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Optical control of electrons in single quantum dots

Optical control of electrons in single quantum dots. Semion K. Saikin. University of California, San Diego. V. Support:. Optical Control of electrons in QDs. Single Electron Devices. Quantum Information Processing. Spintronics. Photonics. Devices: D. Gammon, NRL

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Optical control of electrons in single quantum dots

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  1. Optical control of electronsin single quantum dots Semion K. Saikin University of California, San Diego

  2. V Support: Optical Control of electrons in QDs Single Electron Devices Quantum Information Processing Spintronics Photonics Devices: D. Gammon, NRL Spectroscopy: group of D. G. Steel, U. Michigan Theory & Modeling: group of L. J. Sham, UCSD

  3. Content • Semiconductor quantum dot structures: • Design and Applications • Properties of single dots: • Energy levels structure, Spin states • Interaction with light, excitons • Optical Control: • Goal anddevice design • Optical cooling • Single dot switch • Operations with coupled dots • Conclusions

  4. Semiconductor QDs Artificial Atoms Interface fluctuation QDs Self-assembled dots Vertical QD Gated QD 1 mm Koichiro Zaitsu, et al., APL 92, 033101 (2008) D.Gammon, et al., PRL 76, 3005 (1996) NIST website Elzerman et al. Nature 430, 431(2004) 0.1 mm InAs AlGaAs GaAs GaAs Gate depletion Interface imperfections Lattice mismatch Etching

  5. QD devices Present and Future Past Lasers & Optical Amplifiers Photodetectors Low threshold current Weak temperature dependence Adjustable frequency range Broad frequency spectrum High responsivity High T operation Solar Cells Thermoelectric elements High efficiency of photon to electron conversion Control for electron and phonon mobility Single photon sources & modulators Quantum Information Processing Single Electron Memory Ability to control Long relaxation time Slow relaxation Long coherence time Future

  6. Content • Semiconductor quantum dot structures: • Design and Applications • Properties of single dots: • Energy levels structure, Spin states • Interaction with light, excitons • Optical Control: • Goal anddevice design • Optical cooling • Single dot switch • Operations with coupled dots • Conclusions

  7. Energy Levels quantization EC E3e Infra Red Range ~ 1-100 meV E2e DEC E1e electron Near Infra Red/Visible Range Eg~1.25 eV hole frequency  2 E1h DEV E2h EV Visible light Spacing between the energy levels can controlled using different materials or by design!

  8. Spin up e- Spin down ~ 0.1 - 0.5 meV Ee( ) Ee( ) e- Eh( ) Eh( ) ms at T = 1 K and B = 4 T Spin states EC E1e • An intrinsic angular momentum of a quantum particle • Associated magnetic momentum • Interaction with a magnetic field Bx E1h Spin relaxation time: EV Spin states are long lived!

  9. Spin blockade device Example EF EF Different spins – Current is not zero Same spins – Current is blocked Delft University of Technology

  10. Optical Absorption/Emission exciton Exciton relaxation time ~ 0.1 – 1 ns EC E3e DEC E2e E1e Photon hn=DE DE E1h DEV E2h EV

  11. Selection Rules negative exciton EC E3e V DEC E2e E1e Bx Photon Linear polarization, V[1,0,0] E1h DEV E2h EV

  12. Selection Rules negative exciton EC E3e s+ DEC E2e E1e Photon hn=DE s+ Circular polarization E1h DEV E2h EV

  13. Content • Semiconductor quantum dot structures: • Design and Applications • Properties of single dots: • Energy levels structure, Spin states • Interaction with light, excitons • Optical Control: • Goal anddevice design • Optical cooling • Single dot switch • Operations with coupled dots • Conclusions

  14. Goal Ee( ) Ee( ) Eh( ) Eh( ) Control the spin of an electron spin in a single quantum dot FAST, EFFICIENTLY, PRECISELY. EC EV

  15. Setup Quantum dot is empty InAs QD AlGaAs Laser beam AlGaAs n+ GaAs EF mask V QD layer Quantum dot is filled VB EF

  16. Optical probe of single dot pump capture recombination Photoluminescence X2+ X+ X0 X- X2- X. Xu, et. al., PRL 99, 097401 (2007)

  17. Selection Rules H1 V2 V1 Bx H2 H and V – orthogonal linear polarizations

  18. Whenever an electron is in the state flip it to the state. Optical cooling Pump Relaxation Frequency and polarization selection

  19. Optical cooling EC E1e Photon hn=DE V- E1h EV Z

  20. Optical cooling Model meV Relaxation rate: System evolution: Cooling Rate Relaxation rate, G Cooling rate, g Operation precision Rabi frequency, W Relaxation rate, G Prepared state: Precision, P C. Emary, X. Xu, D. Steel, S.Saikin, L. J. Sham, Phys. Rev. Lett. 98, 047401 (2007) time, ns

  21. Optical cooling Pump-probe measurement H2 V2 Pump Probe State preparation efficiency 98.9% H1 Pump Probe V1 X. Xu, et. al., PRL 99, 097401 (2007)

  22. If an electron in the state then flip it to the state and reverse. Single Quantum Dot Switch d detuning Pump1 Pump2 Use frequency and polarization selection

  23. T = 20 ps T = 50 ps T = 100 ps B = 2 T B = 4 T B = 8 T Model Effects of pulse length Optimization of detuning B = 8 T C. Emary, L. J. Sham J. Phys.: Cond. Matter 19, 056203 (2007) Dynamics of an electron state Classical vs. Quantum

  24. If two electrons are in a same state or flip both of them. Operation with electrons in two dots

  25. Origin of interaction Bi-trion binding energy D D ~ 1 meV

  26. Energy levels H(dot1) H(dot2) V(dot2) V(dot1)

  27. Model Dynamics of electrons Pulse timing To minimize incoherent pumping and losses due to relaxation S.Saikin, et. al.,arXiv:0802.1527

  28. Conclusions • An electron in a single quantum dot can be prepared to a given state with precision ~99% on the timescale of 1 nanosecond using resonant optical pumping. • States of a single electron in a QD can be switched coherently on a timescale of 0.1 nanosecond using a Raman process. • Simple logical operations can be designed with coupled quantum dots. The operation timescale is ~ 0.5 nanosecond Thank you!

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