1. Single Photon Emitters and their use in Quantum Cryptography Presentation by: Bram Slachter Supervision: Dr. Ir. Caspar van der Wal

2. Contents The Ideal single photon emitter Example of their use: Quantum Cryptography in a nutshell Experimental setups Overview of various single photon emitters:  Quantum dot single photon emitters  Quantum ‘well’ single photon emitters*  Molecule single photon emitters*  Colour Centre single photon emitters* Conclusion

3. The ideal single photon emitter Single photon pulses on demand Pulses have identical wavepackets Room temperature operation Easy to create Frequency tuneable

4. The ideal single photon emitter: States of light Maxwell eqs for a cube give EM modes with discrete and polarization EM modes behave as H.O. When quantized these give traditional QM H.O. levels with energy. For these EM modes: well defined and undefined due to number phase Heisenberg minimum uncertainty Classical light (laser light, thermal light) in superposition of these states: (Super)Poissonian

5. The ideal single photon emitter: States of light In reality: ‘infinite’ cube -> quantization becomes continuous -> discrete goes to continuous. Continuous mode excitation now localized in wavepackets with distribution in : Wavepacket excitation still defined by number and phase but also has a distribution

6. The ideal single photon emitter Single photon wavepackets: lowest excitation possible Consecutive wavepackets emitted -> same wavepackets

7. Quantum Cryptography in a Nutshell Modern cryptography: encryption and decryption procedures depend on a secret key This key consists of a randomly chosen string of bits which needs to be shared once in a while: key distribution problem Mathematical solution: public key – private key insecure when quantum computer becomes available Quantum key distribution  Entangled states  Non orthogonal states*

8. Quantum Cryptography in a Nutshell Sender sends a random key with each bit encoded in a random basis Detection basis random for each bit Over a public channel the bases chosen for each bit are compared and the ones with the right bases are kept Randomly chosen part of the remaining key is publicly checked for errors No errors -> safe key established

9. Experimental Setups: Hanbury Brown Twist experiment Determination multiple photon suppression: HBT experiment Calculation: Two photon suppression Santori et al, Nature 419 pg 595 (2002) Classical:

10. Experimental Setups: two photon interference Indistinguishability consecutive photons in experiments -> wavepacket overlap Two photon interference: When two photons enter a 50-50 beam splitter from each side they can only leave together: known as the ‘bunching’ of photons non entangled input:

11. Experimental Setups: two photon interference Santori et al, Nature 419 pg 595 (2002)

12. Overview Single Photon Emitters: Quantum Dot SPE Semiconductor quantum dot  Discrete levels  Charging effects Created by MBE, Etching and E-beam Excited with a laser: 1) Santori et al, Nature 419 pg 595 (2002) 2) Michler et al, Science 290 pg 2282 (2000) 1) 2)

13. Overview Single Photon Emitters: Quantum Dot SPE Semiconductor quantum dot  Discrete levels  Charging effects Excited with a laser. Charging effects used for single photon selection Michler et al, Science 290 pg 2282 (2000)

14. Overview Single Photon Emitters: Quantum Dot SPE Wavepacket overlap by two photon interference 0.7-0.8. Problem: Room temperature operation hard due to optical phonon emission in the bulk Performance reasonable: lifetime limited Big advantage: electrical excitation possible with p- i-n junction with quantum dots in intrinsic region. Yuan et al, Science 295, pg 102 (2002)

15. Overview Single Photon Emitters: Quantum Well SPE Post structures created with MBE, E- Beam Lithography and plasma etching Uses simultaneous Coulomb blockade for electrons and holes Intrinsic quantum well separated by tunnel barriers from an n- and p-doped quantum well lying in a host material Operating at 20 mK Kim et al, Nature 397, pg 500 (1999)

16. Overview Single Photon Emitters: Quantum Well SPE

17. Frequency controlled current Conductance quantization Kim et al, Nature 397, pg 500 (1999)

18. Overview Single Photon Emitters: Quantum Well SPE No HBT experiment but those are probably pretty good. Room temperature operation hard:  Smaller quantum dots needed -> bigger energy spacing and coulomb effects  Higher potential barriers to suppress non radiative decay

19. Overview Single Photon Emitters: Molecule SPE Laser targeted at a single molecule: Laser light filtered Highly Fluorescent and temperature stable molecules needed

20. Overview Single Photon Emitters: Molecule SPE Molecules have been reported which work at room temperature. Lounis & Moerner, Nature 407, pg 491 (2000) Reasonable two photon suppression but not always easy to process

21. Overview Single Photon Emitters: Molecule SPE Also a setup possible based on adiabatic following: Brunel et al, Phys. Rev. Lett. 83, pg 2722 (1999)

22. Overview Single Photon Emitters: Colour Centre SPE Same 4 level principle as before Diamond nanocrystals grown from diamond powder. Nitrogen impurities naturally present By electron bombardment vacancies produced which move next to nitrogen impurities by annealing Nitrogen-Vacancy colour centre produced Reasonable two photon suppression and room temperature stable Can be spincoated but the difficultly of targeting the nanocrystals remains

23. Conclusion All structures in principle capable of producing room temperature stable ‘ideal’ SPE All structures have their drawbacks:  Quantum dot/well SPE have a fight against non radiative decay  Molecule/NV Colour Centre SPE less easy to process but have already been proven to work at RT Of all these structures NV Colour Centre looks most easiest to implement