  We show how imaginary numbers in quantum physics can be eliminated by
enlarging the Hilbert Space followed by an imposition of - what effectively
amounts to - a superselection rule. We illustrate this procedure with a qubit
and apply it to the Mach-Zehnder interferometer. The procedure is somewhat
reminiscent of the constrained quantization of the electromagnetic field,
where, in order to manifestly comply with relativity, one enlargers the Hilbert
Space by quantizing the longitudinal and scalar modes, only to subsequently
introduce a constraint to make sure that they are actually not directly
observable.
