Stanford Synchrotron Radiation Lightsource An Office of Science User Facility

Precision measurements could be brought to their ultimate limit by harnessing the principles of quantum mechanics. Particularly useful, to this end, are multi-particle entangled states of the “Schrödinger’s cat” type, known as NOON states. Photonic NOON states with N entangled photons acquire a phase at a rate N times faster than classical light, and they can be used to obtain high-precision phase measurements and super-resolving imaging, becoming more and more advantageous as the number of photons grows. However, such large-N states are notoriously difficult to create. We have generated “high-NOON” states via the multiphoton interference of “quantum” down-converted light with a “classical” coherent state. Super-resolving phase measurements with up to five entangled photons were demonstrated with a visibility higher than obtainable without entanglement. Most importantly, this approach is inherently scalable to arbitrarily high photon numbers, with implications to several areas of quantum measurements.