Rhine Samajdar, PQI Postdoctoral Fellow

Title: Ferromagnetism in the Hubbard model

Abstract: The Hubbard model, a key descriptor of strongly interacting electronic systems, typically features antiferromagnetic spin alignment. However, in certain idealized limits, the celebrated Nagaoka theorem predicts a ferromagnetic ground state, realizing which has been a long-standing challenge in condensed matter physics. In this talk, we describe how itinerant ferromagnetism arises in a variety of systems without the (infeasible) constraints required by the Nagaoka theorem. Using large-scale density-matrix renormalization group calculations, we first identify high-spin ground states of the Hubbard model on finite-sized triangular and square lattices and uncover their microscopic kinetic origins. Furthermore, we develop a universal mechanism for such Nagaoka ferromagnetism based on the formation of ferromagnetic polarons consisting of a dopant dressed with polarized spins. Probing the polaronic structure and dynamics using multibody correlation functions and pinning fields, we establish the crucial role of mobile polarons in the birth of global long-range order from local ferromagnetic correlations. Finally, we consider a generalization of the Hubbard model which includes the effects of Coulomb interactions and show how long-range interactions induce an instability of high-spin states towards phase separation and competing stripe orders. Our results find immediate application to several experimental systems including moiré materials, ultracold atoms, and quantum dot arrays, providing new insights into recent observations as well as proposals for future experiments.

Yves Hon Kwan, Sam B. Treiman Postdoctoral Fellow, Bernevig Group 

Title: Strongly-interacting topological phases in rhombohedral graphene/hBN superlattices

Abstract: Moiré superlattices of rhombohedral graphene twisted with hBN have attracted significant attention owing to the recent experimental observations of various strongly correlated states, including integer and fractional Chern insulators. This talk will cover our theoretical investigations to explain the emergence of such topological phenomena. First, I will address the subtle role of the moire potential, and its implications regarding the stacking orientation, the phase diagram at integer filling, and collective excitations. I will also describe an idealized model that allows for analytical results and sheds light onto the competition between different phases. Second, I will outline our efforts to understand the stabilization of fractional Chern insulators. Throughout, the unique settings and opportunities inherent in this quantum material platform will be highlighted.

Bichen Zhang, Postdoctoral Research Associate, Thompson Group

Title: Error-Corrected Qubits via Erasure Conversion

Abstract: Achieving fault tolerance through quantum error correction is essential for implementing large-scale quantum algorithms with practical advantages. However, the associated overhead presents a significant challenge. This overhead can be mitigated by engineering physical qubits with reduced error rates and by tailoring residual errors to make them more easily correctable.

In this talk, we present progress on both fronts. We introduce a new neutral atom qubit using the nuclear spin of a long-lived metastable state in 171Yb. The long coherence time and fast excitation to the Rydberg state allow high fidelity one- and two-qubit [1, 2]. Notably, a significant fraction of gate errors result in decays from the qubit subspace to the ground state. By applying fast mid-circuit detection, we convert these errors into erasure errors, which are much easier to be corrected [1].

Based on it, we demonstrate quantum error-correcting codes and logical qubit operations with a noise bias favoring erasure errors. Using a [[4,2,2]] code, we demonstrate error-corrected logical qubits with a resource-efficient overhead of just two physical qubits per logical qubit. Even though the code is too small to correct general Pauli errors, error correction is accomplished through mid-circuit erasure measurements during decoding. We further show how soft decoding can mitigate detection errors in repeated erasure checks. Finally, we demonstrate logical qubit teleportation between multiple logical blocks, using conditionally selected ancilla blocks informed by mid-circuit erasure checks. This approach provides a blueprint for leakage-robust error correction with neutral atom systems.

[1] Ma, S., et al. "High-fidelity gates and mid-circuit erasure conversion in an atomic qubit." Nature 622.7982: 279-284 (2023)

[2] Peper, M., et al. "Spectroscopy and modeling of 171Yb Rydberg states for high-fidelity two-qubit gates." arXiv preprint arXiv:2406.01482 (2024).

A light lunch will be served outside of Bowen Auditorium at noon.