Optical state truncation

We investigate the effects of photodetector imperfections on a recently pro posed device designed to truncate physically a state of light. We find that the mixed state produced in the presence of the imperfections has a surpri singly good fidelity with the required state. For convenience and mathemati cal simplicity, we make use of the quantum-mechanical retrodictive formalis m but show that the results are equivalent to those obtainable from more us ual predictive formalism. [S1050-2947(99)07811-7].