Optical control of electrons in single quantum dots Semion K. Saikin University of California, San...
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Transcript of Optical control of electrons in single quantum dots Semion K. Saikin University of California, San...
Optical control of electronsin single quantum dots
Semion K. Saikin
University of California, San Diego
2
Optical Control of electrons in QDs
Quantum InformationProcessing
PhotonicsSingle 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 QDsArtificial Atoms
Vertical QD
Elzerman et al. Nature 430, 431(2004)
Gated QDSelf-assembled
dots
1 m
Interfacefluctuation QDs
D.Gammon, et al.,PRL 76, 3005 (1996)
NIST websiteKoichiro Zaitsu, et al.,APL 92, 033101 (2008)
Lattice mismatch
GaAs
InAs
GaAs
AlGaAs
Interfaceimperfections
GatedepletionEtching
0.1 m
5
QD devicesPresent and Future
Lasers & Optical Amplifiers Photodetectors
Solar Cells
Thermoelectric elements
Single Electron Memory
Single photon sources & modulators
Quantum Information Processing
Future
Past
Low threshold currentWeak temperature dependenceAdjustable frequency range
Broad frequency spectrumHigh responsivityHigh T operation
High efficiency ofphoton to electronconversion
Slow relaxationLong coherence time
Control for electron and phonon mobility
Ability to controlLong 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 Levelsquantization
E1e
E2e
E1h
E2hEV
EC
EC
EV
E3e~ 1-100 meV
Spacing between the energy levels can controlledusing different materials or by design!
electron
hole
Eg~1.25 eV Near Infra Red/Visible Range
frequency 2
Visible light
Infra Red Range
8
Spin states
e-
Spin up
e-
Spin down
• An intrinsic angular momentum of a quantum particle
• Associated magnetic momentum
• Interaction with a magnetic field
E1e
E1h
EV
EC
Bx
Sμ BgμBH
~ 0.1 - 0.5 meVEe( )
Ee( )
Eh( )
Eh( )Spin relaxation time:
20sT ms at T = 1 K and B = 4 T
Spin states are long lived!
9
Spin blockade deviceExample
EF
EF
Different spins – Current is not zeroDelft University of Technology Same spins – Current is blocked
10
Optical Absorption/Emission
E1e
E3e
E2e
E1h
E2hEV
EC
EC
EV
Photon hEE
Exciton relaxation time ~ 0.1 – 1 nsexciton
11
Selection Rules
E1e
E3e
E2e
E1h
E2hEV
EC
EC
EV
Photon
V
Linear polarization, V[1,0,0]
Bx
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
EV
EC
Ee( )
Eh( )
Ee( )
Eh( )
Control the spin of an electron spin in a single quantum dot FAST, EFFICIENTLY, PRECISELY.
15
Setup
AlGaAs
n+ GaAs
EF
AlGaAs
InAs QD
Quantum dot is empty
EF
Quantum dot is filled
VB
QD layer
mask
Laser beam
V
16
Optical probe of single dot
X0
X+
X- X2-
X2+
X. Xu, et. al., PRL 99, 097401 (2007)
Photoluminescence
pump
capture
recombination
18
Optical cooling
RelaxationPump
Whenever an electron is in the state flip it to the state.
Frequency and polarization selection
20
Optical coolingModel
Rel
axat
ion
rate
,
Coo
ling
rate
,
Rabi frequency, Relaxation rate,
Pre
cisi
on,
P
time, ns
System evolution:
}{],[)( LHitt
tePPPtP γ))()(()()( 0
Prepared state:
C. Emary, X. Xu, D. Steel, S.Saikin, L. J. Sham, Phys. Rev. Lett. 98, 047401 (2007)
Relaxation rate: 21. eV
Cooling Rate
Operation precision
21
Optical coolingPump-probe measurement
State preparation efficiency 98.9%
PumpH2 V2
Probe
PumpH1
V1Probe
X. Xu, et. al., PRL 99, 097401 (2007)
22
Single Quantum Dot Switch
If an electron in the state then flip it to the state and reverse.
Pump1 Pump2
detuning
Use frequency and polarization selection
23
Model
Optimization of detuning Effects of pulse length
B = 8 T
T = 20 psT = 50 psT = 100 ps
B = 2 TB = 4 TB = 8 T
-600 -400 -200 0 200 400 600
0.0
0.2
0.4
0.6
0.8
1.0
Ele
ctro
n st
ate
time (ps)
Dynamics of an electron state
Classical vs. Quantum
C. Emary, L. J. Sham J. Phys.: Cond. Matter 19, 056203 (2007)
27
t4t3t2t1
E4
E5
E3
E1
0
ener
gy
time
0
E2
H(t)V(t)
ener
gy
Pulse timing
Model
To minimize incoherent pumping and losses due to relaxation
Dynamics of electrons
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40.0
0.2
0.4
0.6
0.8
1.0
Ele
ctro
ns
sta
te
pulse width (ns)
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!