CQT Talk by Jinzhao Wang, Stanford University
Title: Some information-theoretic fine prints on the Bekenstein bound
Date/Time: 14-Jul, 03:00PM
Venue: CQT Level 3 Seminar Room, S15-03-15
Abstract: The Bekenstein bound posits a maximum entropy of matter that is of finite energy and confined in a region. It’s often interpreted as a fundamental limit of the information carried by physical objects. In this work, we examine this interpretation by asking if the Bekenstein bound imposes constraints on the channel capacity of communication in spacetime, a scenario in which information can be put on a mathematically rigorous and operationally meaningful footing. Specifically, we study the ``Unruh channel’’ that describes a stationary Alice sending information by exciting different species of free scalar fields to an accelerating Bob, who is confined in a Rindler wedge and is exposed to the noise of Unruh radiation. We show that the unassisted classical capacity of the Unruh channel obeys the Bekenstein bound, and it asymptotes to some small constant value at the large temperature limit. Surprisingly, however, the entanglement-assisted classical capacity is as large as the input dimension (i.e. the number of species) even in arbitrarily hot Unruh radiation. Such separation also shows up for quantum capacities.
To better understand this phenomenon, we directly examine the error recoverability of the Unruh channel and find that, irrespective of the input dimension, it always preserves well the fidelity for any pair of codewords. It hints that the Unruh channel is capable of sending ``zero-bits’’ with an error that doesn’t scale worse with the number of species. Zero-bits are communication resources that can be used as the minimal substitute for the classical/quantum bits needed for many primitive information processing protocols, such as dense coding and teleportation. We then show a stronger result that the Unruh channel has a large zero-bit capacity even at the infinite temperature limit, which explains the capacity boost with entanglement assistance. Therefore, unlike bits and qubits, zero-bits and their associated information processing capability are not constrained by the Bekenstein bound. (Based on joint work with Patrick Hayden.)