Abstract: We discuss the mathematical definition of quantum bit ( qubit) and its physical properties and find

discrepancies between them. In the context of quantum information，the definition of quantum bit is purely mathematical，

which greatly differs from its physical properties. Major differences include: the readability and writeability

of the qubit; the requirement of inifinit procision in typical algrorithms such as Shor’s algorithm; voilation of

classic entropy definition，etc

Key words: qubits, quantum information, measurement