Exploring the nature of gravity with quantum information methods

TL;DR

The article surveys how quantum-information tools can illuminate the interface between quantum mechanics and gravity by focusing on two complementary directions: gravitationally induced entanglement (GIE) and indefinite causal order (ICO). It discusses concrete concepts and experiments from quantum information—Mach-Zehnder interferometry, Stern-Gerlach measurements, Bell inequalities, and quantum circuits—and shows how these can be adapted to probe gravity, either by testing whether gravity can mediate entanglement or by exploring nonclassical causal structures. It critically examines the interpretations and debates surrounding GIE (e.g., gravity as a quantum mediator versus semiclassical gravity) and ICO (e.g., genuine gravity-induced switches versus simulations), and it highlights proposed experiments and theoretical frameworks, such as the quantum switch and process-matrix formalisms. Overall, the work frames a roadmap for using quantum-information methods to empirically and conceptually advance our understanding of gravity at the quantum level and of spacetime causality.

Abstract

The aim of this article is to provide an introduction to the use of quantum information methods for investigating the interface between quantum theory and gravity. To this end, we discuss the basic principles of two current research streams that use this approach. The first one explores a phenomenon known as gravitationally induced entanglement, which aims to infer whether the gravitational field responsible for the interaction between two massive bodies must be quantized or not. The second stream investigates causal structures, thereby providing indirect evidence that spacetime may exhibit non-classical behavior. Before presenting these topics, we briefly review some fundamental concepts and experiments from quantum information theory, such as the Mach-Zehnder interferometer, the Stern-Gerlach experiment, Bell inequalities and entanglement, and the language of quantum circuits.

Figures (14)

• Figure 1: Basic configuration of a Mach-Zehnder interferometer. Items (A)-(C) represent classical light, that is, a wave, and (D) represents a light particle, that is, a photon. The size difference between the two possible paths for the light defines the detection pattern obtained in D$_1$ and D$_2$. (A) Paths of identical size lead to total constructive interference in D$_1$ and total destructive interference in D$_2$. (B) A path difference is generated by the introduction of a deviation in the lower path, which leads to partial interference in the detectors. (C) The path difference is generated by a phase shifter, which also generates partial interference. (D) Mach-Zehnder interferometer for a particle. The statistics generated after several detections are the same as for a classical wave.
• Figure 2: Stern-Gerlach experiment. (A) Theoretical prediction using classical physics. (B) Experimental observations.
• Figure 3: Spacetime diagram for an implementation of the CHSH inequality. Boxes A and B indicate, respectively, the intervals in which Alice and Bob receive systems $s_1$ and $s_2$, perform a measurement, and send the information of which measurement they made and the outcome to Claire. Claire is represented by box C.
• Figure 4: Quantum circuits. (A) Not gate; (B) Hadamard gate; (C) CNot gate; (D) controlled $Z$ operation; (E) composition of Not gates with controlled $Z$ operation; (F) composition of Hadamard gates and a CNot.
• Figure 5: Illustration of the experiment to test GIE using two Mach-Zehnder interferometers of massive particles.