The best observation yet of the energy-time entanglement of photon pairs has been made by physicists in Canada. The feat was possible because Jean-Philippe MacLean, John Donohue and Kevin Resch of the University of Waterloo could measure the arrival times of photons to sub-picosecond precision.
Entanglement is a result of quantum mechanics that allows the properties of two or more photons (or other tiny particles) to be correlated more strongly than allowed by classical physics. Once seen as a quirky aspect of the quantum world, entanglement is now being used to create practical systems for quantum cryptography and quantum communications.
Most photon-entanglement experiments so far have looked at correlations between the polarizations of pairs of photons, but there are other ways that entanglement can occur. For example, a pair can be entangled in terms of the photons’ energies and the times at which the photons are detected in an experiment.
Physicists have struggled to measure energy-time entanglement because it requires a detector that can pinpoint the detection time of a single photon to a very high degree of precision. Now, MacLean and colleagues have built such a device.
Their experiment begins with a laser pulse being fired at a nonlinear crystal, creating a pair of entangled photons that are sent along two different paths. Each path can measure the energy of the photon or its arrival time – but not both properties at the same time.
The energy of the photon is measured to high precision using a grating-based monochromator. The arrival time is determined using a relatively new technique that resembles strobe photography. It involves combining the photon with a 120 fs-long laser timing pulse in a nonlinear crystal. If the photon and pulse overlap temporally, a higher-energy photon can be emitted from the nonlinear crystal. The detection of this higher-energy photon signals the entangled photon’s overlap with the timing pulse – and hence its arrival time is known to sub-picosecond precision.
By adjusting what each arm measures, the team can determine several different correlations between the properties of pairs of photons. These are correlations between the energies of the photon pairs; correlations between the arrival times of photon pairs; and cross-correlations between the arrival time and energy of photon pairs. These data are then used to calculate a mathematical inequality that should be violated when the photon pairs are entangled – which is exactly what MacLean and colleagues found.
“In the last 10 to 20 years, researchers have been interested in exploring and exploiting energy-time entanglement for communication,” says MacLean. “By being able to measure ultrafast entangled photons, our measurement technique opens the door to exploiting entanglement in a whole new regime.”
The experiment is described in Physical Review Letters.