Transmon-based simulator of nonlocal electron-phonon coupling: A platform for observing sharp small-polaron transitions

TL;DR

This paper proposes a superconducting quantum simulator using transmon qubits to observe sharp small-polaron transitions in a nonlocal electron-phonon coupling model, enabling exploration of polaron dynamics with momentum dependence.

Contribution

It introduces a novel analog quantum simulator platform for nonlocal electron-phonon interactions, allowing direct observation of nonanalytic polaron transitions in a controlled setting.

Findings

Able to prepare small-polaron Bloch states with arbitrary quasimomentum

Demonstrates the critical coupling strength for sharp polaron transition

Shows nonanalyticities in ground-state energy and quasiparticle residue

Abstract

We propose an analog superconducting quantum simulator for a one-dimensional model featuring momentum-dependent (nonlocal) electron-phonon couplings of Su-Schrieffer-Heeger and "breathing-mode" types. Because its corresponding vertex function depends on both the electron- and phonon quasimomenta, this model does not belong to the realm of validity of the Gerlach-L\"{o}wen theorem that rules out any nonanalyticities in single-particle properties. The superconducting circuit behind the proposed simulator entails an array of transmon qubits and microwave resonators. By applying microwave driving fields to the qubits, a small-polaron Bloch state with an arbitrary quasimomentum can be prepared in this system within times several orders of magnitude shorter than the typical qubit decoherence times. We demonstrate that in this system -- by varying the circuit parameters -- one can readily reach the critical coupling strength required for observing the sharp transition from a nondegenerate (single-particle) ground state corresponding to zero quasimomentum ($K_{\textrm{gs}}=0$) to a twofold-degenerate small-polaron ground state at nonzero quasimomenta $K_{\textrm{gs}}$ and $-K_{\textrm{gs}}$. Through exact numerical diagonalization of our effective Hamiltonian, we show how this nonanalyticity is reflected in the relevant single-particle properties (ground-state energy, quasiparticle residue, average number of phonons). The proposed setup provides an ideal testbed for studying quantum dynamics of polaron formation in systems with strongly momentum-dependent electron-phonon interactions.