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arXiv:2507.12131

Interaction-Induced Topological Transport of Extended Doublons

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

Published Jul 16, 2025• 0 citations
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Abstract

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.

DOI
10.48550/arxiv.2507.12131

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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.
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 potentialUUU[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 couplingsccc[6]. The driving scheme preserves the mapping and adds a long-range interaction between the two particles in the 1D lattice.
Higher-order topological insulators have attracted significant interest in both static single-particle and many-body lattice systems. While periodically driven (Floquet) higher-order topological phases have been explored at the single-particle level, the role of interactions in such systems remains less understood. In this paper, we extend previous studies by investigating interaction-induced higher-order topological phases through Floquet engineering. To achieve this, we construct an extended Bose-Hubbard model on a square lattice subjected to periodic driving. We demonstrate the emergence of interaction-induced normal Floquet second-order topological corner states for doublons (i.e., bound boson pairs) from a trivial phase, which exhibit robustness against disorder. Notably, beyond the normal phase, we reveal an interaction-induced anomalous Floquet second-order topological phase, where in-gap corner states of doublons emerge within the $\ensuremath{\pi}/T$ gap (TTT being the driving period). Our model, accessible with state-of-the-art ultracold atom techniques, provides a platform for realizing interaction-driven higher-order topological phases uniquely enabled by periodic driving, with no direct counterparts in static or single-particle systems.
Topological pumping is a paradigmatic realization of quantized transport in band systems, yet its fate in strongly correlated regimes, especially with long-range interactions, remains largely unexplored. Here we report the experimental observation of interaction-enabled topological pumping of correlated Rydberg electrons in a synthetic lattice. We show that dipolar exchange interactions induce a controllable shift of the underlying topological singularity in parameter space, such that a fixed pumping trajectory can be driven through successive topological transitions by tuning the interaction strength alone. This leads to the emergence and breakdown of quantized transport. The observations are consistent with an effective Rice-Mele description with interaction-renormalized onsite potentials and are supported by characterizing the adiabaticity and robustness to control trajectory imperfections. Our results establish a platform for exploring interaction-controlled topological transport beyond perturbative regimes and open a route toward engineering correlated topological matter in synthetic quantum systems.
As the basis for generating multi-particle quantum correlations, inter-particle interaction plays a crucial role in collective quantum phenomena, quantum phase transitions, and quantum information processing. It can profoundly alter the band structure of quantum many-body systems and give rise to exotic topological phenomena. Conventional topological pumping, which has been well demonstrated in driven linear or noninteracting systems, may break down in the presence of strong interaction. However, the interplay between band topology and interaction could also induce emergent topological pumping of interacting particles, but its experimental realization has proven challenging. Here we demonstrate interaction-induced topological pumping in a solid-state quantum system comprising an array of 36 superconducting qubits. With strong interaction inherent in the qubits and site-resolved controllability of the lattice potential and hopping strength, we realize the topological Thouless pumping of single and two bounded particles. Beyond these topological phenomena with linear or noninteracting counterparts, we also observe topologically resonant tunneling and asymmetric edge-state transport of interacting particles. Our work creates a paradigm for multi-particle topological effects, and provides a new pathway to the study of exotic topological phenomena, many-body quantum transport, and quantum information transfer.
Topological states of matter are peculiar quantum phases showing different edge and bulk transport properties connected by the bulk-boundary correspondence. While noninteracting fermionic topological insulators are well established by now and have been classified according to a tenfold scheme, the possible realization of topological states for bosons has not been explored much yet. Furthermore, the role of interactions is far from being understood. Here, we show that a topological state of matter exclusively driven by interactions may occur in the p band of a Lieb optical lattice filled with ultracold bosons. The single-particle spectrum of the system displays a remarkable parabolic band-touching point, with both bands exhibiting non-negative curvature. Although the system is neither topological at the single-particle level nor for the interacting ground state, on-site interactions induce an anomalous Hall effect for the excitations, carrying a nonzero Chern number. Our work introduces an experimentally realistic strategy for the formation of interaction-driven topological states of bosons.