44 research outputs found
Shot noise in superconducting junctions with weak link formed by Anderson impurity
A theory is developed to study shot noise in superconducting (SAS) and hybrid
(SAN) junctions with singly occupied Anderson impurity (A) as a weak link. The
zero-frequency DC component of the shot noise spectral density is calculated at
zero temperature as a function of the bias at different Coulomb repulsion
strengths U, and show a remarkable structure resulting from combination of
electron-electron interaction and Andreev reflections.Comment: 4 two column pages including 4 .eps figure
Low-T_c Josephson junctions with tailored barrier
Nb/Al_2O_3/Ni_{0.6}Cu_{0.4}/Nb based
superconductor-insulator-ferromagnet-superconductor (SIFS) Josephson tunnel
junctions with a thickness step in the metallic ferromagnetic
\Ni_{0.6}\Cu_{0.4} interlayer were fabricated. The step was defined by optical
lithography and controlled etching. The step height is on the scale of a few
angstroms. Experimentally determined junction parameters by current-voltage
characteristics and Fraunhofer pattern indicate an uniform F-layer thickness
and the same interface transparencies for etched and non-etched F-layers. This
technique could be used to tailor low-T_c Josephson junctions having controlled
critical current densities at defined parts of the junction area, as needed for
tunable resonators, magnetic-field driven electronics or phase modulated
devices.Comment: 6 pages, 6 figures, small changes, to be published by JA
Terahertz Bloch oscillator with a modulated bias
Electrons performing Bloch oscillations in an energy band of a dc-biased
superlattice in the presence of weak dissipation can potentially generate THz
fields at room temperature. The realization of such Bloch oscillator is a
long-standing problem due to the instability of a homogeneous electric field in
conditions of negative differential conductivity. We establish the theoretical
feasibility of stable THz gain in a long superlattice device in which the bias
is quasistatically modulated by microwave fields. The modulation waveforms must
have at least two harmonics in their spectra.Comment: 5 page
Coherent Rabi response of a charge-phase qubit under microwave irradiation
We report on radio-frequency measurements of the charge-phase qubit being
under continuous microwave irradiation in the state of weak coupling to a
radio-frequency tank circuit. We studied the rf impedance dependence on the two
important parameters such as power of microwave irradiation whose frequency is
close to the gap between the two lowest qubit energy levels, and temperature of
the internal heat bath. We have found that backaction effects of the qubit on
the rf tank, and vice versa, tank on the qubit, lead to a negative as well as a
positive real part of the qubit impedance Re seen by the tank. We
have implemented noise spectroscopy measurements for direct impedance readout
at the extreme points corresponding to maximum voltage response and obtained
absolute values of about 0.017 for the negative and positive
Re. Our results demonstrate the existence and persistence of the
coherent single- and multi-photon Rabi dynamics of the qubit with both negative
and positive dynamic resistance inserted into the tank in the temperature range
of 10 to 200 mK.Comment: 11 pages, 9 figure
Josephson junction microwave amplifier in self-organized noise compression mode
The fundamental noise limit of a phase-preserving amplifier at frequency is the standard quantum limit . In the microwave range, the best candidates have been amplifiers based on superconducting quantum interference devices (reaching the noise temperature at 700 MHz), and non-degenerate parametric amplifiers (reaching noise levels close to the quantum limit at 8 GHz). We introduce a new type of an amplifier based on the negative resistance of a selectively damped Josephson junction. Noise performance of our amplifier is limited by mixing of quantum noise from Josephson oscillation regime down to the signal frequency. Measurements yield nearly quantum-limited operation, at 2.8 GHz, owing to self-organization of the working point. Simulations describe the characteristics of our device well and indicate potential for wide bandwidth operation
Thermal fluctuations in resonant motion of fluxons on a Josephson transmission line: Theory and experiment
The radiation emission from long and narrow Josephson tunnel junctions dc-current biased on zero-field steps has been ascribed to resonant motion of fluxons on the transmission line. Within this dynamic model a theoretical expression for the radiation linewidth is derived from a full statistical treatment of thermal fluctuations in the fluxon velocity. The result appears to be very general and is corroborated by experimental determination of linewidth and frequency of radiation emitted from overlap Nb-I-Pb junctions