Probing BCS pairing and quasiparticle formation in ultracold gases by Rydberg atom spectroscopy

TL;DR

This paper demonstrates how Rydberg atom spectroscopy can be used to probe pairing and quasiparticle formation in ultracold fermionic superfluids, providing a new local measurement technique for many-body quantum states.

Contribution

It introduces a novel spectroscopic method using Rydberg impurities to directly measure superfluid gaps and pairing properties in ultracold fermionic gases.

Findings

Rydberg spectra encode superfluid gap information

Spectroscopy distinguishes broken vs. intact Cooper pairs

Formation of well-defined polaron quasiparticles observed

Abstract

Locally probing pairing in fermionic superfluids, ranging from micro- to macroscopic scales, has been a long-standing challenge. Here, we investigate a new approach that uses Rydberg impurities as a spectroscopic sensor of the surrounding strongly correlated state of ultracold paired fermions. The extended wavefunction of the Rydberg electron induces a finite-range potential that can bind atoms from the BCS medium, forming molecular states. As a consequence, the optical absorption spectrum of the impurity encodes key many-body properties. Using the functional determinant approach, we provide a direct measure of the superfluid gap through frequency shifts of dimer and trimer peaks. The spectra also reveal whether the Cooper pairs are broken or trapped intact. For static Rydberg atoms, we relate this signature of pairing to the suppression of the orthogonality catastrophe due to the superconducting gap resulting in the formation of well-defined polaron quasiparticles. Our work establishes Rydberg atom spectroscopy as a powerful local probe of strongly correlated matter.