One of the attempts is directed towards a speedup of computation due to quantum parallelism. Another application is quantum cryptography, making use of complementarity. The following features are important, but not sufficient qualities of quantum computers. Input, output, program and memory are qbits.
Any computation is reversible.
Compared to classical irreversible computation, this may result in a space and time overheads. Thus, unless classical, qbits cannot be copied. Unless classical,qbits are context-dependent. That is, their value may depend on the method by which they have been inferred, and on the co-measured qbits. Measurements may be carried out on any qbit at any stage of the computation.