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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Topological Extended Doublons Formed by Drive-Induced Non-Local Interactions

J. Beck, Helena Drüeke, Marcus J. Meschede, Matthias Heinrich, Francesco S. Piccioli, Sebastian Weidemann, Joshua Feis, D. Bauer, Alexander Szameit

Published Jun 23, 2025• 0 citations

Abstract

When studying systems of interacting particles, few-particle systems are of particular interest as the mean-field approximation breaks down for a small number of particles. Two particles with attractive or repulsive long-range interactions may form a bound state [1], the so-called doublon, showing a number of peculiar features: topologically protected doublon states can not only persist outside the parameter regime for topological single particles [2], but also break the bulk boundary correspondence for large interactions [3]. So far doublons were only considered as doubly-occupied lattice sites [1], [4], whereas this work focuses on the general form of quasi-particles extending over multiple lattice sites.

DOI: 10.1109/cleo/europe-eqec65582.2025.11110479

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When studying systems of interacting particles, few-particle systems are of particular interest as the mean-field approximation breaks down for a small number of particles. Two particles with attractive or repulsive long-range interactions may form a bound state [1], the so-called doublon, showing a number of peculiar features: topologically protected doublon states can not only persist outside the parameter regime for topological single particles [2], but also break the bulk boundary correspondence for large interactions [3]. So far doublons were only considered as doubly-occupied lattice sites [1], [4], whereas this work focuses on the general form of quasi-particles extending over multiple lattice sites.

While topological single-particle systems have been extensively studied [1], topologically protected entangled two-boson states [2] and many-boson states [3] have recently come into the focus of research. However, interacting particle pairs in these systems have yet to be studied experimentally. This work focuses on the topological center-of-mass motion of bound two-particle states - so-called doublons [4], arising in a 1D lattice for discrete potentials with repulsive interaction. The tight-binding dynamics of a repulsive interacting two-particle state on such a 1D chain can be mapped onto a single particle evolving in a 2D lattice (Fig. 1a), where the main diagonal sites experience an on-site potential

U

[5]. For the 2D case, we choose a square lattice driven by a conventional anomalous Floquet protocol with a full wave packet transfer in each hopping and uniformly distributed couplings

c

[6]. The driving scheme preserves the mapping and adds a long-range interaction between the two particles in the 1D lattice.

Topological boundary states are a defining feature of topological systems, providing robust transport protected against perturbations. While such states are commonly engineered in non-interacting systems, how to realize interaction-induced topological states composed of only few particles has remained experimentally unexplored thus far. In this work, we realize such states using a driven photonic lattice as a linear analogue platform. We observe the formation of spatially extended bound states of two interacting particles, in which the particles remain correlated while occupying different lattice sites. At discrete values of the local interaction, these bound states support robust transport governed by a topological phase arising from drive-induced correlated hopping. Our results establish an experimentally accessible route to interaction-driven topology beyond single-particle physics and provide a versatile framework for exploring correlated few-particle phenomena across different physical platforms.

The controlled transfer of particles from one site of a spatial lattice to another is essential for many tasks in quantum information processing and quantum communication. In this work we study how to induce long-range transfer between the two ends of a dimer chain, by coupling states that are localized just on the chain's end-points. This has the appealing feature that the transfer occurs only between the end-points - the particle does not pass through the intermediate sites-making the transfer less susceptible to decoherence. We first show how a repulsively bound-pair of fermions, known as a doublon, can be transferred from one end of the chain to the other via topological edge states. We then show how non-topological surface states of the familiar Shockley or Tamm type can be used to produce a similar form of transfer under the action of a periodic driving potential. Finally we show that combining these effects can produce transfer by means of more exotic topological effects, in which the driving field can be used to switch the topological character of the edge states, as measured by the Zak phase. Our results demonstrate how to induce long range transfer of strongly correlated particles by tuning both topology and driving.