Lowest laser intensity for observable dynamically assisted Schwinger pairs
In plain words
Pulling pairs out of empty space with a field alone needs a field about a thousand times stronger than the best lasers give, which is several million times their intensity. Adding a weak fast-oscillating field could lower the barrier, and the question is how much.
Precise statement
The Schwinger rate in a constant field, $w = (e E)^2/(4\pi^3 \hbar^2 c)\operatorname{exp}(-\pi E_S/E)$ with $E_S \sim 4.4e13\,\mathrm{statvolt}/\mathrm{cm}$, is negligible at the $E/E_S \sim 1e-3$ of present peak laser fields (peak intensity about $1e30\,\mathrm{erg}\,\mathrm{s}^{-1}\,\mathrm{cm}^{-2}$, approximate), where $\operatorname{exp}(-\pi E_S/E) \sim \operatorname{exp}(-3000)$. Superposing a weak high-frequency field (dynamically assisted Schwinger mechanism, Schuetzhold, Gies and Dunne 2008) reduces the exponent. For realistic focused optical pulses combined with XFEL or gamma-ray photons, determine the minimal strong-field intensity at which the nonperturbative assisted yield reaches 1 pair per shot, and specify an observable that separates it from perturbative multiphoton Breit-Wheeler pairs.
What would settle it
Calculations of assisted pair yields in realistic 3D focused field geometries beyond the locally constant field approximation, followed by a measurement at the predicted threshold.