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Generation of ultraviolet entangled photons in a semiconductor


Entanglement is one of the key features of quantum information and communications technology. The method that has been used most frequently to generate highly entangled pairs of photons1,2 is parametric down-conversion. Short-wavelength entangled photons are desirable for generating further entanglement between three or four photons, but it is difficult to use parametric down-conversion to generate suitably energetic entangled photon pairs. One method that is expected to be applicable for the generation of such photons3 is resonant hyper-parametric scattering (RHPS): a pair of entangled photons is generated in a semiconductor via an electronically resonant third-order nonlinear optical process. Semiconductor-based sources of entangled photons would also be advantageous for practical quantum technologies, but attempts to generate entangled photons in semiconductors have not yet been successful4,5. Here we report experimental evidence for the generation of ultraviolet entangled photon pairs by means of biexciton resonant RHPS in a single crystal of the semiconductor CuCl. We anticipate that our results will open the way to the generation of entangled photons by current injection, analogous to current-driven single photon sources6,7.

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Figure 1: Schematic diagram of the resonant hyper-parametric scattering (RHPS) via biexciton.
Figure 2: Experimental set-up to measure the photon correlation of the RHPS. A sample (CuCl thin crystal) was mounted in the cryostat, and its temperature was kept at 4 K.
Figure 3: Emission spectrum of the CuCl crystal at 4 K.
Figure 4: Photon correlation histogram between the photons emitted via the RHPS without polarization analysers.
Figure 5: Photon correlation histogram for linear polarization combinations HH, HV, VH, VV, DD, D , D, and , are shown where the first and second letters represent the polarizations of the two photons involved.
Figure 6: Graphical representation of the two-photon polarization density matrix reconstructed from the photon correlation measurements for 19 polarization combinations.


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We thank M. Hasegawa for his help in preparing samples. K.E. is grateful to P. G. Kwiat, M. Kuwata-Gonokami and H. Ishihara for discussions. This work was supported in part by the programme “Strategic Information and Communications R & D Promotion Scheme” of the Ministry of Public Management, Home Affairs, Posts and Telecommunications of Japan.

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Correspondence to Keiichi Edamatsu.

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Edamatsu, K., Oohata, G., Shimizu, R. et al. Generation of ultraviolet entangled photons in a semiconductor. Nature 431, 167–170 (2004).

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