A strong loophole-free test of local realism (1511.03189v2)

Abstract: We present a loophole-free violation of local realism using entangled photon pairs. We ensure that all relevant events in our Bell test are spacelike separated by placing the parties far enough apart and by using fast random number generators and high-speed polarization measurements. A high-quality polarization-entangled source of photons, combined with high-efficiency, low-noise, single-photon detectors, allows us to make measurements without requiring any fair-sampling assumptions. Using a hypothesis test, we compute p-values as small as $5.9\times 10^{-9}$ for our Bell violation while maintaining the spacelike separation of our events. We estimate the degree to which a local realistic system could predict our measurement choices. Accounting for this predictability, our smallest adjusted p-value is $2.3 \times 10^{-7}$. We therefore reject the hypothesis that local realism governs our experiment.

• The paper demonstrates a loophole-free violation of Bell’s theorem using entangled photon pairs to rigorously test local realism.
• It employs high-efficiency detectors, spacelike separation, and fast random number generators to close locality, fair-sampling, and freedom-of-choice loopholes.
• The results, with p-values as low as 5.9×10⁻⁹, provide statistically significant support for quantum mechanics over local hidden variable theories.

A Strong Loophole-Free Test of Local Realism

The paper presents a comprehensive experiment that addresses the loophole-free violation of local realism, using entangled photon pairs. This paper serves as an empirical test of Bell’s theorem, which challenges the classical notion of local realism by affirming quantum mechanical predictions.

The authors embarked on addressing the challenge of closing all significant loopholes that had previously cast doubt on the interpretation of Bell test results. Specifically, these loopholes include locality, fair-sampling, and freedom-of-choice. Historically, while individual experiments have closed some of these loopholes, it has been a significant challenge to close all of them simultaneously, which this experiment achieves.

Methodology and Experimental Setup

• Spacelike Separation: To ensure spacelike separation of events, the researchers placed parties far apart enough such that any signal couldn't reach the other party faster than the speed of light. They used high-speed random number generators and rapid polarization measurements to decide the measurement settings for each photon. This arrangement prevents local hidden variables from influencing both measurement outcomes, effectively closing the locality loophole.
• High-Efficiency Detectors: The experiment utilized high-efficiency, low-noise single-photon detectors, which obviated the need for any fair-sampling assumption, thus addressing the fair-sampling loophole effectively.
• Randomness in Measurement Choices: The research also aimed to address the freedom-of-choice loophole. This loophole suggests that the choices in measurement settings could be influenced by hidden variables correlating with the particles. To counteract this, the authors implemented fast and independent random number generators to determine measurement settings, reducing predictability to within a statistically negligible range.

Results and Statistical Analysis

The authors report p-values as low as $5.9 \times 10^{-9}$ for their Bell test violation, taking into account possible influences from local realistic systems. Adjustments for predictability result in an adjusted p-value of $2.3 \times 10^{-7}$. This statistically significant result strongly rejects the hypothesis that local realism could govern the outcomes of their experiment.

Implications and Future Directions

The successful demonstration of a loophole-free Bell test offers significant implications for both the foundational understanding of quantum mechanics and practical applications such as quantum cryptography. The demonstration verifies that quantum mechanics can't be supplemented with local hidden variables while still producing the predictions observed in this and similar experiments.

In practice, this work bolsters the reliability of quantum systems in cryptographic applications, particularly in the generation of secure random numbers. The authors plan to integrate their Bell test machine into the National Institute of Standards and Technology’s public random number beacon for practical implications.

Looking forward, this research lays the groundwork for further empirical studies that could explore even subtler aspects of quantum nonlocality and the potential development of novel quantum technologies. The authors have established a robust foundation for further exploration of the principle of locality, potentially leading to new quantum communications protocols that take advantage of these confirmed quantum mechanical effects.