Gravitationally-induced entanglement between two massive particles is sufficient evidence of quantum effects in gravity

Abstract: All existing quantum gravity proposals share the same deep problem. Their predictions are extremely hard to test in practice. Quantum effects in the gravitational field are exceptionally small, unlike those in the electromagnetic field. The fundamental reason is that the gravitational coupling constant is about 43 orders of magnitude smaller than the fine structure constant, which governs light-matter interactions. For example, the detection of gravitons -- the hypothetical quanta of energy of the gravitational field predicted by certain quantum-gravity proposals -- is deemed to be practically impossible. In this letter we adopt a radically different, quantum-information-theoretic approach which circumvents the problem that quantum gravity is hard to test. We propose an experiment to witness quantum-like features in the gravitational field, by probing it with two masses each in a superposition of two locations. First, we prove the fact that any system (e.g. a field) capable of mediating entanglement between two quantum systems must itself be quantum. This argument is general and does not rely on any specific dynamics. Then, we propose an experiment to detect the entanglement generated between two masses via gravitational interaction. By our argument, the degree of entanglement between the masses is an indirect witness of the quantisation of the field mediating the interaction. Remarkably, this experiment does not require any quantum control over gravity itself. It is also closer to realisation than other proposals, such as detecting gravitons or detecting quantum gravitational vacuum fluctuations.

• The paper argues that detecting entanglement between two masses via gravitational interaction confirms the quantum properties of the gravitational field.
• It employs a Mach-Zehnder interferometer setup with spatial superpositions to isolate gravitational effects from other interactions.
• The findings imply that observing gravitationally-induced entanglement can substantiate quantum gravity theories without relying on direct graviton detection.

Quantum Gravity via Gravitationally-Induced Entanglement

The paper "Gravitationally-induced entanglement between two massive particles is sufficient evidence of quantum effects in gravity" discusses a novel experimental approach to test for quantum-like features in the gravitational field through a quantum-information-theoretic framework. This paper sets forth a theoretically innovative proposal to indirectly verify the quantization of gravity by demonstrating entanglement induced solely through gravitational interaction.

Theoretical Foundation

Central to this research is the assertion that any system capable of mediating entanglement between two quantum systems must itself possess quantum characteristics, specifically the existence of non-commuting observables. The gravitational field is postulated as the mediating system that can entangle two quantum masses through Newtonian interaction, suggesting the field's quantum nature.

The essence of the proof involves interactions between two quantum masses and an interceding gravitational field. Under the locality assumption, an entanglement observed between the two masses, achieved through gravitational interaction alone, implies that the field must feature quantum non-commuting observables. This argument is general and independent of any specific quantum gravity model or detailed dynamics, marking a significant departure from traditional direct detection or graviton-based methods.

Experimental Design

The proposed experiment employs a Mach-Zehnder interferometer setup with two masses in spatial superposition, interacting solely via gravity. The field's quantization is inferred through the generation of entanglement characterized by specific relative phases acquired as a function of the interaction distance and arm length in the interferometers.

The experimental setup is designed to measure the entanglement degree of freedom between the two masses, serving as a quantum witness to field quantization. The experiment is not contingent on coherent quantum control over the gravitational field but relies on current matter-wave interferometry capabilities.

Technological and Feasibility Considerations

The viability of such an experiment, considering technology today, hinges on isolating gravitational interaction as the dominant force between the masses. Technologies such as massive molecule interference, split Bose-Einstein condensates, and nano-mechanical oscillators provide potential experimental platforms. Critical challenges include achieving an adequate signal-to-noise ratio and ensuring gravitational interactions outweigh electromagnetic or other potential entanglement sources.

Implications and Further Research

If successful, the experimental validation would provide empirical support for quantum gravity, offering insights independent of the specifics of model dynamics. Nonetheless, caution is advised, as observational failures might be attributed to model inadequacies or fundamental decoherence of the gravitational channel rather than a lack of field quantization.

Future research directions include refining the experimental design to enhance sensitivity, conducting comparative studies across various quantum gravity models, and potentially extending this framework to scenarios such as the Aharonov-Bohm effect.

Conclusion

This research opens new conceptual and experimental pathways for associating quantum-information principles with gravity. The suggestion that gravitational interactions between spatially superposed masses could illustrate quantum field characteristics highlights the innovative potential in merging quantum mechanics with relativity. As methodologies evolve, the hypothesis of a quantized gravitational field may transition from a theoretical construct to an experimentally substantiated reality, reshaping our understanding of fundamental physics.