  The emergence of quantum mechanics and general relativity has transformed our
understanding of the natural world significantly. However, integrating these
two theories presents immense challenges, and their interplay remains untested.
Recent theoretical studies suggest that the single-photon interference covering
huge space can effectively probe the interface between quantum mechanics and
general relativity. We developed an alternative design using unbalanced
Michelson interferometers to address this and validated its feasibility over an
8.4 km free-space channel. Using a high-brightness single-photon source based
on quantum dots, we demonstrated single-photon interference along this
long-distance baseline. We achieved a phase measurement precision of 16.2 mrad,
which satisfied the measurement requirements for a gravitational redshift at
the geosynchronous orbit by five times the standard deviation. Our results
confirm the feasibility of the single-photon version of the
Colella-Overhauser-Werner experiment for testing the quantum effects in curved
spacetime.
