  So far, quantum-limited power meters are not available in the microwave
domain, hindering measurement of photon number in itinerant quantum states. On
the one hand, single photon detectors accurately detect single photons, but
saturate as soon as two photons arrive simultaneously. On the other hand, more
linear watt meters, such as bolometers, are too noisy to accurately detect
single microwave photons. Linear amplifiers probe non-commuting observables of
a signal so that they must add noise and cannot be used to detect single
photons, either. Here we experimentally demonstrate a microwave
photon-multiplication scheme which combines the advantages of a single photon
detector and a power meter by multiplying the incoming photon number by an
integer factor. Our first experimental implementation achieves a n = 3-fold
multiplication with 0.69 efficiency in a 116 MHz bandwidth up to a input photon
rate of 400 MHz. It loses phase information but does not require any dead time
or time binning. We expect an optimised device cascading such multipliers to
achieve number-resolving measurement of itinerant photons with low dark count,
which would offer new possibilities in a wide range of quantum sensing and
quantum computing applications.
