  A pseudogap (PG) was introduced by Mott to describe a state of matter which
has a minimum in the density of states at the Fermi level, deep enough for
states to become localized. It can arise either from Coulomb repulsion between
electrons, or due to an incipient charge or spin order, or a combination of the
two. These states are rapidly fluctuating in time with random phase, so they
are hard to observe experimentally. Here we present the first coherent quench
measurements of the dynamical transition to the pseudogap state in the
prototype high temperature superconductor
Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$, revealing a marked absence of
incipient collective ordering beyond a few coherence lengths on short
timescales at any level of doping. Instead we find evidence for sub-picosecond
carrier localization favouring a picture of pairing resulting from the
competing Coulomb interaction and strain, enhanced by a Fermi surface
instability.
