in Coupled Quantum Dot Devices Stefan Ludwig Aachen, 06.03.2008 O - - PowerPoint PPT Presentation

Stefan Ludwig

Aachen, 06.03.2008

From Equilibrium- to Non-Equilibrium Interaction in Coupled Quantum Dot Devices

O U T L I N E

• few electron serial triple quantum dot
• backaction of a quantum point contact charge detector
• counterflow in 2 quantum point contacts

α

VL VR VC

500 nm Q P C

electrostatically defined quantum dots (QDs)

tunnel barriers

2 D E G

G a A s A l G a A s plunger gate

x energy EF

250 nm

transport through a serial double QD

current flows on triple points of the stability diagram

I V

L R

1m

current

VR(V) VL(V)

I(nA)

L R

1m

charge spectroscopy using a quantum point contact (QPC) in its tunneling regime

GT = dIQPC dU gL

transconductance of QPC has extrema on charging lines of the double QD

I V

GT

VL(V) VR(V)

S D

rN  lN 

a serial few-electron triple quantum dot

3 QDs ↔ 3 main slopes

• f charging lines

α

VL VR VC

500 nm Q P C

PRB 76, 075306 (2007)

triple quantum dot stability diagram

bistable region

bistable region – telegraph noise

(011) is the ground state !

α

VL VR VC

500 nm Q P C

boundaries of telegraph noise

0,0,2 1,0,0

what is the non-equilibrium energy source ?

bistable region in a DQD

arXiv:0801.4002, in cooperation with

• M. Pioro-Ladriere & A. Sachrajda (CIAR)

TQD DQD

α α

VL VR Vp

300 nm QPC

α

VL VR VC

500 nm QPC

multiple bistable regions in a DQD

α

VL VC

500 nm QPC

molecular states at triple points

in

the driven QPC is the energy source

the nearby QPC used as charge detector provides non-equilibrium energy !

in = out ;

in ≫ out ⇒ QD is permanently charged

• position of charging line is always determined by
• equilibrium: at resonance condition
• non-equilibrium energy source: at resonance condition possible

in = out in ≠ out

transition between atomic and molecular behavior

molecular states

in out

atomic states

at resonance

gap for smaller than Lock-In frequency in = out

noise spectroscopy of excited states

arXiv:0801.4002, in cooperation with M. Pioro-Ladriere & A. Sachrajda (CIAR)

Гin and Гout change at interdot resonances

TQD DQD

backaction of QPC

• a driven QPC always provides energy and causes telegraph noise

(fluctuations between charge configurations) in coupled QDs

• the noise will limit the coherence time of a qubit
• the fluctuations can be directly observed only if slow enough !

α α

VL VR Vp

300 nm QPC

α

VL VR VC

500 nm QPC

is the energy mediated by phonons or by photons ?

interaction between two electrically separated QPCs

the two QPCs are electrically separated: no leakage current !

Idet Idrive

?

drive QPC:

strong electron-hole asymmetry

S D

detector QPC:

counterflow current ICF flows through the detector QPC in the opposite direction than IDRIVE

counterflow current in the unbiased QPC

PRL 99, 096803 (2007)

counterflow current as a function of Gdet

maximal effect for Gdet between plateaus gcf ≡ d I cf d V drive

detector QPC:

counterflow current in the pinch-off regime (Gdet = 0)

gCF is enhanced compared to 4T0(1-T0) even in the pinch-off regime of the detector QPC → hot electrons (E-EF ~ 0.5 meV) ! transmission function: 4T 0 1−T 0 ≡ 4 Gdet G0 1−Gdet G 0 

~0.5 meV

model in terms of asymmetric phonon based heating

phonons electron-hole pairs

1.) hot electrons (E-EF ~ 4 meV) scatter with Fermi-see (EF ~ 8 meV) only in the right lead of the drive-QPC 2.) hot electrons emit phonons with maximal momentum of 2ke → maximal Eph ~ 0.5 meV 3.) acoustic phonons are absorbed in the detector circuit by exciting electron-hole pairs with max. Ee ~ 0.5 meV 4.) the resulting hot electrons pass the detector-QPC, but the “cold” holes are reflected → counterflow current

S D

S U M M E R Y

thank you !!

• full control of charge states in few electrons triple QDs achieved

→ triple QDs provide rich spectrum of physics going beyond that of double QDs (e.g. QCA- processes,...)

• bounded (bistable) regions of telegraph noise observed in triple and double QDs

→ quantum point contact acts as non-equilibrium energy source → noise is always present, but usually too fast for charge detection → consequence: switch off charge detector during qubit operation for longer coherence times

• counterflow through quantum point contacts

→ strongly driven quantum point contact provides phonons

the team wafers are grown by

Jörg Kotthaus Vadim Khrapai Daniel Schröer

• W. Wegscheider

(Uni Regensburg)

• K. Eberl

(MPI for Solid State Physics)

Daniel Harbusch Daniela Taubert