  Holography has always held special appeal, for it is able to record and
display spatial information in three dimensions. Here, we show how to augment
the capabilities of digital holography by using a large number of narrow laser
lines at precisely-defined optical frequencies simultaneously. Using an
interferometer based on two frequency combs of slightly different repetition
frequencies and a lens-less camera sensor, we record time-varying spatial
interference patterns that generate spectral hypercubes of complex holograms,
revealing, for each comb line frequency, amplitudes and phases of scattered
wave-fields. Unlike with previous multi-color holography and low-coherence
holography (including with a frequency comb), the unique synergy of broad
spectral bandwidth and high temporal coherence in dual-comb holography opens up
novel optical diagnostics, such as precise dimensional metrology over large
distances without interferometric phase ambiguity, or hyperspectral
3-dimensional imaging with high spectral resolving power, as we illustrate by
molecule-selective imaging of an absorbing gas.
