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Information causality as a physical principle


Quantum physics has remarkable distinguishing characteristics. For example, it gives only probabilistic predictions (non-determinism) and does not allow copying of unknown states (no-cloning1). Quantum correlations may be stronger than any classical ones2, but information cannot be transmitted faster than light (no-signalling). However, these features do not uniquely define quantum physics. A broad class of theories exist that share such traits and allow even stronger (than quantum) correlations3. Here we introduce the principle of ‘information causality’ and show that it is respected by classical and quantum physics but violated by all no-signalling theories with stronger than (the strongest) quantum correlations. The principle relates to the amount of information that an observer (Bob) can gain about a data set belonging to another observer (Alice), the contents of which are completely unknown to him. Using all his local resources (which may be correlated with her resources) and allowing classical communication from her, the amount of information that Bob can recover is bounded by the information volume (m) of the communication. Namely, if Alice communicates m bits to Bob, the total information obtainable by Bob cannot be greater than m. For m = 0, information causality reduces to the standard no-signalling principle. However, no-signalling theories with maximally strong correlations would allow Bob access to all the data in any m-bit subset of the whole data set held by Alice. If only one bit is sent by Alice (m = 1), this is tantamount to Bob’s being able to access the value of any single bit of Alice’s data (but not all of them). Information causality may therefore help to distinguish physical theories from non-physical ones. We suggest that information causality—a generalization of the no-signalling condition—might be one of the foundational properties of nature.

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Figure 1: The task.
Figure 2: Van Dam’s protocol17.
Figure 3: Information causality identifies the strongest quantum correlations.


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We thank M. Christandl, V. Vedral and S. Wehner for stimulating discussions. This work was supported by the National Research Foundation and the Ministry of Education in Singapore, and by the European Commission through the Integrated Project Qubit Applications. A.W. acknowledges support by the UK Engineering and Physical Sciences Research Council through the Quantum Information Processing Interdisciplinary Research Collaboration and an Advanced Fellowship, by a Royal Society Wolfson Merit Award, and by a Philip Leverhulme Prize.

Author Contributions All authors contributed to the initial conception of the ideas, to the working out of details, and to the writing and editing of the manuscript.

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Correspondence to Marcin Pawłowski.

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Pawłowski, M., Paterek, T., Kaszlikowski, D. et al. Information causality as a physical principle. Nature 461, 1101–1104 (2009).

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