What does this equation mean?

Between those extremes sit two families with a gap between them. Engineered quantum simulators — cold-atom lattices, trapped-ion chains, Rydberg-atom arrays — cluster between 10^{-3} and 3$\times$10^{-2} metres per second, because their dynamics run on tunnelling and spin-exchange rates measured in kilohertz, multiplied by lattice spacings measured in hundreds of nanometres. Natural crystals run several orders faster: longitudinal sound in ordinary crystalline silicon travels at 8.433 kilometres per second [ 12 ] ; the spin excitations of the near-ideal one-dimensional antiferromagnet potassium copper fluoride, with a measured exchange energy of 33.5 millielectronvolts, propagate at roughly 3$\times$10^{4} metres per second [ 11 ] ; and a nine-transmon superconducting circuit — a lattice of qubits wired on a chip rather than atoms trapped in light — shows the same information cone in a purely electronic system, its speed a dimensionless slope in wiring distance converted through the chip’s own pitch rather than a velocity in the ordinary sense [ 14 ] . No platform measured so far sits between about 3$\times$10^{-2} and 2$\times$10^{2} metres per second, and the paper is careful about what that gap means: not a forbidden band, but a statement about what has been built, since nothing yet measured is both slow enough to watch atom by atom and fast enough to fill it.