  The process of heralded noiseless amplification, and the inverse process of
heralded noiseless attenuation, have potential applications in the context of
quantum communications. Although several different physical implementations of
heralded noiseless amplifiers have now been demonstrated, the research on
heralded noiseless attenuators has been largely confined to a beam-splitter
based approach. Here we show that an optical parametric amplifier (OPA),
combined with appropriate heralding, can also serve as a heralded noiseless
attenuator. The counterintuitive use of an optical amplifier as an attenuator
is only possible due to the probabilistic nature of the device.

Quantum Physics