Dresden 2006 – scientific programme

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TT: Tiefe Temperaturen

TT 32: Transport: Quantum Coherence and Quantum Information Systems - Part 2

TT 32.14: Talk

Thursday, March 30, 2006, 17:30–17:45, HSZ 304

Investigation of the ground state of Nb charge-phase qubits — •Hermann Zangerle, J. Könemann, B. Mackrodt, R. Dolata, S. A. Bogoslovsky, M. Götz, and A. B. Zorin — Physikalisch-Technische Bundesanstalt, Braunschweig, Germany

There are several implementations of qubits built of small Josephson junctions. The Bloch transistor included in a superconducting loop can serve as a so-called charge-phase qubit with both electrostatic and magnetic control of its quantum state. For readout the device is inductively coupled to a rf-driven tank circuit. Applying chemical-mechanical polishing technique we have fabricated from Nb/Al/AlO_x/Nb sandwich the charge-phase qubits with junction size of 60 nm by 60 nm and nominal critical current of the single junction of about 50 nA. Our all-Nb samples of gradiometer design include the transistor in the loop and the tank circuit inductance on the same chip. The high critical temperature of Nb and our circuit layout make it possible to precharacterize the samples at 4.2 K. We report the radio-frequency measurements of Nb qubit circuits with characteristic ratio of the Josephson to charging energy E_J/E_c between 1 and 2 carried out at 20 mK. Due to appreciable charging energy E_c of about 50 µeV the transistor critical current was remarkably modulated by the gate. We investigated the ground state of our qubit samples as a function of two control parameters: flux Φ and charge Q. The sample with the energy ratio E_J/E_c∼ 1 allowed to map the complete ground state energy surface E(Φ,Q).