Light-matter interfacing with quantum dots: a polarization tomography approach

Carlos Antón¹, Paul Hilaire¹, Christian Kessler¹, Justin Demory¹, Niccolo Somaschi¹, Carmen Gómez¹, Aristide Lemaître¹, Isabelle Sagnes¹, Oliver Krebs, Norberto-Daniel Lanzillotti-Kimura, Pascale Senellart^1,2 and Loïc Lanco^1,3

¹C2N, CNRS, Université Paris-Saclay, 91460 Marcoussis, France
²Department of Physics, Ecole Polytechnique, F-91128 Palaiseau, France
³University Paris-Diderot, Paris 7, 75205 Paris CEDEX 13, France

The development of quantum networks requires an efficient interface between stationary and flying qubits. A promising approach is a single semiconductor quantum dot (QD) deterministically coupled to a micropillar cavity: such a device performs as a bright single photon emitter [1] as well as an efficient light-matter interface allowing the QD state to be coherently manipulated with few incoming photons [2]. Reciprocally, we have also demonstrated that a giant rotation of photon polarization is induced by a single QD spin qubit [3]. 

Here, we investigate the polarization rotation of coherent light interacting with a QD-cavity system by analysing the photon polarization density matrix in the Poincaré sphere. The superposition of emitted single photons (H-polarized) with reflected photons (V-polarized, see scheme in Fig. 1(a)) leads to a large rotation of the output polarization by 20º both in latitude and longitude [4]. The evolution of the resulting state 𝛼|𝐻 + 𝛽|𝑉 is illustrated in the Poincaré sphere as function of the excitation laser wavelength (λ) scanned across the QD transition wavelength (see Fig. 1(b)). We  demonstrate that the coherent part of the QD emission contributes to polarization rotation, whereas its incoherent part contributes to degrading the polarization purity. This yields crucial information on the ability of our light-matter interface to coherently convert quantum information from a stationary qubit to a flying one.