Broadband energy-entangled photon for high resolution temporal sensing and for quantum information
Broadband energy-time entangled photon pairs are produced by pumping a non-linear crystal with a cw laser. Because of their quantum nature, they exhibit at the same time narrowband and short time features. Indeed the sum energy of both photons is equal to the well defined energy of the pump photon, whereas the correlation time between the two photons is of the order of few tens of femtoseconds. Those properties can be used for measurements beyond the capabilities of classical devices.
Here we make use of those features to study the temporal properties of photons through various media. The propagation of the entangled two-photon quantum states is described by a temporal wavefunction which is comparable for certain aspects to the one of coherent ultrashort laser pulses. However, because this light is in a continuous way regime, femtosecond timing can be performed without relying on ultrashort laser pulses of high intensities. As application, we show a proof of principle experiment where ultrafast optical coincidences of the photon pairs allow selecting only the ballistic photons for imaging through a scattering medium. Using techniques from the ultrafast optics, we are able to manipulate the two-photon wave function with the help of a pulse shaper and reconstruct the dispersion properties of a sample. Ultimately the precise temporal control of the two photon wavefunction can lead to the implementation of proposals for two-photon spectroscopy with entangled photons, allowing to reveal new properties of the investigated molecules.
Finally we present how the same shaping method of the wavefunction allows to encode high dimensional quantum information in the spectrum of the photons and to realize quantum information protocols.