Quantum information is richer than conventional information. Whereas digital information is measured in bits, quantum information is measured in quantum bits, or qubits. The fundamental unit of quantum information is the qubit. A bit of information is either 0 or 1. But a qubit of information is much more crowded. It can be pictured as a sphere.

Qubits encode quantum information. To fully describe the information encoded in a qubit we need to specify two numbers, roughly describing "how much" the qubit is 1 and how much it is 0.

Whenever we represent a number in a computer we will specify it to a particular precision — 32 bits is a pretty conventional approach. So, to represent the state of a qubit we need about 64 bits.