Maximum photon loss tolerable in linear-optical fault-tolerant architectures
In plain words
Photonic quantum computers lose photons in sources, chips and detectors, and their two-photon operations succeed only part of the time. The largest loss rate at which error correction can still work is not known.
Precise statement
In fusion-based quantum computation with linear optics, entangled resource states of bounded size are joined by probabilistic two-photon fusion measurements (success probability $1/2$ without ancilla boosting), and each photon is lost with probability $\eta_L$. Determine the supremum of $\eta_L$ for which a fault-tolerance threshold exists, as a function of resource-state size and fusion boosting, and decide whether loss tolerance well above the $10.4\%$ loss per fusion reported for a ballistic scheme by Bartolucci et al. (arXiv:2101.09310, 2021) is reachable with bounded-size resource states.
What would settle it
An architecture with a computed or proved threshold at higher loss, together with an upper bound on tolerable loss for bounded-size resource states.