Konrad Viebahn

Last Name

Viebahn

First Name

Konrad

ORCID

0000-0002-1311-722X

Organisational unit

03599 - Esslinger, Tilman / Esslinger, Tilman

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Publications1 - 10 of 14

Floquet engineering of individual band gaps in an optical lattice using a two-tone drive
Sandholzer, Kilian; Walter, Anne-Sophie; Minguzzi, Joaquín; et al. (2022)
Physical Review Research
The dynamic engineering of band structures for ultracold atoms in optical lattices represents an innovative approach to understanding and exploring the fundamental principles of topological matter. In particular, the folded Floquet spectrum determines the associated band topology via band inversion. We experimentally and theoretically study two-frequency phase modulation to asymmetrically hybridize the lowest two bands of a one-dimensional lattice. Using quasidegenerate perturbation theory in the extended Floquet space we derive an effective two-band model that quantitatively describes our setting. The energy gaps are experimentally probed via Landau-Zener transitions between Floquet-Bloch bands using an accelerated Bose-Einstein condensate. Separate and simultaneous control over the closing and reopening of these band gaps is demonstrated. We find good agreement between experiment and theory, establishing an analytic description for resonant Floquet-Bloch engineering that includes single- and multiphoton couplings, as well as interference effects between several commensurate drives.
Observing Localization in a 2D Quasicrystalline Optical Lattice
Sbroscia, Matteo; Viebahn, Konrad; Carter, Edward; et al. (2020)
Physical Review Letters
Quasicrystals are long-range ordered but not periodic, representing an interesting middle ground between order and disorder. We experimentally and numerically study the localization transition in the ground state of noninteracting and weakly interacting bosons in an eightfold symmetric quasicrystalline optical lattice. In contrast to typically used real space in situ techniques, we probe the system in momentum space by recording matter wave diffraction patterns. Shallow lattices lead to extended states whereas we observe a localization transition at a critical lattice depth of V0≈1.78(2)Erec for the noninteracting system. Our measurements and Gross-Pitaevskii simulations demonstrate that in interacting systems the transition is shifted to deeper lattices, as expected from superfluid order counteracting localization. Quasiperiodic potentials, lacking conventional rare regions, provide the ideal testing ground to realize many-body localization in 2D.
Mitigating higher-band heating in Floquet-Hubbard lattices via two-tone driving
Chen, Yuanning; Zhu, Zijie; Viebahn, Konrad (2025)
Physical Review A
Multiphoton resonances to high-lying energy levels represent an unavoidable source of Floquet heating in strongly driven quantum systems. In this work, we extend the recently developed two-tone approach of “canceling” multiphoton resonances to shaken lattices in the Hubbard regime. Our experiments show that even for strong lattice shaking, the inclusion of a weak second drive leads to cancellation of multiphoton heating resonances. Surprisingly, the optimal canceling amplitude depends on the Hubbard interaction strength 𝑈, in qualitative agreement with exact diagonalization calculations. Our results call for novel analytical approaches to capture the physics of strongly driven, strongly interacting many-body systems.
Protected quantum gates using qubit doublons in dynamical optical lattices
Kiefer, Yann; Zhu, Zijie; Fischer, Lars; et al. (2026)
Nature
Quantum computing represents a central challenge in modern science. Neutral atoms in optical lattices have emerged as a leading computing platform, with collisional gates offering a stable mechanism for quantum logic¹⁻¹⁰. However, previous experiments have treated ultracold collisions as a dynamically fine-tuned process¹¹⁻²², which obscures the underlying quantum geometry and quantum statistics crucial for realizing intrinsically robust operations. Here we propose and experimentally demonstrate a purely geometric two-qubit SWAP gate by transiently populating qubit doublon states of fermionic atoms in a dynamical optical lattice. The presence of these doublon states, together with fermionic exchange anti-symmetry, enables a two-particle quantum holonomy-a geometric evolution in which dynamical phases are absent²³. This yields a gate mechanism that is intrinsically protected against fluctuations and inhomogeneities of the confining potentials. The resilience of the gate is further reinforced by time-reversal and chiral symmetries of the Hamiltonian. We experimentally validate this exceptional protection, achieving a loss-corrected amplitude fidelity of 99.91(7)% measured across the entire system consisting of more than 17,000 atom pairs. When combined with recently developed topological pumping methods for atom transport¹⁶, our results pave the way for large-scale, highly connected quantum processors. This work introduces a new model for quantum logic that transforms fundamental symmetries, including quantum statistics, into a powerful resource for fault-tolerant computation.
Topological Pumping in a Floquet-Bloch Band
Minguzzi, Joaquín; Zhu, Zijie; Sandholzer, Kilian; et al. (2022)
Physical Review Letters
Constructing new topological materials is of vital interest for the development of robust quantum applications. However, engineering such materials often causes technological overhead, such as large magnetic fields, spin-orbit coupling, or dynamical superlattice potentials. Simplifying the experimental requirements has been addressed on a conceptual level – by proposing to combine simple lattice structures with Floquet engineering – but there has been no experimental implementation. Here, we demonstrate topological pumping in a Floquet-Bloch band using a plain sinusoidal lattice potential and two-tone driving with frequencies ω and 2ω. We adiabatically prepare a near-insulating Floquet band of ultracold fermions via a frequency chirp, which avoids gap closings en route from trivial to topological bands. Subsequently, we induce topological pumping by slowly cycling the amplitude and the phase of the 2ω drive. Our system is well described by an effective Shockley model, establishing a novel paradigm to engineer topological matter from simple underlying lattice geometries. This approach could enable the application of quantized pumping in metrology, following recent experimental advances on two-frequency driving in real materials.
An accordion superlattice for controlling atom separation in optical potentials
Wili, Simon; Esslinger, Tilman; Viebahn, Konrad (2023)
New Journal of Physics
We propose a method for separating trapped atoms in optical lattices by large distances. The key idea is the cyclic transfer of atoms between two lattices of variable spacing, known as accordion lattices, each covering at least a factor of two in lattice spacing. By coherently loading atoms between the two superimposed potentials, we can reach, in principle, arbitrarily large atom separations, while requiring only a relatively small numerical aperture. Numerical simulations of our ‘accordion superlattice’ show that the atoms remain localized to one lattice site throughout the separation process, even for moderate lattice depths. In a proof-of-principle experiment, we demonstrate the optical fields required for the accordion superlattice using acousto-optic deflectors. The method can be applied to neutral-atom quantum computing with optical tweezers, as well as quantum simulation of low-entropy many-body states. For instance, a unit-filling atomic Mott insulator can be coherently expanded by a factor of ten in order to load an optical tweezer array with very high filling. In turn, sorted tweezer arrays can be compressed to form high-density states of ultracold atoms in optical lattices. The method can also be applied to biological systems where dynamical separation of particles is required.
Suppressing Dissipation in a Floquet-Hubbard System
Viebahn, Konrad; Minguzzi, Joaquín; Sandholzer, Kilian; et al. (2021)
Physical Review X
The concept of “Floquet engineering” relies on an external periodic drive to realize novel, effectively static Hamiltonians. This technique is being explored in experimental platforms across physics, including ultracold atoms, laser-driven electron systems, nuclear magnetic resonance, and trapped ions. The key challenge in Floquet engineering is to avoid the uncontrolled absorption of photons from the drive, especially in interacting systems in which the excitation spectrum becomes effectively dense. The resulting dissipative coupling to higher-lying modes, such as the excited bands of an optical lattice, has been explored in recent experimental and theoretical works, but the demonstration of a broadly applicable method to mitigate this effect is lacking. Here, we show how two-path quantum interference applied to strongly correlated fermions in a driven optical lattice suppresses dissipative coupling to higher bands and increases the lifetime of double occupancies and spin correlations by 2 to 3 orders of magnitude. Interference is achieved by introducing a weak second modulation at twice the fundamental driving frequency with a definite relative phase. This technique is shown to suppress dissipation in both weakly and strongly interacting regimes of an offresonantly driven Hubbard system, opening an avenue to realizing low-temperature phases of matter in interacting Floquet systems.
Splitting and Connecting Singlets in Atomic Quantum Circuits
Zhu, Zijie; Kiefer, Yann; Jele, Samuel; et al. (2025)
Physical Review X
Gate operations composed in quantum circuits form the basis for digital quantum simulation and quantum processing. While two-qubit gates generally operate on nearest neighbors, many circuits require nonlocal connectivity and necessitate some form of quantum information transport. Yet, connecting distant nodes of a quantum processor still remains challenging, particularly for neutral atoms in optical lattices. Here, we create singlet pairs of two magnetic states of fermionic potassium-40 atoms in an optical lattice and use a bidirectional topological Thouless pump to transport, coherently split, and separate the pairs, as well as to demonstrate interaction between them via tuneable (SWAP)𝛼-gate operations. We achieve pumping with a single-shift fidelity of 99.78(3)% over 50 lattice sites and split the pairs within a decoherence-free subspace. Gates are implemented by superexchange interaction, allowing us to produce interwoven atomic singlets. For readout, we apply a magnetic field gradient, resulting in single- and multifrequency singlet-triplet oscillations. Our work shows avenues to create complex patterns of entanglement and new approaches to quantum processing, sensing, and atom interferometry.
Matter-Wave Diffraction from a Quasicrystalline Optical Lattice
Viebahn, Konrad; Sbroscia, Matteo; Carter, Edward; et al. (2019)
Physical Review Letters
Interactions Enable Thouless Pumping in a Nonsliding Lattice
Viebahn, Konrad; Walter, Anne-Sophie; Bertok, Eric; et al. (2024)
Physical Review X
A topological "Thouless"pump represents the quantized motion of particles in response to a slow, cyclic modulation of external control parameters. The Thouless pump, like the quantum Hall effect, is of fundamental interest in physics, because it links physically measurable quantities, such as particle currents, to geometric properties of the experimental system, which can be robust against perturbations and, thus, technologically useful. So far, experiments probing the interplay between topology and interparticle interactions have remained relatively scarce. Here, we observe a Thouless-type charge pump in which the particle current and its directionality inherently rely on the presence of strong interactions. Experimentally, we utilize a two-component Fermi gas in a dynamical superlattice which does not exhibit a sliding motion and remains trivial in the single-particle regime. However, when tuning interparticle interactions from zero to positive values, the system undergoes a transition from being stationary to drifting in one direction, consistent with quantized pumping in the first cycle. Remarkably, the topology of the interacting pump trajectory cannot be adiabatically connected to a noninteracting limit, highlighted by the fact that only one atom is transferred per cycle. Our experiments suggest that Thouless charge pumps are promising platforms to gain insights into interaction-driven topological transitions and topological quantum matter.

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Konrad Viebahn

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