  Understanding the real time dynamics of quantum systems without
quasiparticles constitutes an important yet challenging problem. We study the
superfluid-insulator quantum-critical point of bosons on a two-dimensional
lattice, a system whose excitations cannot be described in a quasiparticle
basis. We present detailed quantum Monte Carlo results for two separate lattice
realizations: their low-frequency conductivities are found to have the same
universal dependence on imaginary frequency and temperature. We then use the
structure of the real time dynamics of conformal field theories described by
the holographic gauge/gravity duality to make progress on the difficult problem
of analytically continuing the Monte Carlo data to real time. Our method yields
quantitative and experimentally testable results on the frequency-dependent
conductivity near the quantum critical point, and on the spectrum of
quasinormal modes in the vicinity of the superfluid-insulator quantum phase
transition. Extensions to other observables and universality classes are
discussed.
