## Quantum information

Qubits are the smallest physical carriers of quantum information. The quantum information contained in the quantum state $\Psi$ of a qubit has some truly remarkable properties. Qubits cannot be observed, read or deduced from experimental data as in the case of classical bits stored on a DVD. If we have a quantum version of DVD storing a string of qubits, in general, we also cannot copy, erase, or process with irreversible computational gates any of the stored qubits.

Qubits can be transported from place to place similarly to classical bits, but each qubit cannot be cloned and delivered to multiple recipients. Because qubits cannot be wholly converted into classical bits, they cannot be broadcast. Multiple qubits, however, can be used to carry classical bits. Although $n$ qubits can carry more than $n$ classical bits of information, according to Holevo's theorem the greatest amount of classical information that can be retrieved is $n$ bits. Furthermore, the Bell and Kochen–Specker no-go theorems imply that quantum information is non-local and quantum correlations are enforced with superluminal speed.

The fascinating cross-disciplinary field of quantum information theory applied to the study of brain function has only recently been developed. A recent textbook published by CRC Press offers a self-study guide to probe the problems of consciousness, including a concise but rigorous introduction to classical and quantum information theory, theoretical neuroscience, and philosophy of the mind.