Toward Quantum Computers: New Circuit Technology Enables Execution Of Quantum Algorithm

The quantum computer, when it is finally brought to fruition, is expected to far surpass the capabilities of even the most modern of today's supercomputers. Actual computing in a quantum computer is carried out by manipulating the quantum state of qubits in time sequence by external controls. To achieve such manipulation, it is necessary to control the: 1. States of individual qubits, 2. States of two qubits (logic operation), and 3. Ability to turn on /off the coupling between qubits.

NEC, JST, and RIKEN have already announced successful development of key technologies for the world's first solid-state qubit and the world's first two-qubit logic gate, based on solid-state technology that excels in its ability to integrate qubits. Following these achievements, the research group addressed the controllable coupling of qubits as the next logical step in realization of a practical quantum computer. Their new research result represents the world's first successful demonstration of controllably coupled qubits.

To date, the coupling of qubits has been difficult to control. In order to realize this control, the research group devised an original mechanism that employs another qubit in between the two qubits for coupling. The coupling qubit functions as a non-linear transformer that is able to turn on and off the magnetic coupling between the two qubits, and on/off control is achieved simply by inputting a microwave. Moreover, coupling operation has been achieved without shortening the lifetime of each qubit. Scalability is also realized through the repetition of coupled two-qubit units - a feature necessary for future quantum computers.

To demonstrate the operation feasibility of the controllable coupling scheme, the research group employed a coupled two-qubit system, the smallest quantum logic unit, to carry out a multi-quantum control experiment involving the turning on and off of the coupling. As a result, a simple quantum protocol has been successfully demonstrated, allowing controllable coupling for the execution of quantum algorithms.

In the near future, NEC, JST, and RIKEN, plan to implement a larger-scale, more elaborate quantum computation, aiming for the realization of a practical quantum computer.

The result of this joint research were published in the May 4th issue of the journal Science, published by the American Association for the Advancement of Science.

Article Title: Quantum Coherent Tunable Coupling of Superconducting Qubits

A part of this research has been carried out under the following JST project:

Project Name: Core Research for Evolutional Science and Technology (CREST)

Research Area: "Creation of New Technology Aiming for the Realization of Quantum Information Processing Systems" (Research

Supervisor: Professor Yoshihisa Yamamoto, National Institute of Informatics / Stanford University)

Research Theme: "Superconducting Qubit System"

Research Director: Jaw-Shen Tsai, Fellow, NEC Nano Electronics Research Laboratories / Laboratory Head, Riken Frontier Research System

Research Period: 2003 to 2008