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Quantum spectroscopy with Schrödinger-cat states


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.

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Figure 1: Wigner functions and single-photon increment (SPI).
Figure 2: Input data for cluster-expansion transformation (CET).
Figure 3: Constructed differential quantum absorption.
Figure 4: Zero-photon increment (ZPI).
Figure 5: Quantum-optical spectroscopy with strong quantum-optical excitations.


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We thank R. P. Mirin, NIST-Boulder, for providing the epitaxially grown GaAs sample. The Marburg part is supported by the Deutsche Forschungsgemeinschaft and the work at JILA was supported by the NSF and NIST. S.T.C. acknowledges support from the Alexander von Humboldt Foundation.

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All authors contributed substantially to this work. The Marburg group is predominantly responsible for the development of the theory whereas the experiments have been performed by the JILA group.

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Correspondence to M. Kira.

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The authors declare no competing financial interests.

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Kira, M., Koch, S., Smith, R. et al. Quantum spectroscopy with Schrödinger-cat states. Nature Phys 7, 799–804 (2011).

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