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

Semiconductor Few-Electron Quantum Dots as Spin Qubits 49
spin degree of freedom (which promises a long coherence time, essential for
encoding quantum information).
2.2 Samples
We have fabricated and measured several few-electron double quantum dots,
of three different designs (Fig. 14). The first two types have only been used
once as few-electron single dots. In both cases, one of the gate electrodes was
not functioning, which prevented us from testing if these devices also function
as few-electron double dots. The third type of device (Fig. 14c) did function
as a double dot, and was used for all subsequent few-electron experiments.
To verify that the first device (Fig. 14a) can operate as a few-electron
single quantum dot, we performed a large-bias measurement of the differential
conductance through the dot. Going towards more negative gate voltage, a
series of “Coulomb diamonds” is revealed (Fig. 15a), in which the number of
electrons on the dot, N, is constant. This is followed by a region in which
the “diamond” does not close, even up to a source-drain voltage of 10 mV, i.e.
several times larger than the typical charging energy for a small dot (∼2meV).
Therefore, in this region N =0.
The tunnel coupling between the dot and the source and drain reservoirs
could be changed by simply readjusting the gate voltages. For strong coupling,
a zero-bias peak – hallmark of the Kondo effect – became visible throughout
the one-electron diamond (Fig. 15b). From the width of the zero-bias peak
(Fig. 15c) we found a Kondo temperature of about 0.4 K. The appearance
of a one-electron Kondo effect (unpublished) implies that this quantum dot
design allows the tunnel coupling to be tuned over a wide range, even in the
few-electron regime. In addition, it is striking evidence that we can confine a
single spin in a lateral quantum dot.
In the second quantum dot design (Fig. 14b), the narrow “plunger” gates
approach the dot more from the sides, rather than from below. In this way,
they are further away from the central tunnel barrier, reducing the effect they
have on the tunnel rate. Also, the gate coming from the top of the picture
was made thinner, in order to make the tunnel barriers more easily controllable
[43]. Thirdly, the characteristic gates ending in circles (see Fig. 14a) were
left out. This device was quite easily tunable.
In the rest of this section, we use the third design (Fig. 14c). Two nominally
identical devices are studied, both as shown in Fig. 16a. They consist of a
double quantum dot flanked by two quantum point contacts (QPCs), defined
in a 2DEG that is present below the surface of a GaAs/AlGaAs heterostructure.
The layout of the double quantum dot is an extension of previously
reported single dot devices [43]. The double dot is defined by applying negative
voltages to the 6 central gates. Gate T in combination with the left
(right) gate, L (R), controls the tunnel barrier from the left (right) dot to
drain 1 (source 2). Gate T in combination with the middle gate, M, controls
the tunnel barrier between the two dots. The narrow “plunger” gates, PL and