Graviton physics: Quantum field theory of gravitons, graviton noise and gravitational decoherence -- a concise tutorial

Abstract

The detection of gravitational waves in 2015 ushered in a new era of gravitational wave astronomy capable of probing into the strong field dynamics of black holes and neutron stars. It has opened up an exciting new window for laboratory and space tests of Einstein's theory of classical general relativity. In recent years there are two interesting proposals aimed at revealing the quantum natures of perturbative gravity: 1) theoretical predictions in how graviton noise from the early universe after the vacuum of the gravitational field was strongly squeezed by inflationary expansion; 2) experimental proposals using the quantum entanglement between two masses each in a superposition state. The first proposal invokes the stochastic properties of quantum fields, the second invokes a key concept of quantum information. An equally basic and interesting idea is to ask whether and how gravity might be responsible for a quantum system becoming classical in appearance, known as gravitational decoherence. Decoherence due to gravity is of special interest because gravity is universal. This is an important issue in macroscopic quantum phenomena. To fully appreciate these exciting developments requires a working knowledge in classical GR, QF theory and QI plus some familiarity with stochastic processes, namely, noise in quantum fields. Traditionally a new researcher may be conversant in one or two of these four subjects: GR, QFT, QI, SP, depending on his/her background. This tutorial attempts to provide the necessary connections between them, helping an engaging reader from any one of these four subjects to leapfrog to the frontier of these interdisciplinary research topics. Here we shall treat the three topics listed in the title, save gravitational entanglement, because its nature and implications proclaimed in relation to quantum gravity still contain many controversial elements.