The Emergent Fine Structure Constant of Quantum Spin Ice Is Large

TL;DR

This paper demonstrates that quantum spin ice exhibits an emergent quantum electrodynamics with a significantly larger and tunable fine-structure constant than standard QED, enabling exploration of strong coupling phenomena in condensed matter systems.

Contribution

It reveals that the emergent QED in quantum spin ice has a large, tunable fine-structure constant, unlike standard QED, and can reach strong coupling regimes.

Findings

$oldsymbol{ ext{α}_{QSI}}$ exceeds $rac{1}{137}$ by over an order of magnitude.

$oldsymbol{ ext{α}_{QSI}}$ is tunable from zero to strong coupling.

Quantum spin ice is an ideal platform for studying exotic quantum field theories.

Abstract

Condensed matter systems provide alternative `vacua' exhibiting emergent low-energy properties drastically different from those of the standard model. A case in point is the emergent quantum electrodynamics (QED) in the fractionalized topological magnet known as quantum spin ice, whose magnetic monopoles set it apart from the familiar QED of the world we live in. Here, we show that the two greatly differ in their fine-structure constant $\alpha$, which parametrizes how strongly matter couples to light: $\alpha_{\mathrm{QSI}}$ is more than an order of magnitude greater than $\alpha_{\mathrm{QED}} \approx 1/137$. Furthermore, $\alpha_{\mathrm{QSI}}$, the emergent speed of light, and all other parameters of the emergent QED, are tunable by engineering the microscopic Hamiltonian. We find that $\alpha_{\mathrm{QSI}}$ can be tuned all the way from zero up to what is believed to be the \textit{strongest possible} coupling beyond which QED confines. In view of the small size of its constrained Hilbert space, this marks out quantum spin ice as an ideal platform for studying exotic quantum field theories and a target for quantum simulation. The large $\alpha_{\mathrm{QSI}}$ implies that experiments probing candidate condensed-matter realizations of quantum spin ice should expect to observe phenomena arising due to strong interactions.