  We propose a multidimensional quantum information encoding approach based on
temporal modulation of single photons, where the Hilbert space can be spanned
by an in-principle infinite set of orthonormal temporal profiles. We analyze
two specific realizations of such modulation schemes, and show that error rate
per symbol can be smaller than 1% for practical implementations. Temporal
modulation may enable multidimensional quantum communication over the existing
fiber optical infrastructure, as well as provide an avenue for probing
high-dimensional entanglement approaching the continuous limit.
