Professor, Department of Applied Physics, Tokyo University of Science
Former Fellow/Executive Director, NTT R&D
Former Director, NTT Basic Research Laboratories
The century of quantum engineering
— Aiming for a superconducting quantum computer —
Since becoming an executive director at NTT R&D in July 2003, Dr. Takayanagi has been trying to establish the research organization to enhance quantum information processing and nanobiotechnology. In quantum information processing, four types of quantum bits or qubits are studied to develop a quantum computer. He says, “I personally think superconductors are most appropriate for developing a quantum computer because I have a background in superconductivity.”
A qubit, which is the basic unit of information, has to be able to take a quantum superposition of the "0" and "1" states. A superconducting flux qubit is comprised of an aluminum loop with three Josephson junctions embedded in it and is about five micrometers in size. Dr. Takayanagi says, “The flux that penetrates the loop in a superconductive state is quantized. When a magnetic field which adjusts the number of flux quanta to 0.5 is applied, the number of flux quanta may be one due to a larger current flow in the loop or may be zero due to the current flow in the opposite direction in the loop, and the probability to be either one or zero is the same. Thus, the state at half a flux quantum is a superposition of clockwise and counter-clockwise circulating supercurrent states.” The state of a qubit can be read using a superconducting quantum interference device (SQUID) that surrounds the qubit and is about seven micrometers in size. “When the current direction in the qubit changes, the flux that penetrates the SQUID loop changes. Since a SQUID is extremely sensitive to magnetism, the maximum superconducting current of the SQUID changes depending on the changes in the flux, and therefore, changes in the state of a qubit can be observed,” says Dr. Takayanagi.
Normally, determining a qubit state through one measurement was difficult, and several measurements were carried out and averaged for the determination. However, Dr. Takayanagi and his team were able to distinguish between the ground state and the first excited state, i.e., the "0" and "1" states, respectively, with one measurement by improving the sensitivity of the SQUID, when the number of the flux quanta that penetrate a qubit was 1.5.
One of the advantages of the superconducting flux qubit that Dr. Takayanagi points out is that the decoherence time, within which the coherent superposition of quantum states is destroyed, is longer than that of other solid state qubits. Currently, the longest decoherence time in solid state qubits is 3 microseconds in the superconducting flux qubits prepared at Delft University of Technology. While trying to extend the decoherence time, which is “the key to developing qubits,” Dr. Takayanagi observed multiphoton absorption in the superconducting flux qubits. He says, “This phenomenon in atoms/molecules has been widely observed, but it was the first observation in giant artificial atoms. This observation provided clues to understand how the qubits are bound to the surrounding and how the bonds are weakened to extend the decoherence time.”
There is, as well, decoherence that is induced by Josephson junctions which are the basic building blocks of superconducting flux qubits. So, research on materials in order to prevent decoherence has started. Dr. Takayanagi expects researchers in materials science to launch research on quantum information processing. However, in order to develop a quantum computer, not only researchers in materials science but also researchers involved in information science, mathematics, electrical engineering, electronic engineering and other various fields must work together. “It is contradictory that we want to observe qubit states, but we cannot observe them because the quantum superposition states are destroyed by observation. Therefore, we have to know how much of the noise must be reduced and how high-speed measurements may be conducted. For that, new technology will be required. This won’t be possible only with researchers in physics.”
Dr. Takayanagi was involved in developing a SQUID, when he was in school, and was attracted by Josephson junctions. After he joined NTT, he was committed to doing what he liked. From his experience, he strongly advises young researchers to let themselves do whatever they like and to strive for it. He says, “Although the 20th century was said to be the century of quantum mechanics, quantum mechanics is being truly utilized in the 21st century. The essentials of quantum mechanics are quantum superposition and quantum entanglement. From that viewpoint, we have just started to establish engineering that uses quantum mechanics.” Even if a quantum computer is not developed, the spin-off effects, such as the development of novel materials, metrological engineering and algorithm, are enormous. Now, there is technology with potential for commercialization such as quantum cryptography. “Developing a quantum computer means that quantum mechanics can be experimentally confirmed. So, I want young researchers to be fascinated with developing a quantum computer,” says Dr. Takayanagi.
