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Observation of entanglement between a quantum dot spin and a single photon


Entanglement has a central role in fundamental tests of quantum mechanics1 as well as in the burgeoning field of quantum information processing. Particularly in the context of quantum networks and communication, a main challenge is the efficient generation of entanglement between stationary (spin) and propagating (photon) quantum bits2. Here we report the observation of quantum entanglement between a semiconductor quantum dot spin and the colour of a propagating optical photon. The demonstration of entanglement relies on the use of fast, single-photon detection, which allows us to project the photon into a superposition of red and blue frequency components. Our results extend the previous demonstrations of single-spin/single-photon entanglement in trapped ions3, neutral atoms4,5 and nitrogen–vacancy centres6 to the domain of artificial atoms in semiconductor nanostructures that allow for on-chip integration of electronic and photonic elements7,8. As a result of its fast optical transitions and favourable selection rules, the scheme we implement could in principle generate nearly deterministic entangled spin–photon pairs at a rate determined ultimately by the high spontaneous emission rate. Our observation constitutes a first step towards implementation of a quantum network with nodes9 consisting of semiconductor spin quantum bits10,11,12.

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Figure 1: Quantum dot transitions and experimental set-up.
Figure 2: Measurement of classical spin–photon correlations.
Figure 3: Demonstration of quantum correlation between the electron spin and the emitted single-photon pulse.


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We acknowledge discussions with W. Chin, M. Kroner, A. Srivastava, J. Elzerman, A. Reinhard, T. Volz, P. Maletinsky and D. Gershoni. This work is supported by NCCR Quantum Science and Technology, a research instrument of the Swiss National Science Foundation; the Swiss NSF (grant no. 200021-140818); an ERC Advanced Investigator Grant (A.I.); and a Marie Curie International Incoming Fellowship within FP7 (W.B.G.).

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Correspondence to W. B. Gao or A. Imamoglu.

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The authors declare no competing financial interests.

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Gao, W., Fallahi, P., Togan, E. et al. Observation of entanglement between a quantum dot spin and a single photon. Nature 491, 426–430 (2012).

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