  We present a second quantization description of frequency-based continuous
variables quantum computation in the subspace of single photons. For this, we
define frequency and time operators using the free field Hamiltonian and its
Fourier transform, and show that these observables, when restricted to the one
photon per mode subspace, reproduce the canonical position-momentum commutation
relations. As a consequence, frequency and time operators can be used to define
a universal set of gates in this particular subspace. We discuss the physical
implementation of these gates as well as their effect on single photon states,
and show that frequency and time variables can also be used to implement
continuous variables quantum information protocols, in the same way than
polarization is currently used as a two-dimensional quantum variable.
