# On the reversible extraction of classical information from a quantum source

Howard Barnum, Patrick Hayden, Richard Jozsa, Andreas Winter, On the reversible extraction of classical information from a quantum source. *Proc. R. Soc. Lond. A*, 457. ISSN 1364-5021, pp. 2019–2039. May 2001. No electronic version available.
## Abstract

Consider a source E of pure quantum states with von Neumann entropy S. By the quantum source coding theorem, arbitrarily long strings of signals may be encoded asymptotically into S qubits/signal (the Schumacher limit) in such a way that entire strings may be recovered with arbitrarily high fidelity. Suppose that classical storage is free while quantum storage is expensive and suppose that the states of E do not fall into two or more orthogonal subspaces. We show that if E can be compressed with arbitrarily high fidelity into A qubits/signal plus any amount of auxiliary classical storage then A must still be at least as large as the Schumacher limit S of E. Thus no part of the quantum information content of E can be faithfully replaced by classical information. If the states do fall into orthogonal subspaces then A may be less than S, but only by an amount not exceeding the amount of classical information specifying the subspace for a signal from the source.

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