Letter

Nature 461, 1101-1104 (22 October 2009) | doi:10.1038/nature08400; Received 8 May 2009; Accepted 13 August 2009

Information causality as a physical principle

Marcin Pawl strokeowski1, Tomasz Paterek2, Dagomir Kaszlikowski2, Valerio Scarani2, Andreas Winter2,3 & Marek Z dotukowski1

  1. Institute of Theoretical Physics and Astrophysics, University of Gdan acutesk, 80-952 Gdan acutesk, Poland
  2. Centre for Quantum Technologies and Department of Physics, National University of Singapore, 3 Science Drive 2, 117543 Singapore, Singapore
  3. Department of Mathematics, University of Bristol, Bristol BS8 1TW, UK

Correspondence to: Marcin Pawl strokeowski1 Correspondence and requests for materials should be addressed to M.P. (Email: dokmpa@univ.gda.pl).

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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