Prof. Jaw-Shen Tsai

NEC Fundamental and Environmental Research Laboratories
Tsukuba, JAPAN

Toward scalable superconducting quantum bits

Realization of practical quantum algorithms requires integration of a large number of quantum bits. By coupling Josephson charge qubits, we have so far demonstrated a conditional gate operation (controlled-NOT) [1]. In this CNOT gate, the coupling was achieved by a fixed electrostatic coupling, and the coupling strength could not be altered in this operation scheme. However, in order to achieve unconditional gate operations that required for quantum computing, a controllable coupling between qubits is desired. Recently we have succeeded in demonstrating controllable coupling of two flux qubits, coupled via a third qubit [2]. When coupled, it properly functioned as a double controlled-NOT gate; and when decoupled, unconditional 1-bit control was demonstrated [3]. Further more, the coupling scheme has an advantage that the 2-qubit system can be operated at the so called optimal bias point where decoherence is strongly suppressed. In principal, this 2-qubit gate can be viewed as a prototype universal gate of the quantum computer, although many difficult issues still remain unsolved. Recently we have also demonstrate a lasing effect with a single artificial atom - a Josephson-junction charge qubit - embedded in a superconducting resonator. The device is essentially different from the existing lasers and masers and the population inversion was created by current injection to the device. [4].

[1] T. Yamamoto et al, Nature 425, 941, 2003.
[2] A. O. Niskanen et al, Physical Review B 73, 094506, 2006.
[3] A. O. Niskanen et al, to be published in Science.
[4] O. Astafiev et al, not published.

Seminar schedule: http://www.physics.rutgers.edu/qcg/Seminars.html