Quantum spectroscopy with Schrödinger-cat states

• Published in: Nature physics 2011-10, Vol.7 (10), p.799-804
• Main Authors: Kira, M., Koch, S. W., Smith, R. P., Hunter, A. E., Cundiff, S. T.
• Format: Article
• Language: English
• Subjects: Absorption; Atomic; Classical and Continuum Physics; Clusters; Complex Systems; Compressing; Condensed Matter Physics; Entanglement; Excitons; Lasers; Mathematical and Computational Physics; Molecular; Optical and Plasma Physics; Physics; Physics and Astronomy; Quantum physics; Quantum wells; Semiconductors; Spectroscopy; Spectrum analysis; Theoretical; Theoretical analysis; Transformations
• ISSN: 1745-2473; 1745-2481
• DOI: 10.1038/nphys2091

Laser-spectroscopic techniques that exploit light–matter entanglement promise access to many-body configurations. Their practical implementation, however, is hindered by the large number of coupled states involved. Here, we introduce a scheme to deal with this complexity by combining quantitative experiments with theoretical analysis. We analyse the absorption properties of semiconductor quantum wells and present a converging cluster-expansion transformation that robustly projects a large set of quantitative classical measurements onto the true quantum responses. Classical and quantum sources are shown to yield significantly different results; Schrödinger-cat states can enhance the signal by an order of magnitude. Moreover, squeezing of the source can help to individually control and characterize excitons, biexcitons and electron–hole complexes. Experiments that exploit non-classical properties of light promise to provide unique information about many-body systems. The limited availability of non-classical light sources, however, makes their implementation challenging. A method to calculate the quantum-optical response of a material from signals measured by using coherent-light excitation might provide an alternative route.