Optical control of electrons in single quantum dots Semion K. Saikin University of California, San Diego

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

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 and device design Optical cooling Single dot switch Operations with coupled dots Conclusions

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

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

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 and device design Optical cooling Single dot switch Operations with coupled dots Conclusions

7 Energy Levels quantization E 1e E 2e E 1h E 2h EVEV ECEC ECEC EVEV E 3e ~ meV Spacing between the energy levels can controlled using different materials or by design! electron hole E g ~1.25 eV Near Infra Red/Visible Range frequency  2 Visible light Infra Red Range

8 Spin states e-e- Spin up e-e- Spin down An intrinsic angular momentum of a quantum particle Associated magnetic momentum Interaction with a magnetic field E 1e E 1h EVEV ECEC BxBx ~ meV E e ( ) E h ( ) Spin relaxation time: ms at T = 1 K and B = 4 T Spin states are long lived!

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

10 Optical Absorption/Emission E 1e E 3e E 2e E 1h E 2h EVEV ECEC ECEC EVEV Photon h  E EE Exciton relaxation time ~ 0.1 – 1 ns exciton

11 Selection Rules E 1e E 3e E 2e E 1h E 2h EVEV ECEC ECEC EVEV Photon V Linear polarization, V[1,0,0] BxBx negative exciton

12 Selection Rules E 1e E 3e E 2e E 1h E 2h EVEV ECEC ECEC EVEV Photon h  E   Circular polarization negative exciton

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 and device design Optical cooling Single dot switch Operations with coupled dots Conclusions

14 Goal EVEV ECEC E e ( ) E h ( ) E e ( ) E h ( ) Control the spin of an electron spin in a single quantum dot FAST, EFFICIENTLY, PRECISELY.

15 Setup AlGaAs n + GaAs EFEF AlGaAs InAs QD Quantum dot is empty EFEF Quantum dot is filled VBVB QD layer mask Laser beam V

16 Optical probe of single dot X0X0 X+X+ X-X- X 2- X 2+ X. Xu, et. al., PRL 99, (2007) Photoluminescence pump capture recombination

17 V2V2 V1V1 H1H1 H2H2 H and V – orthogonal linear polarizations Selection Rules BxBx

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

19 Z E 1e E 1h EVEV ECEC Photon h  E V-V- Optical cooling

20 Optical cooling Model Relaxation rate,  Cooling rate,  Rabi frequency,  Relaxation rate,  Precision, P time, ns System evolution: Prepared state: C. Emary, X. Xu, D. Steel, S.Saikin, L. J. Sham, Phys. Rev. Lett. 98, (2007) Relaxation rate:  eV Cooling Rate Operation precision

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

22 Single Quantum Dot Switch If an electron in the state then flip it to the state and reverse. Pump1Pump2  detuning Use frequency and polarization selection

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

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

25 Origin of interaction Bi-trion binding energy   meV

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

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

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!