  Precision interferometry with quantum states has emerged as an essential tool
for experimentally answering fundamental questions in physics. Optical quantum
interferometers are of particular interest due to mature methods for generating
and manipulating quantum states of light. The increased sensitivity offered by
these states promises to enable quantum phenomena, such as entanglement, to be
tested in unprecedented regimes where tiny effects due to gravity come into
play. However, this requires long and decoherence-free processing of quantum
entanglement, which has not yet been explored for large interferometric areas.
Here we present a table-top experiment using maximally path-entangled quantum
states of light in an interferometer with an area of 715 m$^{2}$, sensitive
enough to measure the rotation rate of Earth. A rotatable setup and an active
area switching technique allow us to control the coupling of Earth's rotation
to an entangled pair of single photons. The achieved sensitivity of 5
$\mu$rad/s constitutes the highest rotation resolution ever achieved with
optical quantum interferometers, surpassing previous work by three orders of
magnitude. Our result demonstrates the feasibility of extending the utilization
of maximally entangled quantum states to large-scale interferometers. Further
improvements to our methodology will enable measurements of
general-relativistic effects on entangled photons opening the way to further
enhance the precision of fundamental measurements to explore the interplay
between quantum mechanics and general relativity along with searches for new
physics.
