Quantum Light Sources

A step towards a quantum internet based on optical fibers would rely on quantum repeater networks. For this architecture, quantum memories connected by entangled photons are necessary. One proposal to overcome this challenge involves sources of entangled pairs of photons, one in resonance with the quantum memory transition and the other at telecom wavelength, where the loss in fibers are minimized. This would allow storing one of the photons while the other travels over long distances to an intermediate station to interact with another photon from a second pair produced in a neighbouring mode.

In our group, the system of choice as solid-state quantum memory is the Y2SiO5 crystal doped with rare earth ions of Pr3+ that has demonstrated to be a good candidate for such a role. In order to have photons compatible with Pr3+ optical transitions, we need a photon pair source with at least one of the photons at 606 nm and with a linewidth smaller than 4MHz.

To fulfil such a requirements we embed a PPLN crystal inside a more than 1 meter long optical cavity. If properly pumped with coherent light, it will generate via spontaneous parametric down conversion a pair of photons, one at 606 nm and the other at 1436 nm. With a sophisticated double lock system, we make sure that both photons are resonant with the cavity [1] [2]. This provides an enhancement of the photon pair production at specific frequencies and photon linewidths as narrow as the cavity linewidth.

One of the efforts of this experiment goes in the direction of storing in the Pr3+ quantum memory this heralded single photons, aiming for long storage times and high storage efficiency [3] [4]. On the other hand, a consequence of having the cavity resonant with both the photons is that our single photon pair are produced in a superposition of many frequencies (i.e. all the frequencies compatible with both wavelength resonances at the same time). This brings us many interesting possibilities. The main work where we take advantage of this feature is the so called “Frequency-bin entanglement” [5].

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