Interrogating the void : the difficulty of extracting information from many-body systems

Abstract

In this thesis, I will explore some of the ways the information-theoretic properties of quantum many-body systems can be analyzed. I do this in two different settings. First, I will describe an approach to the "scrambling time problem," a conjecture of Susskind and Sekino that asserts that black holes can thermalize the information of objects that are dropped into them at the fastest rate consistent with unitarity. Specifically, I will analyze the dynamics of the Iizuka-Polchinksi model, a matrix model of a black hole whose response functions can be calculated exactly. Second, I will study the average information content of subsystems of a larger system. In particular, I will improve a result of Page giving the average entanglement entropy of such a subsystem in the ensemble of random, Haar-distributed states by refining it to a smaller, more physically relevant ensemble of states known as "matrix product states," which encode a notion of locality. In both these examples, fundamental obstacles arise that impede our analysis; I explain how these roadblocks are related to the difficulty of understanding the interactions between the exponentially large number the degrees of freedom such many-body systems contain.