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• PUBLIC
  LECTURE

• Department of Physics
• Brown University
• Box 1843
• Barus Holley Building
• 182 Hope Street
• Providence, RI 02912
• physics @ physics.brown.edu

The BCS Theory of
Superconductivity

J.S. Tsai
Research Fellow
NEC Fundamental and Environmental Research Labs

Dr. Tsai's NEC Profile


Lecture
"Toward Scalable Superconducting Quantum Bits"

Abstract
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, 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. 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.

[1] T. Yamamoto et al, Nature 425, 941, 2003
[2] Antti O. Niskanen et al, Physical Review B 73, 094506, 2006