Transcript Document

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.
Quantized Conductance
(data from vanWees, 1988)
In-plane magnetic field dependence
temperature dependence
0.7 feature
gets stronger
at higher
temperatures!
Nonlinear Transport
T =T = 80mK
75 mK, B = 0 TB=0
T T== 600
0.6K
mK, B =B=0
0T
||
T=80mK
T = 75 mK, BB=8T
=8T
||
||
3
3
2
2
2
g
g (2e2/h)
g (2e2/h)
g (2e2/h)
3
g
1
0
g
1
-1
0
Vsd (mV)
Vs
1
0
1
-1
0
Vsd (mV)
Vs
1
0
-1
0
Vsd (mV)
Vs
1
Nonlinear Transport
T =T = 80mK
75 mK, B = 0 TB=0
T T== 600
0.6K
mK, B =B=0
0T
||
T=80mK
T = 75 mK, BB=8T
=8T
||
||
3
3
2
2
2
g
g (2e2/h)
g (2e2/h)
g (2e2/h)
3
g
1
0
g
1
-1
0
Vsd (mV)
Vs
1
0
1
-1
0
Vsd (mV)
Vs
1
0
-1
0
Vsd (mV)
Vs
1
Nonlinear Transport
T =T = 80mK
75 mK, B = 0 TB=0
T T== 600
0.6K
mK, B =B=0
0T
||
T=80mK
T = 75 mK, BB=8T
=8T
||
||
3
3
2
2
2
g
g (2e2/h)
g (2e2/h)
g (2e2/h)
3
g
1
0
g
1
-1
0
Vsd (mV)
Vs
1
0
1
-1
0
Vsd (mV)
Vs
1
0
-1
0
Vsd (mV)
Vs
1
Nonlinear Transport
T =T = 80mK
75 mK, B = 0 TB=0
T T== 600
0.6K
mK, B =B=0
0T
||
T=80mK
T = 75 mK, BB=8T
=8T
||
||
3
3
2
2
2
g
g (2e2/h)
g (2e2/h)
g (2e2/h)
3
g
1
0
g
1
-1
0
Vsd (mV)
Vs
1
0
1
-1
0
Vsd (mV)
Vs
1
0
-1
0
Vsd (mV)
Vs
1
Lifting spin degeneracy due to interactions
quantum dot
quantum point contact
gate
gate
2DEG
2DEG
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 Field
Dependence of
Zero Bias Anomaly
B|| = 0
B|| = 3T
B|| = 8T
g
Vsd
quantum dot
quantum point contact
gate
gate
2DEG
Charging energy lifts spin
degeneracy. Kondo effect
results from interaction of
unpaired state with leads.
2DEG
Interaction energy lifts spin
degeneracy. Kondo effect
results from interaction of
unpaired mode with bulk
2DEG.
entanglement
of 1 and 2
propagation of
entanglement
exact numerical
for N=31
long-chain limit
A single quantum point contact acts as a free spin with a
Kondo-like screening cloud at low temperature
KONDO
what happens when2 more
than one point contact are in proximity?
mm
Depending on parameters, the quasibound spins should become
entangled with each other, mediated by conduction electrons.
KONDO
RKKY
KONDO
This is the famous RKKY interaction, the physical effect
that gives rise to spin glasses in 2D and 3D.
2 mm
We can use this to construct spin chains with controllable local Kondo temperatures
KONDO
RKKY
KONDO
RKKY
KONDO
RKKY
KONDO
2 mm
first experimental results:
two point contacts in series
B||
striking dependence on
in-plane magnetic field
indicates spin-related effect,
but they are not understood.
2) quantum dot as gate-tunable spin filter
Point contact at 1e2/h plateau as spin detector
B|| = 8T
1 mm
N even to N odd
SS+1/2
N odd to N even
SS-1/2
Aligned spins transmitted
Anti-aligned spins transmitted
B||
A spin separator
and spin-bridge detector
g (e2/h)
g (e2/h)
First Data on Spin Injection and Detection from a QD
Vg(mV)
Vg(mV)
Telectron~150mK
Bparallel = 7T
conductance
focusing
g (e2/h)
gQPC ~ 1e2/h
Vg(mV)
conductance
Vg(mV)
focusing
g (e2/h)
gQPC ~ 2e2/h
Vg(mV)
Vg(mV)
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.