 Unitary gates manipulate information stored in the “quantum register”—a quantum system—and in this sense ordinary (unitary) quantum evolution can be regarded as a computation. In order to read the result of this computation, however, the quantum register must be measured. The measurement gate is a non-unitary gate that “collapses” the quantum superposition in the register onto one of its terms with a probability corresponding to its complex coefficient. Usually this measurement is done in the computational basis (see the previous section), but since quantum mechanics allows one to express an arbitrary state as a linear combination of basis states, provided that the states are orthonormal (a condition that ensures normalisation) one can in principle measure the register in any arbitrary orthonormal basis. This, however, doesn’t mean that measurements in different bases are equivalent complexity-wise. Indeed, one of the difficulties in constructing efficient quantum algorithms stems exactly from the fact that measurement collapses the state, and some measurements are much more complicated than others.