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.