Controllable coupling of superconducting qubits and implementation of quantum gate protocols
Doctoral thesis, 2006

The concept of a quantum computer was invented in the beginning of the 1980s as a quantum generalization of the reversible classical computer. After discoveries in the middle of the 1990s of quantum algorithms, which would solve some problems considered intractable for classical computers, the field of quantum computing has developed rapidly. A wide range of quantum systems are investigated for their possible ability to implement quantum algorithms in practice. The most promising solid state implementations are superconducting electrical circuits based on the Josephson effect, and the Coulomb blockade effect. Superconducting circuits are macroscopic quantum systems which can be fabricated using standard lithography technologies. Superconducting qubits - basic building blocks of a quantum computer - have been developed at several research laboratories, among others the MC2 here at Chalmers. This thesis presents a theoretical investigation of controllable coupling of superconducting qubits based on the single Cooper-pair box. Two coupling schemes are investigated in detail: current-controlled coupling via large Josephson junctions, and coupling via a superconducting stripline cavity. Both coupling designs are scalable and suitable for the charge regime of the single Cooper-pair box, as well as for the charge-phase regime where the qubit is better protected from environmental noise. Quantum gate protocols which are relevant for these physical couplings are discussed. The investigations show that, for both considered designs, the simplest two-qubit gate is the conditional phase-shift gate. This gate is universal for quantum computing, and it can create maximally entangled qubit pairs in only one run. The investigation combines methods from electrical engineering, quantum mechanics and quantum computer science.

Controllable Qubit-Coupling

Single Cooper-Pair Box

Josephson Effect

Quantum Gate

Superconducting Circuit

Quantum Computer


Quantum Electrical Circuit

10.00 Kollektorn, Kemivägen 9, Chalmers
Opponent: Docent Dr. Jens Siewert, Institute of Theoretical Physics, University of Regensburg, Germany


Margareta Wallquist

Chalmers, Microtechnology and Nanoscience (MC2), Applied Quantum Physics

Current-controlled coupling of superconducting charge qubits

Quantum Computation: solid state systems, Kluwer Academic Plenum Publishers,; (2004)

Conference contribution

Superconducting qubit network with controllable nearest neighbor coupling

New Journal of Physics,; Vol. 7(2005)

Journal article

Josephson junction qubit network with current-controlled interaction

Physical Review B,; Vol. 70(2004)

Journal article

Subject Categories

Other Computer and Information Science

Electrical Engineering, Electronic Engineering, Information Engineering

Condensed Matter Physics



Technical report MC2 - Department of Microtechnology and Nanoscience, Chalmers University of Technology: 81

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 2511

10.00 Kollektorn, Kemivägen 9, Chalmers

Opponent: Docent Dr. Jens Siewert, Institute of Theoretical Physics, University of Regensburg, Germany

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