  A time lens allows one to stretch or compress optical waveforms in time,
similar to the conventional lens in space. However, a single-time-lens imaging
system always imparts a residual temporal chirp on the image, which may be
detrimental for quantum networks, where the temporal image interacts with other
fields. We show that a two-time-lens imaging system satisfying the telescopic
condition, a time telescope, is necessary and sufficient for creating a
chirpless image. We develop a general theory of a time telescope, find the
conditions for loss minimization, and show how an erecting time telescope
creating a real image of a temporal object can be constructed. We consider
several applications of such a telescope to making indistinguishable the
photons generated by spontaneous parametric downconversion or single emitters
such as quantum dots.

Quantum Physics