We report on the operation of a single-electron pump (comprising three Al/AlOx/Al tunnel junctions and two gates) connected to the bias electrodes through the compact on-chip Cr resistors, R≈60 kΩ>Rk=h/e2≈26 kΩ. The function of the resistors in this so-called R pump was to suppress electron cotunneling, the process which otherwise severely deteriorates the performance of few-junction single-electron devices. When a harmonic ac drive of frequency f of several MHz was applied to the gates, the current–voltage curve of the R pump exhibited remarkably horizontal current steps at I=ef. We show that the use of the resistors is capable of substantially increasing the accuracy of the pump in comparison to operation of the pump without resistors.

We observed current-voltage characteristics of superconducting single charge transistors with on-chip resistors of R approximately R(Q)=h/4e(2) approximately 6.45 kOmega, which are explained in terms of Cooper pair cotunneling. Both the effective strength of Josephson coupling and the cotunneling current are modulated by the gate-induced charge on the transistor island. For increasing values of the resistance R we found the Cooper pair current at small transport voltages to be dramatically suppressed.

We present investigations of Josephson charge-phase qubits inductively coupled to a radio-frequency driven tank-circuit enabling the readout of the states by measuring the Josephson inductance of the qubit. The circuits including junctions with linear dimensions of 60 nm and 80 nm are fabricated from Nb trilayer and allowing the determination of relevant sample parameters at liquid helium temperature. The observed partial suppression of the circulating supercurrent at 4.2 K is explained in the framework of a quantum statistical model. We have probed the ground-state properties of qubit structures with different ratios of the Josephson coupling to Coulomb charging energy at 20 mK, demonstrating both the magnetic control of phase and the electrostatic control of charge on the qubit island.Comment: 8 pages, 8 figure

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We investigate low-temperature and low-voltage-bias charge transport in a superconducting Al single electron transistor in a dissipating environment, realized as on-chip high-ohmic Cr microstrips. In samples with relatively large charging energy E c ϾE J , where E J is the Josephson coupling energy, two transport mechanisms were found to be dominant, both based on discrete tunneling of individual Cooper pairs: Depending on the gate voltage V g , either sequential tunneling of pairs via the transistor island ͓in the conducting state of the transistor around the points Q g ϵC g V g ϭe mod (2e), where C g is the gate capacitance͔ or their cotunneling through the transistor ͑for Q g away of these points͒ was found to prevail in the net current. As the conducting state of our transistors had been found to be subject to quasiparticle poisoning, high-frequency gate cycling ͑at f ϳ1 MHz) was applied to study the sequential tunneling mechanism. A simple model based on the master equation was found to be in a good agreement with the experimental data.