 Quantum information can be processed, but the accessibility of this information is limited by the Holevo bound (mentioned in Section 3). David Deutsch (1985) first showed how to exploit quantum entanglement to perform a computational task that is impossible for a classical computer. Suppose we have a black box or oracle that evaluates a Boolean function \(f\), where the arguments or inputs of \(f\) are either 0 or 1, and the values or outputs of \(f\) are also 0 or 1. The outputs are either the same for both inputs (in which case \(f\) is said to be constant), or different for the two inputs (in which case \(f\) is said to be balanced). Suppose we are interested in determining whether \(f\) is constant or balanced. Classically, the only way to do this is to run the black box or query the oracle twice, for both arguments 0 and 1, and to pass the values (outputs of \(f\)) to a circuit that determines whether they are the same (for ‘constant’) or different (for ‘balanced’). Deutsch showed that if we use quantum states and quantum gates to store and process information, then we can determine whether \(f\) is constant or balanced in one evaluation of the function \(f\). The trick is to design the circuit (the sequence of gates) to produce the answer to a global question about the function in an output qubit register that can then be read out or measured.