A Selective and Long-distance Coupling Scheme for Plural Flux Quantum Bits
Hayato Nakano1, Kosuke Kakuyanagi1, Kouichi Semba1, and Masahito Ueda2
1Physical Science Laboratory, 2Tokyo Inst. Tech./NTT Research Professor
A quantum computer is composed of many two-state elements called quantum bits (qubits). A quantum calculation should be completed within the coherence time (during which, the quantum superposition is maintained). To achieve this, two-qubit operations where we can choose two arbitrary qubits from a large number are advantageous. Moreover, there is a strong need for an operation that can be performed even if the two qubits are not next to each other. However, to achieve two-qubit operation, we employ the physical interaction between the two qubits, therefore performing a qubit operation on a spatially remote pair is usually very difficult.
We developed a structure and an operating principle to enable such two-bit operation for superconducting qubit systems, and clarified the physical characteristics by theoretical analysis (Fig. 1) .
Every qubit interacts with the same superconducting resonance circuit (e.g. an LC circuit or a transmission line), but the resonance frequencies of individual qubits are sufficiently different to ignore direct interactions between qubits. We use indirect interaction through this resonance circuit and realize the function described above.
Assume that a two-bit operation is performed only between a specific qubit pair (with frequencies of ω1, and ω2). It is important that the resonance frequency ωr can be varied by changing the dc bias-current (Ib) supplied to the Josephson junction in the resonance circuit. When the resonator frequency is tuned to ωr〜(ω1 + ω2)/2 and a microwave with the frequency ωex〜(ω2 − ω1)/2 is applied from the outside, the two qubits are operated without affecting a state of a resonance circuit via a two-photon Rabi process. This means that it is possible to operate two qubits that do not interact directly.
Because only a qubit pair that simultaneously satisfies the two above ωr and ωex conditions can react, we can choose two of specified qubits by tuning the resonator and the microwave (Fig. 2). In addition, this enables the two-bit operation of spatially longer-distance remote qubits because the resonance circuit can be 1 mm for an LC circuit , or more for a transmission line. This work was partially supported by CREST-JST, JSPS-KAKENHI (18201018, 18001002).
 H. Nakano, K. Kakuyanagi, M. Ueda, and K. Semba, Appl. Phys. Lett. 91 (2007) 032501.
 J. Johansson, S. Saito, T. Meno, H. Nakano, M. Ueda, K. Semba, and H. Takayanagi, Phys. Rev. Lett. 96 (2006) 127006.
Fig. 1. Schematic structure of our coupler.
Fig. 2. Example of a two- operation (entanglement formation).
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