Quantum spectroscopy of semiconductors with Schr\"odinger's cat states

Quantum investigations on simple systems --- such as atoms or quantized light modes --- have reached a level where one can read and write information directly onto the density matrix itself. Currently, the same level of quantum-information control seems inconceivable in condensed-matter systems simply because the many-body states involved are unimaginably complicated. In this talk, I will present the first steps in realizing targeted access of many-body states within condensed-matter systems by combining quantum-optics and many-body theory [1] with classical high-precision laser spectroscopy. The light--matter interaction has an inherent capability to directly excite targeted many-body states through the light source's quantum fluctuations [2]. The related quantum-optical responses can be projected from the classical data set by applying the cluster-expansion transformation [3] (CET). As a proof of principle, we CET project the measured nonlinear absorption of semiconductor quantum wells [4] into the quantum absorption generated by Schr\"{o}dinger's cat-state sources. The results expose a completely new level of many-body physics that remains otherwise hidden. Especially, the investigations reveal an anomalous reduction of Coulomb scattering of excitons, the excitation-induced narrowing of the exciton-molecule resonance, and the formation of electron--hole complexes (multi-exciton clusters) [5]. \\[4pt] [1] M. Kira and S.W. Koch, {\it Semiconductor quantum optics}, (Cambridge University Press, 2011). \\[0pt] [2] M.~Kira and S.W.~Koch, Phys.~Rev.~A {\bf 73}, 013813 (2006); S.W.~Koch, M.~Kira, G.~Khitrova, and H.M.~Gibbs, Nature Mat.~{\bf 5}, 523 (2006); M.~Kira and S.W.~Koch, Prog.~Quantum Electr.~{\bf 30}, 155 (2006). \\[0pt] [3] M.~Kira and S.W.~Koch, Phys.~Rev.~A {\bf 78}, 022102 (2008). \\[0pt] [4] R.P.~Smith {\it et al.}, Phys.~Rev.~Lett.~{\bf 104}, 247401 (2010). \\[0pt] [5] M. Kira {\it et al.}, Nature Physics {\bf 7}, 799-804 (2011).