Two qubits in the Dirac representation - art. no. 024303

The Dirac-matrix representation of a general two-qubit system is shown to e xhibit quite interesting features. The relativistic symmetries of time reve rsal T, charge conjugation C, parity P, and their products are reinterprete d here by examining their action on the Bell states, It is shown that only C does not mix the Bell states whereas all others do. The various logic gat es of quantum information theory are also expressed in terms of the Dirac m atrices. For example, the NOT gate is related to the product of T and P. A two-qubit density matrix is found to be entangled if it is invariant under C.