# Projects

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Fault-tolerant quantum computing requires high-fidelity qubits in a scalable system.
This project focuses on single electron spins in silicon quantum dots, a system with potential for excellent coherence and manufacturability.
Our approach is to meet the antithetic requirements for spin coherence and controllability by engineering a micro-magnet stray field.
We have demonstrated unprecedented qubit performance both in isotopically-natural and -purified silicon quantum dots.
Publication:
J. Yoneda et al., Nature Nanotechnology (2017).
K. Takeda et al., Science Advances, **

**2**, e1600694 (2016).

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Electron spins in semiconductor quantum dots are good candidates of quantum bits for quantum information processing. Basic operations of the qubit have been realized in recent years: initialization, manipulation of single spins, two-qubit entanglement operations, and readout. Now it becomes crucial to demonstrate the scalability of this architecture by conducting spin operations on a scaled up system. We have demonstrated a semiconductor quintuple quantum dot (5QD) or series coupled five QDs with a
concept relevant for further increasing the number of QDs.
Publication:
T. Ito et al., Scientific Reports, **

**6**, 39113 (2017). T. Otsuka et al., Scientific Reports,

**6**, 31820 (2016).

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We focus on the spin-control method utilizing a micro-magnet deposited adjacent to the dots. We use highly tunable GaAs-based lateral double quantum dots with a single electron confined in each dot. We have demonstrated rotations of the single electron spin with electron spin resonance (ESR), by applying microwaves to gate electrodes to oscillate the electrons in a slanting magnetic field. We further showed that two electrons can be controlled independently because each spin feels a different magnetic field in different dots resulting in distinct resolved ESR peaks.
Publication:
T. Nakajima et al., Physical Review Letters, **

**119**, 017701 (2017). M. R. Delbecq et al., Physical Review Letters,

**116**, 046802 (2016).

**Quantum simulation provides a way of searching for solutions to complex quantum problems, among which dynamics of a quantum many body system in the non-equilibrium regime is intriguing. Semiconductor quantum dots (QDs) could well confine electrons with charge and spin degrees of freedom, from which quantum many body interaction can arise. Utilizing a double-QD coupled to an electron reservoir, we investigate the spin dynamics of an open quantum system in non-equilibrium regime.
Publication:
T. Otsuka et al., Scientific Reports, **

**7**, 12201 (2017).