- 03 (2010 Selected)：
- Theory on quantum coherence in hybrid quantum system of superconductor and quantum dot
The supercurrent flowing through a quantum dot can be reversed by changing the charge state of the quantum dot. The number of electrons in the quantum dot, i.e. even or odd, is controlled by gate voltage in the experiment. This behavior in the macroscopic quantity originates from the fermion statistics and the strong repulsion between electrons. The commutation of two electrons, which occurs in the hopping process of Cooper pair through the quantum dot, adds a negative sign due to the fermion statistics. This means that the quantum mechanical spin state in the quantum dot appears in the macroscopic superconducting state.
Due to the controllability mentioned above, the quantum dot is an ideal example to observe the microscopic and macroscopic quantum coherences. On the other hand, the spintronics is now one of hot topics in the condensed matter physics. In this research field, people aim to use the spin degrees of freedom for a new functionality. Under this circumstance, the quantum dot is not only idealistic but also realistic to use for controlling the quantum coherence.
In this study, we consider a quantum system combined superconductor with multiple quantum dots. (1) We will propose a method for controlling the microscopic coherence of spin state and the macroscopic coherence of supercurrent. Especially, when two superconductors couple through double quantum dot arranged in parallel, the orientation of Josephson current would depend on the spin state of double quantum dot. In other words, the macroscopic quantity as Josephson current will be controlled by the microscopic quantity as spin state in double quantum dot. It also means that the microscopic electronic states can be observed through a macroscopic quantity like as Josephson current. The electron spin fluctuates and gradually loses its coherence through the interaction with nuclear spin and so on. By examining in detail the Josephson current through multiple quantum dot, the mechanism of decoherence in the quantum spin would be elucidated. (2) We will propose a feasible qubit system in fusion of multiple quantum dot and superconductor. We think that it must be possible to make a qubit by combining the Josephson junction through the quantum dot and the conventional one. We will try to find a new qubit design.
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