Testing spooky action at a distance

Abstract: In science, one observes correlations and invents theoretical models that describe them. In all sciences, besides quantum physics, all correlations are described by either of two mechanisms. Either a first event influences a second one by sending some information encoded in bosons or molecules or other physical carriers, depending on the particular science. Or the correlated events have some common causes in their common past. Interestingly, quantum physics predicts an entirely different kind of cause for some correlations, named entanglement. This new kind of cause reveals itself, e.g., in correlations that violate Bell inequalities (hence cannot be described by common causes) between space-like separated events (hence cannot be described by classical communication). Einstein branded it as spooky action at a distance. A real spooky action at a distance would require a faster than light influence defined in some hypothetical universally privileged reference frame. Here we put stringent experimental bounds on the speed of all such hypothetical influences. We performed a Bell test during more than 24 hours between two villages separated by 18 km and approximately east-west oriented, with the source located precisely in the middle. We continuously observed 2-photon interferences well above the Bell inequality threshold. Taking advantage of the Earth's rotation, the configuration of our experiment allowed us to determine, for any hypothetically privileged frame, a lower bound for the speed of this spooky influence. For instance, if such a privileged reference frame exists and is such that the Earth's speed in this frame is less than 10^-3 that of the speed of light, then the speed of this spooky influence would have to exceed that of light by at least 4 orders of magnitude.

• The paper demonstrates a robust violation of Bell inequalities using a 24-hour, 18 km experiment on energy-time entangled photons.
• It employs a fiber-optic Michelson interferometer setup and leverages Earth’s rotation to challenge classical local realism.
• The results set a lower bound indicating any hypothetical nonlocal influence would exceed light speed by at least four orders of magnitude.

Testing Spooky Action at a Distance

The paper "Testing Spooky Action at a Distance" by Salart et al. provides an experimental examination of quantum entanglement phenomena that challenge classical understandings of causality and locality. Rooted in the concept of "spooky action at a distance" as described by Einstein, the paper tackles the nonintuitive nature of quantum correlations, particularly those that violate Bell inequalities, which cannot be satisfactorily explained by classical physics' local realistic theories.

Experimental Setup and Methodology

The authors devised a rigorous Bell test conducted over a continuous 24-hour period between two villages 18 km apart in Switzerland, with a photon source located centrally. Leveraging the Earth's rotation, this setup aimed to capture the implications of any hypothetical universally privileged frame, a reference frame where superluminal influences could be quantified. The experiment focused on energy-time entangled photon pairs generated via parametric down-conversion, with detection performed using fiber optic Michelson interferometers.

Key Findings

The experiment provided significant empirical data, indicating sustained violations of Bell inequalities across various frames due to the Earth's rotation. The visibility of interference patterns remained consistent, surpassing the thresholds set by Bell inequalities throughout multiple trials over different periods of the day.

A distinct quantitative outcome of this experimental configuration was the establishment of stringent lower bounds for the speed of any such theoretical influence. Specifically, assuming a hypothetical frame in which Earth's velocity is less than $10^{-3}c$, where $c$ represents the speed of light, the results implied that the speed of any non-classical influence would need to exceed the speed of light by at least four orders of magnitude. This lower speed boundary adds a critical dimension to understanding nonlocal quantum effects.

Implications and Future Directions

The findings confront classical and relativistic perspectives, emphasizing the necessity for interpretations of quantum mechanics that transcend traditional notions of space-time. The results bring more weight to the discussion surrounding nonlocality in quantum mechanics, probing the limits of Lorentz invariance without transgressing the core tenets of relativity.

The implications for future research are profound. This experimental approach could constitute a foundational step in refining or dismissing theoretical constructs like Bohm's pilot-wave theory and others relying on superluminal communication. Moreover, advancing experimental precision and extending the scale of Bell tests could elicit more comprehensive insights into the fabric of quantum mechanics, continuity of causality, and invariant frameworks.

Conclusion

The empirical work conducted by Salart et al. demonstrates rigorous methodology and analysis, contributing valuable data and insights regarding quantum entanglement and the contentious notion of superluminal influences. It lays a robust foundation for subsequent inquiries into the characteristics and parameters governing quantum nonlocality, further delineating the boundaries between classical and quantum physics.