  The quantization of the electromagnetic field has successfully paved the way
for the development of the Standard Model of Particle Physics and has
established the basis for quantum technologies. Gravity, however, continues to
hold out against physicists' efforts of including it into the framework of
quantum theory. Experimental techniques in quantum optics have only recently
reached the precision and maturity required for the investigation of quantum
systems under the influence of gravitational fields. Here, we report on
experiments in which a genuine quantum state of an entangled photon pair was
exposed to a series of different accelerations. We measure an entanglement
witness for $g$ values ranging from 30 mg to up to 30 g - under free-fall as
well on a spinning centrifuge - and have thus derived an upper bound on the
effects of uniform acceleration on photonic entanglement. Our work represents
the first quantum optics experiment in which entanglement is systematically
tested in geodesic motion as well as in accelerated reference frames with
acceleration a>>g = 9.81 m/s^2.
