Professor, Department of Physics, Graduate School of Science, The University of Tokyo
Quest for quantum computing
— The world of physics demonstrated by artificial atoms —
“Even in the field of quantum mechanics, simple problems can be solved.” Just as Prof. Tarucha says, the electron state of a single atom in a vacuum, for example, can be described by a simple Schrödinger equation, which can be solved. However, in order to understand the electron state of a system consisting of many atoms such as a solid, various interactions and external disturbances should be taken into consideration, thus making it complicated problems to describe the phenomena. “Interesting macroscopic phenomena, such as superfluidity of helium and superconductivity, tend to be derived from interactions between many particles. As a result of these interactions, quantum effects appear as macroscopic phenomena, and this is why we can utilize these quantum effects. However, I must say that there is a big gap between the understanding of the micro-world that can be accurately described by quantum mechanics, and the macro-world.” In order to solve the complicated many-body problem, Prof. Tarucha fabricated the artificial atom. He was the first person in the world to do this.
An artificial atom is disk-shaped, and several hundred angstroms in diameter. This is 1000 times larger than a real atom. Electrons within artificial atoms are confined strongly in the vertical direction by a semiconductor heterostructure, and weakly in the in-plane direction by an electrostatic potential. When electrons are injected into the artificial atom one by one, the quantum confinement effect makes the electrons in the artificial atom show a shell structure, and they have energy levels similar to those of electrons in a real atom. Prof. Tarucha fabricated artificial atoms, which require extremely precise control in the semiconductor process, and then established the methods of analyzing phenomena in artificial atoms. He has also verified the basic assumption in quantum mechanics that had been derived from empirical rules, using artificial atoms. That is to say, he succeeded in verifying Hund's rule, the Pauli exclusion principle, and the Tomonaga-Luttinger theory, and observing a novel Kondo effect.
“Conducting researches on physics for scientific purpose is of course important, but I also want to skillfully control the phenomena that have been elucidated by the physical researches and realize interesting applications of the findings. One example of such efforts is the quantum computer that utilizes the quantized spins.” In December 2001, IBM carried out quantum computing that uses the nuclear spins of molecules in a test-tube, and succeeded in detecting the results with NMR. However, Prof. Tarucha is still determined to aim at quantum computing using solid-state devices. “I cannot discard the thought of controlling the quantum state of electrons within solid. When performing basic experiments for quantum computing, solution- and atom/molecule-based systems are good for experimental research on quantum computing but they are unfit for integration. I feel it important to implement a research that would at some point lead to some device, or be helpful to develop devices.”
Prof. Tarucha also says that he does not have enough confidence to declare at this point that it is worth conducting R&D of quantum computing. But many researchers believe that it is worth conducting research on quantum computing and are striving to make progress in their research. “Of course, I also believe that quantum computing is meaningful. Many unknown physical phenomena are related to quantum computing, and elucidating those unknown physical phenomena will lead to realization of quantum computers. Combining quantum bits is just a technical issue, so there is not much that people like us can do there. But when it comes to explicating the quantum coherence and entanglement state within a solid matter where complex quantum interactions exist, and also using them efficiently, there is much for us to research. We have an interesting world in front of us.”
