Quantum nondemolition detection of a propagating microwave photon

Understand

The ability to nondestructively detect the presence of a single, traveling photon has been a long-standing goal in optics, with applications in quantum information and measurement.

• Realising such a detector is complicated by the fact that photon-photon interactions are typically very weak.
• At microwave frequencies, very strong effective photon-photon interactions in a waveguide have recently been demonstrated.
• Here we show how this type of interaction can be used to realize a quantum nondemolition measurement of a single propagating microwave photon.