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The Bose--Marletto--Vedral experiment is a family of proposals to test gravitationally induced entanglement between initially independent masses. Sougato Bose and collaborators, and independently Chiara Marletto and Vlatko Vedral, described the idea in 2017. Each mass occupies two distinguishable spatial branches, encoding a qubit. Different branch separations give different Newtonian gravitational potential energies and hence different relative phases.^[ref]

For stationary branches at separations $r_{ab}$ held for duration $\tau$, the ideal model givesThe positive phase follows from negative Newtonian gravitational potential energy in $e^{-iV\tau /\hslash }$. A direct algebraic check separates local rephasing from genuine entanglement: the determinant of the two-by-two coefficient matrix vanishes exactly whenThus a large common phase alone is insufficient. For this ideal pure state, the concurrence is $|\sin (\Delta \varphi /2)|$, reaching one at an odd multiple of $\pi$.

The spin-based proposal uses two Stern-Gerlach interferometers: embedded spins label the spatial branches, the masses interact, and recombination transfers the branch information into measurable spin correlations. Measurements in different spin bases supply an entanglement witness. Unwanted electromagnetic interactions and quantum decoherence must be controlled for the gravitational interpretation.^[ref]

The mediator inference assumes initially unentangled systems, local coupling through a mediator, and no other entangling interaction. Under those assumptions, a mediator described only by commuting observables cannot generate the observed entanglement; at least two incompatible observables are required. This is an indirect test of nonclassical mediation, without selecting a complete high-energy theory of quantum gravity.^[ref]

A Stern-Gerlach interferometer uses spin-dependent magnetic forces to split matter-wave components and subsequently recombine them coherently. Splitting alone is a spin-position correlation; interference requires restoring sufficient overlap in both position and momentum while controlling the relative phase.

An atom-chip realization uses accurately timed magnetic-field gradients to drive the separating and recombining trajectories. Reading the final spin populations reveals phase-dependent interference after recombination. Residual trajectory mismatch or uncontrolled magnetic noise reduces contrast, explaining why reversing the average force is insufficient by itself to make a complete interferometer.^[ref]

Yair Margalit is an experimental physicist credited as first author of a detailed realization of a complete Stern-Gerlach interferometer. In this experiment, gradients of a magnetic field generated by an atom chip split atomic wave packets according to their internal spin state and then recombined them coherently.

Recombination matters: separating two paths shows state-dependent motion, whereas recovering quantum interference tests preservation of the relative phase. The reported arrangement used controlled microscopic field sources; it should not be confused with the original macroscopic-magnet Stern-Gerlach experiment.^[ref]