Bottom-up grown nanowire quantum devices
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oduction The workhorse of a classical computer is the transistor. Computational power scales linearly with the number of transistors on the chip. Moore’s Law predicts that device dimensions are decreasing in time, and within the next decade or so, we are going to hit a fundamental physical limit, which is the size of a single atom. If we want to continue scaling in computational power, we have to think about alternative concepts that have different scaling laws. One of these alternative concepts could be quantum computation.1 In a quantum computer, quantum mechanical principles, such as superposition and entanglement, are used. The workhorse of a quantum computer is a quantum bit, or qubit. Thanks to the principle of superposition, a qubit can be ON and OFF at the same time (unlike the conventional transistor, which can either be in an ON or OFF position), and it can also take any value in between. In a quantum computer, an ensemble of entangled (coupled) qubits is envisioned, and if the quantum state of one of these qubits is changed, then the states of all of the qubits will be simultaneously changed. Due to these quantum mechanical principles, the computational power scales exponentially with the number of qubits entangled in the system. This also means that the computational power is doubled every time an entangled qubit is added to the system.
This is the promise of quantum computing and also the reason why a number of companies in the United States such as IBM, Microsoft, Intel, Google, and also in China such as Baidu, Tencent, and Alibaba are investing in quantum computing research. Many different principles can be used to make a qubit.1 One possibility is the spin qubit. A spin qubit can be realized in an indium antimonide (InSb) nanowire, as shown in Figure 1a,2,3 where five gate electrodes are located below the nanowire. Two quantum dots can be defined in series in the wire by applying a negative potential to gates 1, 3, and 5, such that three potential barriers are formed, with two potential wells (dots) in between. In each of the dots, exactly one electron is captured by tuning the potential on gates 2 and 4. If both electrons have spin-up, then the electron in one cannot tunnel to the other quantum dot because a triplet state has to be formed, which is a higher-energy state. As a consequence, current is blocked by the Pauli exclusion principle. A short voltage pulse can be given to one of the gates, in the presence of a magnetic field, such that one of these spin-up electrons is rotated. If the situation is reached where one electron has spin-down and the other one has spin-up, then the left electron can tunnel to the right quantum dot,
Erik Bakkers, Department of Applied Physics, Eindhoven University of Technology, The Netherlands; [email protected] doi:10.1557/mrs.2019.102
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