  A single photon prepared in a time-energy state described by a frequency comb
combines the extreme precision of energy defined by a single tooth of the comb
with a high sensitivity to small shifts in time defined by the narrowness of a
single pulse in the long sequence of pulses that describe the frequency comb
state in the time domain. We show how this simultaneous suppression of time and
energy uncertainties can be described by a separation of scales and compare
this with the suppression of uncertainties in the two particle correlations of
an entangled state. To illustrate the sensitivity of the frequency comb states
to small shifts in time and frequency, we consider the Hong-Ou-Mandel dips
observed in two-photon interference when both time- and frequency shifts
between the input photons are varied. It is shown that the interference of two
photons in equivalent frequency comb states results in a two dimensional
Hong-Ou-Mandel dip that is narrow in both time and frequency, while the
corresponding entangled photon pairs are only sensitive to temporal shifts.
Frequency comb states thus represent a unique and different approach towards
quantum operations beyond the uncertainty limit.
