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TT: Fachverband Tiefe Temperaturen

TT 20: Focused Session: 50 Years of Flux Quantization

TT 20.4: Invited Talk

Tuesday, March 15, 2011, 12:10–12:40, HSZ 03

Quantum information with quantized fluxoids: flux qubits — •Johan E. Mooij — Kavli Institute of Nanoscience, Delft University of Technology

A closed superconducting ring which contains one or more Josephson junctions can exist in a quantum superposition of two states with different fluxoid number, when it is biased near half a flux quantum. The ring behaves as a two-level system and can be manipulated with microwave pulses. With three or four junctions in the ring, the size can be as small as several micrometers. The level splitting can be tuned with the applied magnetic flux, in addition one of the junctions can be converted into a SQUID leading to tunable quantum coupling between the fluxoid states . Relative to other types of superconducting quantum bits, flux qubits have the advantage that the next-higher level is separated by a high energy. Quantum operations can be performed in nanoseconds, coherence times go up to microseconds. Readout is performed with a SQUID that senses the generated flux of order 0.001 flux quantum. Inductive coupling between qubits is typically h times 200 MHz, allowing the realization of quantum entanglement. With flux qubits it is possible to achieve ultrastrong coupling between a two-level system and a harmonic oscillator.