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Compact source of polarization-entangled photon pairs

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We present a compact source of polarization-entangled photon pairs at a wavelength of 805 nm using a violet single-mode laser diode as the pump source of type-II spontaneous parametric down-conversion. The source exhibits entanglement and pair-rate comparable to conventional systems utilizing large frame ion lasers thus significantly increases the practicality of novel quantum information or quantum metrology applications.

©2004 Optical Society of America

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Figures (4)

Fig. 1.
Fig. 1. Schematic set-up of the source. The single-mode cw laser diode operating at 402.6 nm pumps a 2 mm thick BBO crystal and produces polarization-entangled photon pairs at 805.2 nm collected into single-mode fibres. For compensation of the walk-off, two additional BBO crystals preceded by a half-wave retarder are used.
Fig. 2.
Fig. 2. Single (oe-12-2-276-i001) and coincidence (oe-12-2-276-i002) count rates depending on the pump power measured right behind the violet laser diode.
Fig. 3.
Fig. 3. Spectral distributions of the down-converted light collected into single-mode fibres. Both photons of the pair have nearly the same wavelength with separation only 0.22 nm around the degeneracy wavelength of 805.2 nm. The least-square Gaussian fit (solid lines) to measured data points shows FWHMs of 6.03±0.14 nm (oe-12-2-276-i003) and 6.24±0.56 (oe-12-2-276-i004), respectively. The offset of 44000 counts is due to the dark counts of the Si APDs.
Fig. 4.
Fig. 4. Polarization correlations between photons of a pair measured in the H/V (oe-12-2-276-i005) and +45°/-45° (oe-12-2-276-i006) polarization basis. The visibilities obtained from a sin2 fit (solid lines) to the measured coincidence count rates are 98.3±0.1% and 94.3±0.2%, respectively.
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