The Quantum Space-Time Loophole in Quantum Foundations: An Interferometric Probe with Quantum Metrology*

The core tenet of quantum mechanics is its rejection of local realism, exemplified by Bell tests. Despite impressive recent constraints on the locality and freedom-of-choice loopholes, including the "cosmic Bell test," one significant loophole remains entirely unaddressed: Every Bell test so far assumed a definite background fabric of space-time, which is in foundational conflict with the background independence of general relativity, our standard theory of space-time and gravity. This intersection of QM and GR is perhaps best characterized by the holographic principle, in which the 2D entropy of black holes, the densest objects possible, sets a universal 2D upper bound on the quantum information of 3D empty space-time. Because information density decreases with scale, there must be some nonlocal correlations in the background space-time, reformulating the locality loophole. Such a bandwidth limit is predicted to cause a precise ruler or clock to see "pixelation," an irreducible indeterminacy of space-time itself, at the scale of a 10m lab fluctuating by a billionth of a single atom. This incredible level of precision is now reachable by use of novel quantum metrology in laser interferometers, which can improve on the latest LIGO technology by as much as an order of magnitude. We present a research program to study this frontier, and a brief overview of our experiment currently being commissioned at Cardiff University.