hermitian Schrödinger dynamics onto a non-reciprocal master equation, unveiling a noise-induced point gap in the system’s energy spectrum. This point gap is crucial, as it enables delocalization and directional transport even when the static non-Hermitian skin effect is absent, offering a novel route for controlling transport in open quantum systems. The random fluctuation fundamentally alters system behaviour by creating this ‘point gap’ within the energy spectrum, allowing wave packet delocalization and directional movement even without the usual non-Hermitian skin effect, potentially controlling particle transport in complex quantum systems. The significance of this lies in the potential to overcome limitations imposed by disorder and boundary effects in quantum materials and devices.

Restoring directed quantum transport via correlated Ornstein-Uhlenbeck noise

Ornstein-Uhlenbeck (OU) noise proved key to restoring directional transport in these systems. The technique involves adding a fluctuating, yet statistically predictable, perturbation to the quantum system, as the noise isn’t random but follows specific correlations over time. This correlated nature distinguishes it from white noise, which would likely exacerbate localization. With parameters set at J = 3/2, ∆= 1/2, and a system size of L = 100, investigation focused on a one-dimensional non-reciprocal Aubry-Andr e-Harper model, a system exhibiting the non-Hermitian skin effect where states accumulate at boundaries. The Aubry-Andr e-Harper model is a paradigmatic example of a quasiperiodic system, known for its sensitivity to perturbations and its ability to exhibit both localization and extended states depending on the strength of the quasiperiodic potential. This approach differed from simply increasing the quasiperiodic strength, which induces localization rather than reviving the dynamic skin effect. Perturbative analysis, a mathematical technique used to approximate solutions to complex problems, mapped the complex, non-Hermitian dynamics onto a simplified non-reciprocal master equation. This mapping allowed the researchers to clearly demonstrate how the noise alters the system’s behaviour and restores directional transport by counteracting the quasiperiodic potential. The non-reciprocal master equation describes the evolution of the system’s density matrix, accounting for the non-Hermitian nature of the Hamiltonian and the influence of the environment. This provides a more tractable framework for understanding the underlying physics.

Calibrated noise restores energy transport despite material disorder and edge effects

Controlling energy movement through materials is fundamental to advances in photonics and quantum computing, but strong disorder often traps energy, hindering its flow. This energy trapping arises from the localization of energy eigenstates, preventing efficient energy transfer. A new demonstration shows that precisely calibrated noise can surprisingly reverse this effect, restoring directional transport even when conventional barriers would normally prevent it. The ability to overcome disorder is particularly important for realising robust quantum devices, as real materials inevitably contain imperfections and impurities. However, the benefit isn’t limitless, as the demonstration reveals a delicate balance between noise assisting delocalization and, counterintuitively, noise increasing decoherence, the loss of quantum information. It is vital to acknowledge that increasing noise ultimately degrades quantum information; this isn’t a free energy boost. Decoherence arises from interactions between the quantum system and its environment, leading to the loss of quantum coherence and the destruction of quantum effects. Specifically, controlling the ‘dynamical skin effect’, where energy concentrates at the edges of materials, offers potential for designing more efficient photonic devices and quantum circuits. Reversing the effects of strong quasiperiodic potentials which usually induce localisation and suppress the dynamical skin effect, causing wave packets to drift to the edge of a material, opens up possibilities for novel device architectures. The implications extend to areas such as energy harvesting, where efficient transport of energy is crucial, and quantum sensing, where maintaining coherence is paramount. Further research will focus on optimising the noise parameters and exploring the applicability of this technique to higher-dimensional systems and different types of quantum materials.

The research demonstrated that the addition of Ornstein-Uhlenbeck noise can restore directional energy transport in quasiperiodic systems where it is normally suppressed. This is significant because disorder and edge effects typically hinder energy movement within materials, impeding advances in areas like photonics and quantum computing. The study revealed that noise effectively counteracts localisation, reviving the ‘dynamical skin effect’ and enabling energy to flow despite imperfections. Researchers found this benefit is limited by a competition between noise-assisted delocalization and noise-induced decoherence, highlighting a complex relationship between noise strength and system performance.