Probing the non-classicality of temporal correlations

Martin Ringbauer1,2,3 and Rafael Chaves4

1Centre for Engineered Quantum Systems, School of Mathematics and Physics, University of Queensland, Brisbane, QLD 4072, Australia
2Centre for Quantum Computer and Communication Technology, School of Mathematics and Physics, University of Queensland, Brisbane, QLD 4072, Australia
3Institute of Photonics and Quantum Sciences, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK
4International Institute of Physics, Federal University of Rio Grande do Norte, 59070-405 Natal, Brazil

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Correlations between spacelike separated measurements on entangled quantum systems are stronger than any classical correlations and are at the heart of numerous quantum technologies. In practice, however, spacelike separation is often not guaranteed and we typically face situations where measurements have an underlying time order. Here we aim to provide a fair comparison of classical and quantum models of temporal correlations on a single particle, as well as timelike-separated correlations on multiple particles. We use a causal modeling approach to show, in theory and experiment, that quantum correlations outperform their classical counterpart when allowed equal, but limited communication resources. This provides a clearer picture of the role of quantum correlations in timelike separated scenarios, which play an important role in foundational and practical aspects of quantum information processing.

Explaining observations in terms of cause and effect is central to all of empirical science. Yet, the correlations between measurements on remote parts of entangled quantum systems can be stronger than any classical notion of cause and effect can explain. In practice, moreover, we typically face situations where communication between the measurements cannot be excluded, and with enough communication a cause-and-effect explanation would become possible. Here, we aim provide a fair comparison of classical and quantum resources and find that even with limited communication, quantum correlations still outperform their classical counterpart. These results contribute to our understanding of the non-classical nature of quantum correlations, which power a wide range of today's quantum technologies, and might thus be important for future quantum information processing applications.

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