Progress Report: Tools for Quantum Information Processing in Microelectronics ARO MURI (Rochester-Stanford-Harvard-Rutgers) Third Year Review, Harvard University, February 25-26, 2001 C. M. Marcus, Harvard University http://marcuslab.harvard.edu 1) Understanding (finally) how 0.7 structure in quantum point contacts can be used as a natural spin system. 2) First results on multiple point contact systems - toward spin entangled chains. 3) Using a quantum dot as a gate-tunable spin filter. First experiments. 4) The next steps.
30
Embed
Progress Report: Tools for Quantum Information Processing in Microelectronics
Progress Report: Tools for Quantum Information Processing in Microelectronics ARO MURI (Rochester-Stanford-Harvard-Rutgers) Third Year Review, Harvard University, February 25-26, 2001 C. M. Marcus, Harvard University http://marcuslab.harvard.edu - PowerPoint PPT Presentation
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Progress Report:Tools for Quantum Information Processing in
Microelectronics
ARO MURI (Rochester-Stanford-Harvard-Rutgers)
Third Year Review, Harvard University, February 25-26, 2001C. M. Marcus, Harvard University
http://marcuslab.harvard.edu
1) Understanding (finally) how 0.7 structure in quantum point contacts can be used as a natural spin system.
2) First results on multiple point contact systems - toward spin entangled chains.
3) Using a quantum dot as a gate-tunable spin filter. First experiments.
4) The next steps.
Quantized Conductance
(data from vanWees, 1988)
In-plane magnetic field dependence
temperature dependence
0.7 feature gets stronger at higher temperatures!
3
2
1
0-1 0 1
Vsd (mV)
T = 75 mK, B|| = 8 T3
2
1
0-1 0 1
Vsd (mV)
T = 75 mK, B|| = 0 T
3
2
1
0-1 0 1
Vsd (mV)
T = 600 mK, B|| = 0 T T=80mK B=8TT = 0.6K B=0T = 80mK B=0
g g g
Vsd Vsd Vsd
Nonlinear Transport
3
2
1
0-1 0 1
Vsd (mV)
T = 75 mK, B|| = 8 T3
2
1
0-1 0 1
Vsd (mV)
T = 75 mK, B|| = 0 T
3
2
1
0-1 0 1
Vsd (mV)
T = 600 mK, B|| = 0 T T=80mK B=8TT = 0.6K B=0T = 80mK B=0
g g g
Vsd Vsd Vsd
Nonlinear Transport
3
2
1
0-1 0 1
Vsd (mV)
T = 75 mK, B|| = 8 T3
2
1
0-1 0 1
Vsd (mV)
T = 75 mK, B|| = 0 T
3
2
1
0-1 0 1
Vsd (mV)
T = 600 mK, B|| = 0 T T=80mK B=8TT = 0.6K B=0T = 80mK B=0
g g g
Vsd Vsd Vsd
Nonlinear Transport
3
2
1
0-1 0 1
Vsd (mV)
T = 75 mK, B|| = 8 T3
2
1
0-1 0 1
Vsd (mV)
T = 75 mK, B|| = 0 T
3
2
1
0-1 0 1
Vsd (mV)
T = 600 mK, B|| = 0 T T=80mK B=8TT = 0.6K B=0T = 80mK B=0
g g g
Vsd Vsd Vsd
Nonlinear Transport
quantum dot quantum point contact
gate
2DEG2DEG
gate
Lifting spin degeneracy due to interactions
Kondo Effect in Metals
Kondo Effect in Quantum Dots
Kondo Effect in Quantum Dots
Cronenwett, et al (Delft)
Now, back to our quantum point contact
Kondo-like scaling in a quantum point contact
Kondo Temperature and Transport Features
In-Plane FieldDependence ofZero Bias Anomaly
g
Vsd
B|| = 0
B|| = 3T
B|| = 8T
quantum dot quantum point contact
Charging energy lifts spin degeneracy. Kondo effect results from interaction ofunpaired state with leads.
Interaction energy lifts spin degeneracy. Kondo effect results from interaction ofunpaired mode with bulk 2DEG.
gate
2DEG2DEG
gate
entanglement of 1 and 2
propagation ofentanglement
exact numericalfor N=31
long-chain limit
2 m
KONDO
A single quantum point contact acts as a free spin with a Kondo-like screening cloud at low temperature
what happens when more than one point contact are in proximity?
2 m
RKKY
KONDO
KONDO
Depending on parameters, the quasibound spins should become entangled with each other, mediated by conduction electrons.
This is the famous RKKY interaction, the physical effectthat gives rise to spin glasses in 2D and 3D.
2 m
RKKY
RKKY
RKKY
KONDO
KONDO
KONDO
KONDO
We can use this to construct spin chains with controllable local Kondo temperatures
B||
first experimental results:two point contacts in series
striking dependence onin-plane magnetic fieldindicates spin-related effect,but they are not understood.
Point contact at 1e2/h plateau as spin detector
B|| = 8T
N even to N oddS→ +1/2S
N odd to N evenS→ -1/2S
Aligned spins transmitted - Anti aligned spins transmitted
B||
A spin separatorand spin-bridge detector
2) quantum dot as gate-tunable spin filter
1 m
2.5
2.0
1.5
1.0
0.5
-200 -100 0 100 200Gate Voltage (mV)
Vg(mV)
g (e
2 /h)
0.25
0.20
0.15
0.10
0.05
0.00403020100
Gate Voltage (mV)
g (e
2 /h)
Vg(mV)
First Data on Spin Injection and Detection from a QD
Telectron~150mKBparallel = 7T
0.25
0.20
0.15
0.10
0.05
0.00403020100
Gate Voltage (mV)
200x10-9
150
100
50
0
403020100Gate Voltage (mV)
gQPC ~ 1e2/h
0.25
0.20
0.15
0.10
0.05
0.00403020100
Gate Voltage (mV)
200x10-9
150
100
50
0
403020100Gate Voltage (mV)
gQPC ~ 2e2/h
Vg(mV)
Vg(mV) Vg(mV)
Vg(mV)
conductance
conductance
focusing
focusing
g (e
2 /h)
g (e
2 /h)
Significant Results in the last 12 months:
Breakthrough in understanding of 0.7 structure in a quantum point contact: Free spin, due to interactions, capable of undergoing Kondo screening. (Cronenwett, et al., PRL, in press.)
First results on arrays of quantum point contacts, clear evidence of spin physics, but still lacking a good physical picture. Arrays of point contacts can be used to realize propagation of spin entanglement.
Focusing from a quantum dot into a quantum point contact as a demonstration of gate-controlled spin filtering has first hurdle passed: strong focusing signal from a quantum dot. Experiments underway.
The next year:
•Construct spin chains with gated regions between point contacts to change density and multiple ohmic contact points.
•Develop noise measurement technology in our lab. Measure noise cross-correlation to investigate correlations between quantum point contacts.
•Complete first dot-focusing experiments, investigate size and temperature dependence. Compare to direct ground state spin measurements to see if multi-electron dots can operate as spin filters and spin storage devices.
•Begin to investigate variable g-factor materials with simple point contacts and quantum dots.