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SPDC – Spontaneous parametric down-conversion

Animated Entangled Photon Source

LaseriThe pump laser diode emits light with a wavelength of λ = 405 nm and vertical polarization.
HWPiThe half waveplate rotates the polarization of linearly polarized light. Here, the vertical pump light can be rotated to +45° polarized light.
YVOiThe group-velocity mismatch between the pump and the down-conversion light in the birefringent BBO crystals causes the photon pairs born in the first crystal to be advanced with regard to those originating from the second. This birefringent YVO4 pre-compensation crystal negates this effect.
YVOiThe photons originating from the first BBO crystal experience higher dispersive delay due to their pass through the second BBO crystal. Consequently, a compensation using another birefringent YVO4 crystal has to be applied.
BBOiThe optical axis of the second BBO crystal is oriented at a 90° angle in comparison to the optical axis of the first crystal. This makes the process of down-converting horizontal pump photons to vertical output photon pairs possible.
BBOiA BBO crystal is a highly non-linear medium, making spontaneous parametric down conversion possible. Conservation of energy and momentum hold when a pump photon (vertically polarized) is converted into two output photons (horizontally polarized) with half the energy at λ = 810 nm.

Optical FibersiThe fiber couplers spatially select the photons to be detected. For best entanglement quality, their position and orientation has to be fine tuned such that photons from both BBO crystals are coupled with the same probability.

Dirac Notation:|V⟩|V⟩+|H⟩|V⟩|V⟩|V⟩+|H⟩|H⟩|HH⟩|VV⟩|VV⟩+|HH⟩|VV⟩|HH⟩

SPDC generates entangled photon pairs

The heart of the quED and Quantenkoffer is made out of β-Barium-Borate (BBO), a special optical non-linear crystal. A high power UV diode laser at 405 nm wavelength, called the pump laser, is focused in this crystal. If the polarization of the pump beam and the axis of the BBO crystal are matched in a way enabling energy and momentum conservation, some of the pump photons are converted into two lower energy near infrared photons at 810 nm. These down con-verted photons then emerge at opposite sides of a so-called emission cone and form a photon pair.

Even though the conversion rate is low (only about 1 of pump photons is converted), these photon pairs are quite useful, since whenever you observe a photon on one side, you know that there must be one on the other side, too! Therefore, you call them heralded single photons, and they can then be used in further experiments.

It becomes even more interesting if you add another BBO crystal with the optical axis perpendicular to the first one, condition the pump laser polarization by inserting a half wave plate (HWP) and compensate some temporal shifts and dispersion effects by two Ytterbium Vanadate (YVO) crystals: The photon pairs from the two crystals are coherently overlapped, so one cannot distinguish between pairs originated from the first or second crystal.

This is the condition for creating polarization entanglement between the photons of one such pair.

See also the brochure of the quED for more information.