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
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
Semiconduc<strong>to</strong>r Few-Electron <strong>Quantum</strong> Dots<br />
as Spin Qubits<br />
J.M. Elzerman 1,2 ,R.Hanson 1 , L.H.W. van Beveren 1,2 , S. Ta<strong>ru</strong>cha 2,3 ,<br />
L.M.K. Vandersypen 1 , and L.P. Kouwenhoven 1,2<br />
1<br />
Kavli Institute of Nanoscience Delft, PO Box 5046, 2600 GA Delft,<br />
The Netherlands<br />
elzerman@qt.tn.tudelft.nl<br />
2<br />
ERATO Mesoscopic Correlation Project, University of Tokyo, Bunkyo-ku, Tokyo<br />
113-0033, Japan<br />
3<br />
NTT Basic Research Labora<strong>to</strong>ries, Atsugi-shi, Kanagawa 243-0129, Japan<br />
The spin of an electron plac<strong>ed</strong> in a magnetic field provides a natural twolevel<br />
system suitable as a qubit in a quantum computer [1]. In this work, we<br />
describe the experimental steps we have taken <strong>to</strong>wards using a single electron<br />
spin, trapp<strong>ed</strong> in a semiconduc<strong>to</strong>r quantum dot, as such a spin qubit [2].<br />
The outline is as follows. Section 1 serves as an introduction in<strong>to</strong> quantum<br />
computing and quantum <strong>dots</strong>. Section 2 describes the development of the<br />
“hardware” for the spin qubit: a device consisting of two coupl<strong>ed</strong> quantum<br />
<strong>dots</strong> that can be fill<strong>ed</strong> with one electron (spin) each, and flank<strong>ed</strong> by two<br />
quantum point contacts (QPCs). The system can be prob<strong>ed</strong> in two different<br />
ways, either by performing conventional measurements of transport through<br />
one dot or two <strong>dots</strong> in series, or by using a QPC <strong>to</strong> measure changes in the<br />
(average) charge on each of the two <strong>dots</strong>. This versatility has proven <strong>to</strong> be<br />
very useful, and the type of device shown in this section was us<strong>ed</strong> for all<br />
subsequent experiments.<br />
In Sect. 3, it is shown that we can determine all relevant parameters of<br />
a quantum dot even when it is coupl<strong>ed</strong> very weakly <strong>to</strong> only one reservoir.<br />
In this regime, inaccessible <strong>to</strong> conventional transport experiments, we use a<br />
QPC charge detec<strong>to</strong>r <strong>to</strong> determine the tunnel rate between the dot and the<br />
reservoir. By measuring changes in the effective tunnel rate, we can determine<br />
the excit<strong>ed</strong> states of the dot.<br />
In Sect. 4, the QPC as a charge detec<strong>to</strong>r is push<strong>ed</strong> <strong>to</strong> a faster regime<br />
(∼100 kHz), <strong>to</strong> detect single electron tunnel events in real time. We also determine<br />
the dominant contributions <strong>to</strong> the noise, and estimate the ultimate<br />
spe<strong>ed</strong> and sensitivity that could be achiev<strong>ed</strong> with this very simple method of<br />
charge detection.<br />
In Sect. 5, we develop a technique <strong>to</strong> perform single-shot measurement of<br />
the spin orientation of an individual electron in a quantum dot. This is done by<br />
J.M. Elzerman et al.: Semiconduc<strong>to</strong>r Few-Electron <strong>Quantum</strong> Dots as Spin Qubits,<br />
Lect. Notes Phys. 667, 25–95 (2005)<br />
www.springerlink.com c○ Springer-Verlag Berlin Heidelberg 2005