We construct a Heisenberg-like algebra for the one dimensional quantum free
Klein-Gordon equation defined on the interval of the real line of length L.
Using the realization of the ladder operators of this type Heisenberg algebra
in terms of physical operators we build a 3+1 dimensional free quantum field
theory based on this algebra. We introduce fields written in terms of the
ladder operators of this type Heisenberg algebra and a free quantum Hamiltonian
in terms of these fields. The mass spectrum of the physical excitations of this
quantum field theory are given by n2π2/L2+mq2, where n=1,2,... denotes the level of the particle with mass mq in an infinite
square-well potential of width L.Comment: Latex, 16 page