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Ultrabright solid state sources of quantum light

Pascale Senellart

A semiconductor quantum dot (QD) is a promising system to achieve a source of single photons or of entangled photon pairs. The main advantage of QD based sources over attenuated classical sources is the possibility to generate exactly one photon for each excitation pulse. However, to be of practical value, one needs to efficiently extract and collect the photons emitted by the QD. Accelerating the QD spontaneous (Purcell effect) is a way to make sure that the photons emitted by the QD are funnelled into a cavity mode. This idea has motivated many works to fabricate coupled QD-cavity devices. For the last few years, the main challenge has been to control both the spectral and spatial matching between a single QD and a cavity mode. We have developed an in-situ lithography technique that allows deterministically coupling a single QD to a pillar cavity mode1 and to fabricate bright single photon sources.

Using the same technique, we recently fabricated a bright single photon source by coupling a QD to a confined hybrid photon-plasmon mode. We show that discrete optical modes can be obtained simply by depositing a 50 nm thick microdisk of gold on an interferential mirror (fig a). The field is confined at the interface with the metal, below the disk area, with no further processing needed. A four-fold acceleration of spontaneous emission into the mode together with an inhibition by a factor 40 of emission in all other mode ensures efficient funnelling of the quantum dot emission into the cavity mode

The radiative recombination of two electron-hole pairs trapped in a QD can also lead to the emission of entangled photon pairs2. Extracting polarization entangled photon pairs requires achieving simultaneous Purcell effect for both optical transitions of the QD as well as gathering several conditions to make sure that the Purcell effect is not detrimental to the entanglement. We show that a photonic molecule consisting in two coupled identical pillars gathers all these requirements. By deterministically inserting a single QD into a photonic molecule, we fabricate the brightest source of entangled photon pairs to date. We also show that the Purcell effect allows increasing the degree of entanglement of the photon pairs.

[1] A. Dousse et al., Phys. Rev. Lett. 101, 267404 (2008)
[2] A. Dousse et al, Nature 466, 217 (2010).