The number and quality of the generated photon pairs depend strongly on the used pump laser diode. Therefore it is necessary to select it according the ordering options. For a stabilized, narrow-bandwidth laser diode, a measured coincidence rate of C=500/(s*mW) at an overall visibility V>98% can be achieved.

The EPR-source is optimized to a length of the nonlinear BBO-crystal of 4mm, cut in a type-II configuration. The spectral width of the entangled photons is less than 10nm.

I) Influence from the coherence length of a pump laser

The wavelength stabilisation has two positive effects: In contrast to free running laser diodes (low coherence length) the wavelength is not determined by the laser diode itself and therefore not temperature-dependent. For generation of (entangled) photon pairs the complex set of energy conservation and birefringent phase-matching conditions leads to an increase of the fraction of non-entangled pairs, increased singles rate without a corresponding pair photon and a decreased measured visibility for a changed pump wavelength. Secondly, only if the coherence length is much longer than the optical crystal, coherent superposition is guaranteed for all generated pairs and so leading to a constant phase with high reachable visibility.

II) Considerations regarding the generated pair rate

In the case of our source, every mW of incident pump for a single-longitudinal-mode diode-laser at 405nm approx. generate approximately N=12.500 pairs per second within the nonlinear crystal. In principle they are thermally distributed, but due to the short coherence length tp ≈ 2ps. the product (N*tp <<1), a common Poisson distribution can be assumed. A coupling efficiency of nc=50% in combination with a detector efficiency of nd=40% can be achieved yielding an overall efficiency of neff=nc*nd=20% Therefore the typical rate for single detection in both arms is S1,2 = neff *N = 2,5kcounts per second per mW. Unfortunately not the same 20% of the generated photons will be detected on the other side, but the detection on the second side is statistically independent, therefore only 20% of the singles are coincidences. We expect C≈500 counts measured coincidence pairs per second per mW incident pump power. The quality of entangled photon pairs are excellent with a visibility V>98%.

For a free-running laser-diode, its spectrum is broader and the generated number of pairs is reduced to C≈200 counts/(s*mW) and their visibility is about V>94%, still far enough to violate Bell’s inequalities.