1. Entangled Photons Anneke Batenburg

2. Overview Entanglement – basics
Making entangled photons by `parametric down conversion’ (Kwiat et al.)
Entanglement after conversion to surface plasmons (Altewischer et al.)

3. Entanglement - basics Hilbert space of two systems: , basis:
Most general state of two systems:
Product state (separable state):

4. Entanglement - basics
Not all states of the two systems can be represented as product states. States that can’t be factored into products of two single-particle states are
entangled states.
Example: two spin-1/2 particles in singlet state:
If Observer 1 measures `+’, Observer 2 measures `-’
If Observer 1 measures `-’, Observer 2 measures `+’

5. Making entangled photons by `parametric down conversion’ (Kwiat et al
Where the cones overlap, the photons are in an entangled state:

6. Making entangled photons
States that can be produced:

7. Making entangled photons
Coincidence fringes:
Visibility of coincidence interference pattern is a measure of entanglement quality.
Visibility: (97.8 ± 1.0)%
Coincidence count rate >1500 s-1

8. Entanglement after a conversion to surface plasmons (Altewischer et al

9. Entanglement after a conversion to SP’s

10. Entanglement after a conversion to SP’s
Plasmon: Quasiparticle resulting from a collective oscillation of a free electron gas.
Surface plasmons occur at interfaces of materials with positive dielectric constant (air, glass) with materials with negative dielectric constant (metals)
A typical SP results from collective movement of 1010 electrons...

11. Entanglement after a conversion to SP’s+ - + -
tunneling
photon
photon SP SP
metal hole array

12. Entanglement after a conversion to SP’s
Etanglement survives the photon → SP → photon conversion!
(though with much smaller coincidence count rate)

13. Entanglement after a conversion to SP’s
Entanglement is only partly removed when one of the beams is focused with a confocal telescope.

14. Entanglement after a conversion to SP’s
SP’s show `quantum’ behaviour, even though they are quite `large’
Combination of research fields: nanostructured metal optics and quantum information

15. Questions?