Continuous Monitoring of Rabi Oscillations in a Josephson Flux Qubit

Under resonant irradiation, a quantum system can undergo coherent (Rabi) oscillations in time. We report evidence for such oscillations in a _continuously_ observed three-Josephson-junction flux qubit, coupled to a high-quality tank circuit tuned to the Rabi frequency. In addition to simplicity, this method of_Rabi spectroscopy_ enabled a long coherence time of about 2.5 microseconds, corresponding to an effective qubit quality factor \~7000.Comment: REVTeX4, 4pp., 4 EPS figure files. v3: changed title, fixed typos; final, to appear in PR

Low-frequency measurement of the tunneling amplitude in a flux qubit

We have observed signatures of resonant tunneling in an Al three-junction qubit, inductively coupled to a Nb LC tank circuit. The resonant properties of the tank oscillator are sensitive to the effective susceptibility (or inductance) of the qubit, which changes drastically as its flux states pass through degeneracy. The tunneling amplitude is estimated from the data. We find good agreement with the theoretical predictions in the regime of their validity.Comment: REVTeX4, 3pp., 3 EPS figures. v2: new sample, textual clarifications. v3: minor polishing; final, to appear in PRB Rapid

Observation of macroscopic Landau-Zener transitions in a superconducting device

A two-level system traversing a level anticrossing has a small probability to make a so-called Landau-Zener (LZ) transition between its energy bands, in deviation from simple adiabatic evolution. This effect takes on renewed relevance due to the observation of quantum coherence in superconducting qubits (macroscopic "Schrodinger cat" devices). We report an observation of LZ transitions in an Al three-junction qubit coupled to a Nb resonant tank circuit.Comment: REVTeX4, 4pp., 4 EPS figures. v2: clarifications added; final, to appear in EP

Radio-Frequency Method for Investigation of Quantum Properties of Superconducting Structures

We implement the impedance measurement technique (IMT) for characterization of interferometer-type superconducting qubits. In the framework of this method, the interferometer loop is inductively coupled to a high-quality tank circuit. We show that the IMT is a powerful tool to study a response of externally controlled two-level system to different types of excitations. Conclusive information about qubits is obtained from the read-out of the tank properties.Comment: 10 pages, 10 figures;to be published in Fizika Nizkikh Temperatur (Low Temperature Physics); v3: minor polishing; fina

Radio-frequency method for investigation of quantum properties of superconducting structures.

We implement the impedance measurement technique (IMT) for characterization of interferometer- type superconducting qubits. In the framework of this method, the interferometer loop is inductively coupled to a high-quality tank circuit. We show that the IMT is a powerful tool to study a response of externally controlled two-level system to different types of excitations. Conclusive information about qubits is obtained from the read-out of the tank properties

Trapped electron coupled to superconducting devices

We propose to couple a trapped single electron to superconducting structures located at a variable distance from the electron. The electron is captured in a cryogenic Penning trap using electric fields and a static magnetic field in the Tesla range. Measurements on the electron will allow investigating the properties of the superconductor such as vortex structure, damping and decoherence. We propose to couple a superconducting microwave resonator to the electron in order to realize a circuit QED-like experiment, as well as to couple superconducting Josephson junctions or superconducting quantum interferometers (SQUIDs) to the electron. The electron may also be coupled to a vortex which is situated in a double well potential, realized by nearby pinning centers in the superconductor, acting as a quantum mechanical two level system that can be controlled by a transport current tilting the double well potential. When the vortex is trapped in the interferometer arms of a SQUID, this would allow its detection both by the SQUID and by the electron.Comment: 13 pages, 5 figure

HTS-SQUIDs im Kraftwerk: Verringerung der Stoerfeldempfindlichkeit des SQUID-Systems Abschlussbericht

The aim of the project was the development of a measurement system, based on SQUIDs (Superconducting Quantum Interference Detector) using high-temperature superconductors, which is usable for NDE (non-destructive evaluation) in industrial environment. To realize this task, both - new SQUID gradiometers and a new SQUID electronics - have been developed. The susceptibility of the SQUID structures to flux penetration by large environmental magnetic fields could be minimized by technology and layout development as well. Using a combination of gradiometer SQUID and magnetometer SQUID new sensors were developed, which are highly balanced, sensitive against magnetic field gradients and, at the same time, resistant against disturbances and movements within the Earth's field. The new SQUID electronics allows the use of bias current reversal with frequencies in the MHz range. Therewith a low-noise operation is possible also at low frequencies obtaining large bandwidth and dynamic range at the same time. The electronics has been integrated, thus allowing a robust operation. The power of the complete SQUID system could be shown exemplary with an NDE, where the SQUID was moved by hand across the workpiece in the disturbed environment of the lab. (orig.)SIGLEAvailable from TIB Hannover: F04B607 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekBundesministerium fuer Bildung und Forschung (BMBF), Bonn (Germany)DEGerman