Detecting Energy-Time Entanglement

Two photons can be entangled in multiple ways, including through their energies and emission times. Such energy-time entanglement can mean, for instance, that the measured frequency of one entangled photon can be used to determine when its partner will arrive at a separate detector. And since this type of entanglement is robust over long distances, it could be useful for quantum information applications. But observing energy-time entanglement directly has proved tricky because experiments have lacked the necessary resolution in both the time and frequency domains. Jean-Philippe MacLean and his colleagues from the University of Waterloo in Canada overcome this problem with an ultrafast single-photon detection system that can measure the quantum correlations between energy-time entangled photons.