  A quantum computer is a multi-particle interferometer that comprises beam
splitters at both ends and arms, where the n two-level particles undergo the
interactions among them. The arms are designed so that relevant functions
required to produce a computational result is stored in the phase shifts of the
2^n arms. They can be detected by interferometry that allows us to utilize
quantum parallelism. Quantum algorithms are accountable for what
interferometers to be constructed to compute particular problems. A standard
formalism for constructing the arms has been developed by the extension of
classical reversible gate arrays. By its nature of sequential applications of
logic operations, the required number of gates increases exponentially as the
problem size grows. This may cause a crucial obstacle to perform a quantum
computation within a limited decoherence time. We propose a direct and
concurrent construction of the interferometer arms by one-time evolution of a
physical system with arbitrary multi-particle interactions. It is inherently
quantum mechanical and has no classical analogue. Encoding the functions used
in Shor's algorithm for prime factoring, Grover's algorithm and Deutsch-Jozsa
algorithm requires only one-time evolution of such a system regardless of the
problem size n as opposed to its standard sequential counterpart that takes
O(n^3), O(n) and O(n2^n).
