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Quantum dot state initialization by control of tunnelling rates


Quantum dots are used in semiconductor quantum technology. They allow to localize and control single electrons in semiconductor devices. Many applications require a fast initialization of a certain quantum state of the electrons. For this purpose, a new technique based on control of tunnelling rates has been developed at PTB.



This development is based on electrically controlled quantum dots, which are also used for the development of single-electron current sources for a future quantum standard of the unit of electrical current: within a narrow semiconductor channel a quantum dot (QD) is defined by applying negative voltages Ventry and Vexit to two metal gates crossing the channel (see inset of figure). These voltages control both the number of electrons in the QD and the tunnel coupling of the QD to the leads. A rapidly changing voltage applied to one of the gates, Ventry(t), first reduces the potential and increases the tunnel coupling to allow loading of one or two electrons onto the QD; thereafter a fast rise of the tunnelling barriers isolates the loaded electrons.

The figure shows the resulting colour coded stability diagram of the average number of loaded and isolated electrons as function of the two voltages applied during the loading phase. In the upper part of the stability diagram one observes a simple dependence on only one voltage, Vexit (see also line graph on top of the stability diagram). However, at low loading voltages Ventry the result is different: the transition from an empty QD after loading at lowest voltage (white region) to the region of successful loading shows complex structures. The analysis shows that these structures result from different quantum states of the one or two electrons confined in the QD. These states are both different in energy and in tunnelling rate, the latter rising strongly with increasing energy. Depending on the exact gate voltages during loading the electrons tunnel preferably into a specific quantum state. If two electrons are loaded, not only their charge, but also their quantum mechanical spin plays an important role. Depending on the combination of the two electron spins, the two-electron quantum state has two possible different symmetries, referred to as singlet (“S” in the figure) and triplet (“T”). A further analysis of the stability diagram has shown that at position (1) in the figure a singlet and at position (2) a triplet state is initialized in the quantum dot. Such selectively prepared quantum states can be used in diverse quantum mechanics experiments and technologies.


stability diagram

The stability diagram shows the colour coded average number of electrons loaded into the quantum dot by a gate voltage sequence. At the positions marked “1” and “2” in the colour plot selective initialization of two-electron states with singlet (S) respectively triplet (T) symmetry is possible. A scanning electron micrograph of the semiconductor device hosting the quantum dot (QD) in between the two metal gate electrodes is shown in the inset in the lower left corner.




Tobias Wenz, Jevgeny Klochan, Frank Hohls, Thomas Gerster, Vyacheslavs Kashcheyevs, Hans W. Schumacher, "Quantum dot state initialization by control of tunneling rates" Phys. Rev. B 99, 201409(R) (2019)