Curator's Take
AI Commentary
This article provides the first gauge‑invariant formula for how soft gravitons—ubiquitous low‑frequency ripples of spacetime—decohere matter superpositions, showing that even in empty space the infrared cutoff drives the coherence to zero unless the branches emit identical radiation. By extending the analysis to squeezed graviton states, it links decoherence rates to the phase and occupation of relic gravitational‑wave backgrounds, a connection that could set fundamental limits for macroscopic qubits or interferometric sensors operating over long timescales. The work builds on recent soft‑theorem and quantum‑gravity studies and gives a concrete quantitative bridge between cosmological graviton spectra and practical decoherence in future quantum technologies.
— Mark Eatherly
Summary
We revisit gravitational decoherence of matter superpositions using the leading soft sector of the graviton $S$-matrix. Starting from Weinberg's soft theorem and the overlap of coherent radiation states, we derive, for branch configurations with the same total four-momentum, a gauge-invariant, nonnegative leading-soft decoherence exponent and evaluate the polarization sum and angular integrals exactly at this order. In the vacuum, coherence between branches with distinguishable soft radiation vanishes as the infrared cutoff is removed. The leading nonrelativistic term is purely quadrupolar as a consequence of energy--momentum conservation. We extend the analysis to single- and two-mode squeezed graviton states, including the two-mode case motivated by inflation. Squeezing can enhance or suppress decoherence depending on the relative phase, while phase averaging gives an enhancement governed by the graviton occupation. For a phase-averaged relic background with an approximately flat energy-density spectrum over the relevant soft frequency band, the sharp-cutoff identification of the infrared scale with the inverse observation time gives a contribution that grows with the gravitational-wave energy density and with the fourth power of the observation time. These results connect soft-graviton scattering, infrared quantum information, and decoherence in nonvacuum gravitational backgrounds.