Quantum computers

This paper first considers sequential quantum machines (SQMs). The SQMs that possess an isometric transition operator and the SQMs that are factorizable or strongly factorizable are characterized. Quantum Turing machines (QTMs) are studied next and an alternative proof of the result that characterizes the unitary evolution of a QTM is given. it is shown that any QTM can be represented in terms of two quantum printers which are much simpler than a QTM. Unidirectional QTMs are studied and it is shown that their corresponding quantum printers are closely related to each other A simple method for constructing unidirectional QTMs is given. Finally, a preliminary development of generalized QTMs and quantum pushdown automata is presented.