Heiss W.D. (ed.) Quantum dots.. a doorway to - tiera.ru

Semiconductor Few-Electron Quantum Dots as Spin Qubits 47
grounded. We find that in general samples get more quiet during the first
week of applying the gate voltages. Finally, sample stability also involves an
element of luck: Fig. 13 shows two Coulomb diamonds that were measured immediately
after each other under identical conditions. Measurement Fig. 13a
is reasonably quiet, but in Fig. 13b the effects of an individual two-level fluctuator
are visible. This particular fluctuator remained active for a week, until
the sample was warmed up.
a dI/ dVSD
(arb. units) b
Bias voltage (mV)
2
1
0
-2 -1 0 1 2
Bias voltage (mV)
-1 -1
-2 -2
-0.725 -0.730 -0.735 -0.725 -0.730 -0.735
Gate voltage (V) Gate voltage (V)
2
1
0
dI/ dV (arb. units)
SD
-2 -1 0 1 2
switching
Fig. 13. Charge switching in a large-bias measurement in the few-electron regime,
for B =12T.(a) Differential conductance, dI/dVSD (in grayscale), as a function
of bias voltage and gate voltage. This measurement is considered reasonably stable.
(b) Identical measurement, taken immediately after (a). A single two-level fluctuator
has become active, causing the effective gate voltage to fluctuate between two values
at any position in the figure, and leading to an apparent splitting of all the lines.
This is considered a measurement of poor stability
Switching has made all experiments we performed more difficult, and has
made some experiments that we wanted to perform impossible. Better control
over heterostructure stability is therefore essential for the increasingly difficult
steps towards creating a quantum dot spin qubit.
2 Few-Electron Quantum Dot Circuit
with Integrated Charge Read-Out
In this section, we report on the realization of few-electron double quantum
dots defined in a two-dimensional electron gas by means of surface gates on
top of a GaAs/AlGaAs heterostructure. The double quantum dots are flanked