= Quantum information
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Quantum information is information encoded in <quantum states> and processed through <quantum channels> or <quantum measurements>. A <qubit> supplies a two-dimensional state space; <quantum phase> and <entanglement> affect which operations and outcome statistics are possible. The <Born rule> connects the state description with the <probabilities> of classical measurement records.
The continuously varying <probability amplitudes> of a <qubit> do not supply a readable list of arbitrarily many classical <bits>. A measurement produces a classical record with state-dependent <probabilities>. <Quantum state tomography> estimates the state from many identically prepared systems. For a classical label encoded into one transmitted <qubit>, with no additional shared resource, <Holevo's theorem> bounds the <mutual information> available from a measurement by one <bit>. This bound concerns information about the label, rather than the number of possible outcome labels a measuring device can print. https://jhwatrous.github.io/TQI-notes.pdf#page=116[Watrous's lecture notes, Lecture 12, on accessible information and the Holevo bound]{ref}
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