Quantum interference of single photons without optical superposition: towards high resolution imaging in spatial and spectral domains

Observation of the universe demands telescopes with high resolution. In
the optical band, traditional interference requires to bring interfering
fields together, which limits the resolution due to the restricted length
of baseline. Here we demonstrate the very-long-baseline interferometer
(VLBI) in optical band, where two interfering fields never met each other.
Especially, we report the first quantum interference observation when the
input of VLBI is single photon state. Interference is recovered after
measuring the amplitudes of photon fields and digitally processing the
signals of quantum receivers. Moreover, we analyze interference in time
and spectral domains for broadband thermal light input and show the
ultra-high spectral resolution can improve the precision of radial
velocity to 0.08 cm/s, which is 2 orders of magnitude better than that
achievable at the current stage. Further, we apply the spectrally resolved
interference in distinguishing two independent sources with angular
resolutions beyond the diffraction limit. Our investigations have profound
effect on the VLBI, quantum optics, and precision measurement.