Information Causality as a Physical Principle

TL;DR

This paper introduces the principle of Information Causality, which limits the amount of information that can be gained through classical communication, distinguishing quantum from stronger no-signaling correlations and potentially serving as a fundamental physical principle.

Contribution

It proposes Information Causality as a new principle that constrains correlations in physical theories, extending the no-signaling principle and aligning with quantum mechanics.

Findings

Information Causality is upheld in classical and quantum physics.

Stronger-than-quantum correlations violate Information Causality.

Maximal no-signaling correlations allow unlimited data access, violating the principle.

Abstract

Quantum physics exhibits remarkable distinguishing characteristics. For example, it gives only probabilistic predictions (non-determinism) and does not allow copying of unknown state (no-cloning). Quantum correlations may be stronger than any classical ones, nevertheless information cannot be transmitted faster than light (no-signaling). However, all these features do not single out quantum physics. A broad class of theories exist which share such traits with quantum mechanics, while they allow even stronger than quantum correlations. Here, we introduce the principle of Information Causality. It states that information that Bob can gain about a previously completely unknown to him data set of Alice, by using all his local resources (which may be correlated with her resources) and a classical communication from her, is bounded by the information volume of the communication. In other words, if Alice communicates m bits to Bob, the total information access that Bob gains to her data is not greater than m. For m=0, Information Causality reduces to the standard no-signaling principle. We show that this new principle is respected both in classical and quantum physics, whereas it is violated by all the no-signaling correlations which are stronger that the strongest quantum correlations. Maximally strong no-signalling correlations would allow Bob access to 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 being able to access the value of any single bit of Alice's data (but of course not all of them). We suggest that Information Causality, a generalization of no-signaling, might be one of the foundational properties of Nature.