1Department of Physics, Pontifical Catholic University of Rio de Janeiro, Rio de Janeiro 22451-900, Brazil
2Istituto del Consiglio Nazionale delle Ricerche, OVI, Italy
3Departament de Física Quàntica i Astrofísica, Institut de Ciències del Cosmos, Universitat de Barcelona, Martí i Franquès 1, E-08028 Barcelona, Spain
4Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
5Dipartimento di Matematica e Fisica “E. De Giorgi”, Università del Salento, and Istituto Nazionale di Fisica Nucleare (INFN) sezione di Lecce, via per Arnesano, 73100 Lecce, Italy
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Abstract
The effective quantum field theory description of gravity, despite its non-renormalizability, allows for predictions beyond classical general relativity. As we enter the age of gravitational wave astronomy, an important and timely question is whether measurable quantum predictions that depart from classical gravity, analogous to quantum optics effects which cannot be explained by classical electrodynamics, can be found. In this work, we investigate quantum signatures in gravitational waves using tools from quantum optics. Squeezed-coherent gravitational waves, which can exhibit sub-Poissonian graviton statistics, can enhance or suppress the signal measured by an interferometer, a characteristic effect of quantum squeezing. Moreover, we show that Gaussian gravitational wave quantum states can be reconstructed from measurements over an ensemble of optical fields interacting with a single copy of the gravitational wave, thus opening the possibility of detecting quantum features of gravity beyond classical general relativity.
Featured image: Quantum detection for quantum states of gravitational waves.
Popular summary
Thinking by analogy, however, detecting photons is not the only way of proving the quantum mechanical nature of electromagnetism. Quantum optics has taught us that quantum field fluctuations are measurable in macroscopic states of light – e.g. squeezed and squeezed-coherent states – through linear classical detection such as homodyne and heterodyne measurements. This idea has led us to a search for macroscopic quantum effects of gravitational waves measurable regardless of our ability to detect gravitons. In summary, we ask the question: which predictions of the effective quantum description of gravity departing from classical general relativity could be detected in gravitational wave detectors?
In the present work, we report some of our latest results in the attempt to answer such question. We show that within the low energy effective field theory description of gravity, there exists quantum states of gravitational waves – notably squeezed-coherent states – which could cause non-classical effects measurable using present-day or near-future interferometric detectors such as LIGO and VIRGO. The generation of such quantum states of gravitational waves remains unknown and much still has to be researched, but our work paves the way for a phenomenological search for such effects, which given the non-linear nature of Einstein gravity could be produced in strong field astrophysical events. If detected, the effects we describe provide a smoking gun for the quantum mechanical nature of gravity, thus opening the way to experimental measurements of quantum spacetime.
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