New publication on arXiv!


A new publication on Quantum Superposition of Massive Objects and the Quantization of Gravity on arXiv.

We analyse a gedankenexperiment previously considered by Mari et al. that involves quantum superpositions of charged and/or massive bodies (``particles'') under the control of the observers, Alice and Bob. In the electromagnetic case, we show that the quantization of electromagnetic radiation (which causes decoherence of Alice's particle) and vacuum fluctuations of the electromagnetic field (which limits Bob's ability to localize his particle to better than a charge-radius) both are essential for avoiding apparent paradoxes with causality and complementarity. We then analyze the gravitational version of this gedankenexperiment. We correct an error in the analysis of Mari et al. and of Baym and Ozawa, who did not properly account for the conservation of center of mass of an isolated system. We show that the analysis of the gravitational case is in complete parallel with the electromagnetic case provided that gravitational radiation is quantized and that vacuum fluctuations limit the localization of a particle to no better than a Planck length. This provides support for the view that (linearized) gravity should have a quantum field description.


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FIG. 1. Arrangement of the gedankenexperiment. Alice’s

particle is prepared in a spatial superposition with separation


while Bob’s particle, at distance



, is initially local-

ized by a trap. At the start of the protocol Bob can decide

whether or not to release his particle from the trap, while

Alice starts to recombine the paths of her particle. (When

dividing and recombining the paths of her particle, Alice uses

Stern-Gerlach devices, as discussed in [1, 23].) If Bob can

acquire which-path information in a time



< D

and Al-

ice recombines the superposition in a time



< D


emitting radiation, then inconsistencies with causality or com-

plementarity arise: Assuming complementarity holds, Alice

could, by testing the coherence of her state (e.g., by measur-

ing the additional spin degree of freedom in an appropriate

basis, looking for the interference pattern, etc.) determine

whether or not Bob opened the trap, in violation of causality.

Alternatively, assuming causality holds, Alice could maintain

the coherence of her state while which-path information has

been acquired by Bob, in violation of complementarity.