Josephson tunnel junctions with nonlinear damping for RSFQ-qubit circuit applications

We demonstrate that shunting of Superconductor-Insulator-Superconductor Josephson junctions by Superconductor-Insulator-Normal metal (S-I-N) structures having pronounced non-linear I-V characteristics can remarkably modify the Josephson dynamics. In the regime of Josephson generation the phase behaves as an overdamped coordinate, while in the superconducting state the damping and current noise are strikingly small, that is vitally important for application of such junctions for readout and control of Josephson qubits. Superconducting Nb/AlO${_x}$/Nb junction shunted by Nb/AlO${_x}$/AuPd junction of S-I-N type was fabricated and, in agreement with our model, exhibited non-hysteretic I-V characteristics at temperatures down to at least 1.4 K.Comment: 4 pages incl. 3 figure

Bloch inductance in small-capacitance Josephson junctions

We show that the electrical impedance of a small-capacitance Josephson junction includes besides the capacitive term $-i/\omega C_B$ also an inductive term $i\omega L_B$. Similar to the known Bloch capacitance $C_B(q)$, the Bloch inductance $L_B(q)$ also depends periodically on the quasicharge $q$, and its maximum value achieved at $q=e (\textrm{mod} 2e)$ always exceeds the value of the Josephson inductance of this junction $L_J(\phi)$ at fixed $\phi=0$. The effect of the Bloch inductance on the dynamics of a single junction and a one-dimensional array is described.Comment: 5 pages incl. 3 fig

A hybrid superconductor-normal metal electron trap as a photon detector

A single-electron trap built with two Superconductor (S) - Insulator (I) - Normal (N) metal tunnel junctions and coupled to a readout SINIS-type single-electron transistor A (SET A) was studied in a photon detection regime. As a source of photon irradiation, we used an operating second SINIS-type SET B positioned in the vicinity of the trap. In the experiment, the average hold time of the trap was found to be critically dependent on the voltage across SET B. Starting in a certain voltage range, a photon-assisted electron escape was observed at a rate roughly proportional to the emission rate of the photons with energies exceeding the superconducting gap of S-electrodes in the trap. The discussed mechanism of photon emission and detection is of interest for low-temperature noise spectrometry and it can be of relevance for the ampere standard based on hybrid SINIS turnstiles.Comment: submitted, 3 pages, 3 figure

Pumping properties of the hybrid single-electron transistor in dissipative environment

Pumping characteristics were studied of the hybrid normal-metal/superconductor single-electron transistor embedded in a high-ohmic environment. Two 3 micrometer-long microstrip resistors of CrOx with a sum resistance R=80kOhm were placed adjacent to this hybrid device. Substantial improvement of pumping and reduction of the subgap leakage were observed in the low-MHz range. At higher frequencies 0.1-1GHz, a slowdown of tunneling due to the enhanced damping and electron heating negatively affected the pumping, as compared to the reference bare devices.Comment: 3 pages 4 figure

Storage capabilities of a 4-junction single electron trap with an on-chip resistor

We report on the operation of a single electron trap comprising a chain of four Al/AlOx/Al tunnel junctions attached, at one side, to a memory island and, at the other side, to a miniature on-chip Cr resistor R=50 kOhm which served to suppress cotunneling. At appropriate voltage bias the bi-stable states of the trap, with the charges differing by the elementary charge e, were realized. At low temperature, spontaneous switching between these states was found to be infrequent. For instance, at T=70 mK the system was capable of holding an electron for more than 2 hours, this time being limited by the time of the measurement.Comment: 3 pages of text and 2 figure