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 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!

T=80mK B=8TT = 0.6K B=0T = 80mK B=0 g g g VsdVsd VsdVsd VsdVsd Nonlinear Transport

T=80mK B=8TT = 0.6K B=0T = 80mK B=0 g g g VsdVsd VsdVsd VsdVsd Nonlinear Transport

T=80mK B=8TT = 0.6K B=0T = 80mK B=0 g g g VsdVsd VsdVsd VsdVsd Nonlinear Transport

T=80mK B=8TT = 0.6K B=0T = 80mK B=0 g g g VsdVsd VsdVsd VsdVsd Nonlinear Transport

quantum dotquantum point contact gate 2DEG gate Lifting spin degeneracy due to interactions

Kondo Effect in Metals

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

g V sd B || = 0 B || = 3T B || = 8T

quantum dotquantum point contact Charging energy lifts spin degeneracy. Kondo effect results from interaction of unpaired state with leads. Interaction energy lifts spin degeneracy. Kondo effect results from interaction of unpaired mode with bulk 2DEG. gate 2DEG gate

entanglement of 1 and 2 propagation of entanglement exact numerical for 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 Depending on parameters, the quasibound spins should become entangled with each other, mediated by conduction electrons. This is the famous RKKY interaction, the physical effect that gives rise to spin glasses in 2D and 3D.

2  m RKKY 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 on in-plane magnetic field indicates spin-related effect, but they are not understood.

Point contact at 1e 2 /h plateau as spin detector B || = 8T A spin separator and spin-bridge detector 2) quantum dot as gate-tunable spin filter 1  m

V g (mV) g (e 2 /h) V g (mV) First Data on Spin Injection and Detection from a QD T electron ~150mK B parallel = 7T

g QPC ~ 1e 2 /h g QPC ~ 2e 2 /h V g (mV) conductance focusing 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.