Quantum decoherence of gravitational waves

TL;DR

This paper explores whether gravitational waves can retain their quantum coherence, analyzing how environmental interactions cause decoherence and identifying conditions under which quantum properties might be observable.

Contribution

It derives a model-independent threshold for GW decoherence and examines how different cosmological scenarios affect the preservation of quantum coherence in gravitational waves.

Findings

Decoherence is stronger at lower frequencies and higher reheating temperatures.

A fundamental amplitude threshold determines when decoherence is negligible.

In certain scenarios, GWs can maintain quantum coherence at frequencies above 10^7 Hz.

Abstract

The quantum nature of gravity remains an open question in fundamental physics, lacking experimental verification. Gravitational waves (GWs) provide a potential avenue for detecting gravitons, the hypothetical quantum carriers of gravity. However, by analogy with quantum optics, distinguishing gravitons from classical GWs requires the preservation of quantum coherence, which may be lost due to interactions with the cosmic environment causing decoherence. We investigate whether GWs retain their quantum state by deriving the reduced density matrix and evaluating decoherence, using an environmental model where a scalar field is conformally coupled to gravity. Our results show that quantum decoherence of GWs is stronger at lower frequencies and higher reheating temperatures. We identify a model-independent amplitude threshold below which decoherence is negligible, providing a fundamental limit for directly probing the quantum nature of gravity. In the standard cosmological scenario, the low energy density of the universe at the end of inflation leads to complete decoherence at the classical amplitude level of inflationary GWs. However, for higher energy densities, decoherence is negligible within a frequency window in the range $100\ {\rm Hz} \text{-} 10^8\ {\rm Hz}$, which depends on the reheating temperature. In a kinetic-dominated scenario, the dependence on reheating temperature weakens, allowing GWs to maintain quantum coherence above $10^7\ {\rm Hz}$.