Large coherent states are commonly assumed to be necessary for an observation of quantum effects in gravitational interactions. However, holographic boundary conditions, with an information density that decreases linearly with size, suggest that the Planckian degrees of freedom of space-time must contain significant quantum correlations on all scales, preserving substantial coherence in the "background" itself -- even in the non-excited limit of zero curvature. To probe this, we built the Holometer, a pair of 40m-long, co-located but independent high-power Michelson interferometers. Their signals are sampled at a frequency far exceeding the 8MHz inverse light crossing time of the optical layout, for broad-band cross-correlation measurements of differential position that are both timelike and spacelike across the physical system. This allows sensitivity to nonlocal phenomena such as an entanglement of space-time, beyond the local metric fluctuations or stochastic gravitational waves measured at LIGO or GEO600. With 10^29 photons, the cross spectral density integrates down to 0.1 Planck time in dimensionless strain units.
In this talk, I will describe the phenomenology behind this design concept -- a hypothesis about a relational space-time emergent from a quantum system, with the information delocalized according to EPR-like causal structures. I will present the current status of our analysis and systematics. If our signature is verified, it appears acausal in standard frameworks, and offers a potential pathway to the black hole information problem, the nature of dark energy, and issues in quantum foundations such as indefinite causal structures and observer dependent realities. I will also discuss possible concordant signatures in the latest data on the cosmic microwave background (Planck) and galaxy density distributions (DES).