Semiconductor Few-Electron Quantum Dots as Spin Qubits

The spin of an electron placed in a magnetic field provides a natural twolevel system suitable as a qubit in a quantum computer [1]. In this work, we describe the experimental steps we have taken towards using a single electron spin, trapped in a semiconductor quantum dot, as such a spin qubit [2]. The outline is as follows. Section 1 serves as an introduction into quantum computing and quantum dots. Section 2 describes the development of the “hardware” for the spin qubit: a device consisting of two coupled quantum dots that can be filled with one electron (spin) each, and flanked by two quantum point contacts (QPCs). The system can be probed in two different ways, either by performing conventional measurements of transport through one dot or two dots in series, or by using a QPC to measure changes in the (average) charge on each of the two dots. This versatility has proven to be very useful, and the type of device shown in this section was used for all subsequent experiments. In Sect. 3, it is shown that we can determine all relevant parameters of a quantum dot even when it is coupled very weakly to only one reservoir. In this regime, inaccessible to conventional transport experiments, we use a QPC charge detector to determine the tunnel rate between the dot and the reservoir. By measuring changes in the effective tunnel rate, we can determine the excited states of the dot. In Sect. 4, the QPC as a charge detector is pushed to a faster regime (∼100 kHz), to detect single electron tunnel events in real time. We also determine the dominant contributions to the noise, and estimate the ultimate speed and sensitivity that could be achieved with this very simple method of charge detection. In Sect. 5, we develop a technique to perform single-shot measurement of the spin orientation of an individual electron in a quantum dot. This is done by