Single Photon Emitters and their use in Quantum Cryptography

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: Classical: Two photon suppression Santori et al, Nature 419 pg 595 (2002)

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) 2) • Santori et al, Nature 419 pg 595 (2002) • Michler et al, Science 290 pg 2282 (2000)

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. Overview Single Photon Emitters:Quantum Well SPE • 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. • Reasonable two photon suppression but not always easy to process Lounis & Moerner, Nature 407, pg 491 (2000)

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