  Detecting traveling photons is an essential primitive for many quantum
information processing tasks. We introduce a single-photon detector design
operating in the microwave domain, based on a weakly nonlinear metamaterial
where the nonlinearity is provided by a large number of Josephson junctions.
The combination of weak nonlinearity and large spatial extent circumvents
well-known obstacles limiting approaches based on a localized Kerr medium.
Using numerical many-body simulations we show that the single-photon detection
fidelity increases with the length of the metamaterial to approach one at
experimentally realistic lengths. A remarkable feature of the detector is that
the metamaterial approach allows for a large detection bandwidth. In stark
contrast to conventional photon detectors operating in the optical domain, the
photon is not destroyed by the detection and the photon wavepacket is minimally
disturbed. The detector design we introduce offers new possibilities for
quantum information processing, quantum optics and metrology in the microwave
frequency domain.
