Experimentally tunable QED in dipolar-octupolar quantum spin ice

We propose a readily achievable experimental setting where an external magnetic field is used to tune the emergent quantum electrodynamics (eQED) of dipolar-octupolar quantum spin ice (DO-QSI). In $U(1)_\pi$ DO-QSI -- the proposed ground state of QSI candidates Ce$_2$Zr$_2$O$_7$, Ce$_2$Sn$_2$O$_7$ and Ce$_2$Hf$_2$O$_7$ -- we show that the field can be used to control the emergent speed of light (and, consequently, the emergent fine structure constant). Depending on the field's alignment with the crystal, one may induce different speeds for the two polarizations of the emergent photons, in a fascinating analogue of the electro-optic Kerr effect. In $U(1)_0$ DO-QSI -- yet to be uncovered experimentally -- we find a number of unusual field-induced transitions, including a transition between $0$- and $\pi$-flux QSI phases, as well as phases with frustrated flux configurations. We discuss experimental signatures of these effects in the spinon excitation spectrum, which can be readily accessed for instance in inelastic neutron scattering measurements. Our proposal opens the gate to a plethora of experimentally accessible, engineerable eQED phenomena in the emergent universes of quantum spin ice.