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arXiv:2212.11294 · Physical Review Research

Adiabatic preparation of fractional Chern insulators from an effective thin-torus limit

Benjamin Michen, Cécile Repellin, Jan Carl Budich

Published May 15, 2023• 3 citations
arXivHTMLPDF

Abstract

We explore the quasi-one-dimensional (thin torus, or TT) limit of fractional Chern insulators (FCIs) as a starting point for their adiabatic preparation in quantum simulators. Our approach is based on tuning the hopping amplitude in one direction as an experimentally amenable knob to dynamically change the effective aspect ratio of the system. Similar to the TT limit of fractional quantum Hall systems in the continuum, we find that the hopping-induced TT limit adiabatically connects the FCI state to a trivial charge density wave (CDW) ground state. This adiabatic path may be harnessed for state preparation schemes relying on the initialization of a CDW state followed by the adiabatic decrease of a hopping anisotropy. Our findings are based on the calculation of the excitation gap in a number of FCI models, both on a lattice and consisting of coupled wires. By analytical calculation of the gap in the limit of strongly anisotropic hopping, we show that its scaling is compatible with the preparation of large-size FCIs for sufficiently large hopping anisotropy, where the amenable system sizes are only limited by the maximal hopping amplitude. Our numerical simulations in the framework of exact diagonalization explore the full anisotropy range to corroborate these results.

DOI
10.1103/physrevresearch.5.023100

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We explore the quasi-one-dimensional (thin torus, or TT) limit of fractional Chern insulators (FCIs) as a starting point for their adiabatic preparation in quantum simulators. Our approach is based on tuning the hopping amplitude in one direction as an experimentally amenable knob to dynamically change the effective aspect ratio of the system. Similar to the TT limit of fractional quantum Hall systems in the continuum, we find that the hopping-induced TT limit adiabatically connects the FCI state to a trivial charge density wave (CDW) ground state. This adiabatic path may be harnessed for state preparation schemes relying on the initialization of a CDW state followed by the adiabatic decrease of a hopping anisotropy. Our findings are based on the calculation of the excitation gap in a number of FCI models, both on a lattice and consisting of coupled wires. By analytical calculation of the gap in the limit of strongly anisotropic hopping, we show that its scaling is compatible with the preparation of large-size FCIs for sufficiently large hopping anisotropy, where the amenable system sizes are only limited by the maximal hopping amplitude. Our numerical simulations in the framework of exact diagonalization explore the full anisotropy range to corroborate these results.
We analytically and numerically analyze the one-dimensional "thin-torus" limit of Fractional Topological Insulators in a series of simple models exhibiting exactly flat bands with local hopping. These models are the one-dimensional limit of two dimensional Chern Insulators, and the Hubbard-type interactions projected into their lowest band take particularly simple forms. By exactly solving the many-body interacting spectrum of these models, we show that, just like in the Fractional Quantum Hall effect, the zero modes of the thin-torus limit are CDW states of occupation numbers satisfying generalized Pauli principles. As opposed to the FQH where the thin-torus CDW appear in orbital space, in the thin-torus FCI states, the CDW states are in real-space. We show the counting of the quasihole excitations in the energy spectrum cannot distinguish between a CDW state and a FQH state. However, by exactly computing the entanglement spectrum for the thin-torus states, we show that it can qualitatively and quantitatively distinguish between a CDW and a fractional topological state such as the FCI. We then discover a previously unknown separation of energy scales of the full FQH energy spectrum in the thin torus limit and find that Chern insulator models exhibiting strong isotropic FCI states have a similar structure in their thin-torus limit spectrum. We close by numerically computing the evolution of energy and entanglement spectra from the thin-torus to the isotropic limit. Our results can also be interpreted as an analysis of one-body, 1-dimensional topological insulators stabilized by inversion symmetry in the presence of interactions.
Fractional Chern insulators (FCIs) are lattice analogs of fractional quantum Hall systems, where the interplay of strong interactions with a frustrated tunnelling kinetics leads to the emergence of a gapped ground state with long-range entanglement and anyonic excitations. The highly correlated nature of such systems makes their adiabatic preparation challenging already beyond the minimal system size of two particles. Considering Floquet implementations of the bosonic Harper-Hofstadter-Hubbard model of few photons in superconducting circuits, we design protocols for the driven-dissipative stabilization of its FCI ground state at half filling via quantum bath engineering. Dissipation control is achieved through the coupling to driven leaky cavity modes, which realize a tuneable artificial environment having the Floquet-FCI as its approximate fixed point. For systems of two, three and six particles, we show numerically how the flexibility of the control scheme further allows for the detection of fractional quantum Hall signatures in the stabilized steady states, including bulk incompressibility, Hall response and the trapping of fractional charges. Our results provide a concrete pathway to dissipation-assisted preparation of strongly correlated states in quantum simulators.
This thesis explores multiple ways of nudging quantum matter over the edge of topological order and is divided into two parts. The first part offers an overview of the theoretical foundations for the original results presented in the second part. These results first focus on non-standard dynamical paths between trivial and topological phases with applications to adiabatic state preparation in atomic quantum simulators. We commence with the adiabatic preparation of a fractional Chern insulator from a one-dimensional charge density wave. To achieve this, we show analytically how an anisotropy of the hopping amplitudes can effectively change the aspect ratio of the system and induce a so-called ``thin-torus'' limit. The analytical results are corroborated using numerical exact diagonalization. Subsequently, we investigate the adiabatic preparation of a number-conserving Majorana phase protected by a parity symmetry that emerges at a critical point of a two-leg ladder subject to a pair-hopping interaction. For this purpose, we design a symmetry-breaking flux term that can gap out the critical target state and devise a protocol that ramps down the flux term with a power law adapted to system size so as to enable an optimal (linear) scaling of preparation time. To design the flux term, we treat the system in the framework of constructive bosonization. The results are supported numerically using matrix product state techniques to calculate excitation gaps, correlation functions, and time evolutions. Then, we shift the focus to the identification of minimal requirements for topological effects and demonstrate how an integer quantum Hall phase can form in a topologically completely trivial band structure at the onset of disorder. These results are confirmed through extensive numerical simulations with the software package Kwant. Finally, we investigate a Floquet system in an anomalous topological phase that is subjected to nonlinear effects. Specifically, we demonstrate a nonlinearity induced scattering of an edge state off a circulating bulk mode by calculating the exact dynamics of the system through a numerical solution of the nonlinear Schrödinger equation. Notably, we find that the scattering amplitude is determined by interference effects between the two pulses and therefore highly sensitive to their initial relative phase.
Fractional Chern insulators (FCIs) are lattice analogues of fractional quantum Hall states, hosting topological order and fractionalized anyonic excitations. Their realization poses a demanding target for quantum simulation. Here, we realize a bosonic FCI on an IBM Nighthawk superconducting quantum processor using classically optimized variational circuits. We target a Laughlin-like state that is an exact zero-energy ground state of a long-range parent Hamiltonian on a torus, eliminating physical edges and providing a favorable geometry for accessing fractional quantum Hall physics in a small system. On a 4×44\times44×4 qubit lattice, we prepare both the four-boson FCI ground state and a three-boson state hosting two quasiholes. Measurements of particle densities and correlations reproduce the characteristic structure of the target states, while reconstructed chiral magnetization currents resolve the circulating current pattern associated with the quasiholes. The preparation of these strongly correlated topological states with shallow circuits establishes a route toward the digital simulation and manipulation of fractionalized excitations, including measurements of topological invariants and anyonic braiding.
Topological order, the hallmark of fractional quantum Hall states, is primarily defined in terms of ground-state degeneracy on higher-genus manifolds, e.g., the torus. We investigate analytically and numerically the smooth crossover between this topological regime and the Tao-Thouless thin torus quasi-one-dimensional (1D) limit. Using the wire-construction approach, we analyze an emergent charge density wave (CDW) signifying the breakdown of topological order, and relate its phase shifts to Wilson loop operators. The CDW amplitude decreases exponentially with the torus circumference once it exceeds the transverse correlation length controllable by the interwire coupling. By means of numerical simulations based on the matrix product states (MPS) formalism, we explore the extreme quasi-1D limit in a two-leg flux ladder and present a simple recipe for probing fractional charge excitations in the $\ensuremath{\nu}=1/2$ Laughlin-like state of hard-core bosons. We discuss the possibility of realizing this construction in cold-atom experiments. We also address the implications of our findings to the possibility of producing non-Abelian zero modes. As known from rigorous no-go theorems, topological protection for exotic zero modes such as parafermions cannot exist in 1D fermionic systems and the associated degeneracy cannot be robust. Our theory of the 1D--2D crossover allows us to calculate the splitting of the degeneracy, which vanishes exponentially with the number of wires, similarly to the CDW amplitude.
Topological states of interacting many-body systems are at the focus of current research due to the exotic properties of their elementary excitations. In this paper we suggest a realistic experimental setup for the realization of a simple version of such a phase. We show how $\ensuremath{\delta}$-interacting bosons hopping on the links of a one-dimensional ladder can be used to simulate the thin-torus limit of the two-dimensional (2D) Hofstadter-Hubbard model at one-quarter magnetic flux per plaquette. Bosons can be confined to ladders by optical superlattices, and synthetic magnetic fields can be realized by laser-assisted tunneling. We show that twisted boundary conditions can be implemented, enabling the realization of a fractionally quantized Thouless pump. Using numerical density-matrix-renormalization-group calculations, we show that the ground state of our model is an incompressible symmetry-protected topological charge density wave phase at average filling $\ensuremath{\rho}=1/8$ per lattice site, related to the 1/21/21/2 Laughlin-type state of the corresponding 2D model.