  The photonic temporal degree of freedom is one of the most promising
platforms for quantum communication over fiber networks and free-space
channels. In particular, time-bin states of photons are robust to environmental
disturbances, support high-rate communication, and can be used in
high-dimensional schemes. However, the detection of photonic time-bin states
remains a challenging task, particularly for the case of photons that are in a
superposition of different time-bins. Here, we experimentally demonstrate the
feasibility of picosecond time-bin states of light, known as ultrafast
time-bins, for applications in quantum communications. With the ability to
measure time-bin superpositions with excellent phase stability, we enable the
use of temporal states in efficient quantum key distribution protocols such as
the BB84 protocol.
