We experimentally demonstrate a universal strategy for producing a quantum state that is orthogonal
to an arbitrary, infinite-dimensional, pure input one, even if only a limited amount of
information about the latter is available. Arbitrary coherent superpositions of the two mutually
orthogonal states are then produced by a simple change in the experimental parameters. We use
input coherent states of light to illustrate two variations of the method. However, we show that the
scheme works equally well for arbitrary input fields and constitutes a universal procedure, which
may thus prove a useful building block for quantum state engineering and quantum information
processing with continuous-variable qubits.