Heiss W.D. (ed.) Quantum dots.. a doorway to - tiera.ru
Heiss W.D. (ed.) Quantum dots.. a doorway to - tiera.ru
Heiss W.D. (ed.) Quantum dots.. a doorway to - tiera.ru
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V L (V)<br />
-1.084<br />
-1.082<br />
-1.080<br />
-1.078<br />
a<br />
11<br />
Semiconduc<strong>to</strong>r Few-Electron <strong>Quantum</strong> Dots as Spin Qubits 59<br />
01 00<br />
10<br />
-0.54 -0.55 -0.56<br />
V PR (V)<br />
b<br />
∆V L (mV)<br />
5<br />
4<br />
3<br />
2<br />
1<br />
0<br />
0<br />
10<br />
20<br />
30<br />
40<br />
Frequency (GHz)<br />
Fig. 20. Pho<strong>to</strong>n-assist<strong>ed</strong> tunnelling in a one-electron double quantum dot. (a) Current<br />
through the double dot at the last set of triple points, with zero bias voltage<br />
(VDOT = VSD1 = VSD2 = 0). A microwave signal of 50 GHz is appli<strong>ed</strong> <strong>to</strong> PL. The<br />
microwaves pump a current, IDOT , by absorption of single pho<strong>to</strong>ns [47]. This pho<strong>to</strong>nassist<strong>ed</strong><br />
current shows up as two lines, indicat<strong>ed</strong> by the two arrows. The white line<br />
(bot<strong>to</strong>m) corresponds <strong>to</strong> electrons being pump<strong>ed</strong> from the left <strong>to</strong> the right reservoir,<br />
the dark line (<strong>to</strong>p) corresponds <strong>to</strong> pumping in the reverse direction. In the middle,<br />
around the dott<strong>ed</strong> line separating the 01 from the 10 configuration, a finite current<br />
is induc<strong>ed</strong> by an unwant<strong>ed</strong> voltage drop over the double dot, due <strong>to</strong> asymmetric<br />
coupling of the ac-signal <strong>to</strong> the two leads. (b) Separation between the two pho<strong>to</strong>nassist<strong>ed</strong><br />
tunnelling lines versus microwave frequency. The dependence is linear down<br />
<strong>to</strong> the lowest frequency of about 6 GHz, from which it follows that the inter-dot tunnel<br />
coupling (half the energy difference between bonding and anti-bonding state) is<br />
smaller than ∼12 µeV<br />
voltage pulses appli<strong>ed</strong> <strong>to</strong> a metal gate induces tunnelling of electrons between<br />
the quantum dot and a reservoir. The effective tunnel rate depends on the<br />
number and nature of the energy levels in the dot made accessible by the<br />
pulse. Measurement of the charge dynamics thus reveals the energy spect<strong>ru</strong>m<br />
of the dot, as demonstrat<strong>ed</strong> for a dot in the few-electron regime.<br />
3.1 Introduction<br />
Few-electron quantum <strong>dots</strong> are consider<strong>ed</strong> as qubits for quantum circuits,<br />
where the quantum bit is s<strong>to</strong>r<strong>ed</strong> in the spin or orbital state of an electron in a<br />
single or double dot. The elements in such a device must have functionalities<br />
such as initialization, one- and two-qubit operations and read-out [2]. For all<br />
these functions it is necessary <strong>to</strong> have precise knowl<strong>ed</strong>ge of the qubit energy<br />
levels. Standard spectroscopy experiments involve electron transport through<br />
the quantum dot while varying both a gate voltage and the source-drain<br />
50