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 -gt quantization
  becomes continuous -gt 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 -gt 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 -gt safe key established

9
Experimental SetupsHanbury 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 -gt 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 EmittersQuantum 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 EmittersQuantum 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 EmittersQuantum 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 EmittersQuantum 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 EmittersQuantum Well SPE
17
Overview Single Photon EmittersQuantum Well SPE

• Frequency controlled current
• Conductance quantization

Kim et al, Nature 397, pg 500 (1999)
18
Overview Single Photon EmittersQuantum Well SPE

• No HBT experiment but those are probably pretty
  good.

• Room temperature operation hard
• Smaller quantum dots needed -gt bigger energy
  spacing and coulomb effects
• Higher potential barriers to suppress non
  radiative decay

19
Overview Single Photon EmittersMolecule SPE

• Laser targeted at a single molecule

• Laser light filtered

• Highly Fluorescent and temperature stable
  molecules needed

20
Overview Single Photon EmittersMolecule 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 EmittersMolecule SPE

• Also a setup possible based on adiabatic
  following

Brunel et al, Phys. Rev. Lett. 83, pg 2722 (1999)
22
Overview Single Photon EmittersColour 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