Detection of X-ray photons by niobium Josephson tunnel junctions with trapped Abrikosov vortices

The high atomic number of niobium (Z=41) can be exploited to develop a high efficiency superconducting gamma-ray detector based on a novel detection principle, namely, the interaction of a single gamma-ray photon with Abrikosov vortices trapped inside a niobium bulk absorber. To study the feasibility of this principle, niobium type Josephson tunnel junctions with the aluminium oxide as a tunnel barrier and with a thick (0.3 mm) niobium base electrode have been fabricated. The devices have been tested at T = 4.2 K in terms of the current-voltage characteristic and of the magnetic field dependence of the Josephson critical current. The feasibility of the detection principle has been tested under X-ray irradiation from the 55Fe source. The time dependence of the Josephson critical current of the junction with trapped Abrikosov vortices has been recorded without and with X-ray irradiation. The data analysis of obtained experimental curves has confirmed the effect of the X-ray photon absorption on the Josephson critical current caused by jumping of Abrikosov vortices

Observation of a New Fluxon Resonant Mechanism in Annular Josephson Tunnel Structures

A novel dynamical state has been observed in the dynamics of a perdurbed sine-Gordon system. This resonant state, has been experimentally observed as a singularity in the dc current voltage characteristic of an annular Josephson tunnel junction, excited in the presence of a magnetic field. With this respect, it can be assimilated to self-resonances known as Fiske steps. Differently from these, however, we demonstrate, on the basis of numerical simulations, that its detailed dynamics involves rotating fluxon pairs, a mechanism associated, so far, to self-resonances known as zero-field steps.Comment: 4 pages, 2 figures, submitted to Physical Review Letter

Superconducting Qubit Network as a Single Microwave Photon Detector for Galactic Axion Search

Experimental search of galactic axions requires detection of single photons in the microwave range. We work on a novel approach to detect single microwave photons based on a coherent collective response of quantum states occurring in a superconducting qubit network (SQN) embedded in a low-dissipative superconducting resonator. We propose a two resonators detector configuration with two parallel resonators without common part and with separated input and output terminals. The device consists of a low-dissipative resonator with embedded SQN in which microwave photons arrive (“signal resonator”), and a transmission line for measuring the frequency dependent transmission coefficient demonstrating resonant drops at the qubit frequencies (“readout resonator”). In comparison with T-type three terminal device recently proposed and investigated by us, the device with two resonators with separated input and output terminals doesn’t contain common part of both resonators and exclude an unwanted noise from measurement readout circuits to the signal resonator. A layout of two resonators four terminal SQN detectors containing 5 flux qubits weakly coupled to a low-dissipative signal and readout resonator was developed and optimized. The samples were fabricated by Manhattan Al-based technology with Nb resonator circuits. The SQN detector was experimentally tested in terms of microwave measurements of scattering parameters of both resonators and crosstalk properties. Comparison of experimental data with results of the simulations permits one to conclude that the electromagnetic conditions of the fundamental resonant peak of 8.5 GHz of both resonators aren’t affected by the crosstalk phenomenon and their performances provided by the design remain not altered for correct device operation

Fiske Steps and Abrikosov Vortices in Josephson Tunnel Junctions

We present a theoretical and experimental study of the Fiske resonances in the current-voltage characteristics of "small" Josephson junctions with randomly distributed misaligned Abrikosov vortices. We obtained that in the presence of Abrikosov vortices the resonant interaction of electromagnetic waves, excited inside a junction, with the ac Josephson current manifests itself by Fiske steps in a current-voltage characteristics even in the absence of external magnetic field. We found that the voltage positions of the Fiske steps are determined by a junction size, but the Fiske step magnitudes depend both on the density of trapped Abrikosov vortices and on their misalignment parameter. We measured the magnetic field dependence of both the amplitude of the first Fiske step and the Josephson critical current of low-dissipative small $Nb$ based Josephson tunnel junctions with artificially introduced Abrikosov vortices. A strong decay of the Josephson critical current and a weak non-monotonic decrease of the first Fiske step amplitude on the Abrikosov vortex density were observed. The experimentally observed dependencies are well described by the developed theory.Comment: 21 pages, 7 figures, submitted to Physical Review

Composite excitation of Josephson phase and spin waves in Josephson junctions with ferromagnetic insulator

Coupling of Josephson-phase and spin-waves is theoretically studied in a superconductor/ferromagnetic insulator/superconductor (S/FI/S) junction. Electromagnetic (EM) field inside the junction and the Josephson current coupled with spin-waves in FI are calculated by combining Maxwell and Landau-Lifshitz-Gilbert equations. In the S/FI/S junction, it is found that the current-voltage (I-V) characteristic shows two resonant peaks. Voltages at the resonant peaks are obtained as a function of the normal modes of EM field, which indicates a composite excitation of the EM field and spin-waves in the S/FI/S junction. We also examine another type of junction, in which a nonmagnetic insulator (I) is located at one of interfaces between S and FI. In such a S/I/FI/S junction, three resonant peaks appear in the I-V curve, since the Josephson-phase couples to the EM field in the I layer.Comment: 16 pages, 5 figure

Coherent Quantum Network of Superconducting Qubits as a Highly Sensitive Detector of Microwave Photons for Searching of Galactic Axions

We propose a novel approach to detect a low power microwave signal with a frequency of the order of several GHz based on a coherent collective response of quantum states occurring in a superconducting qubits network (SQN). An SQN composes of a large number of superconducting qubits embedded in a low-dissipative superconducting resonator. Our theory predicts that an SQN interacting with the off-resonance microwave radiation, demonstrates the collective alternating current Stark effect that can be measured even in the limit of single photon counting. A design of the layout of three terminals SQN detectors containing 10 flux qubits weakly coupled to a low-dissipative R-resonator and T-transmission line was developed. The samples were fabricated by Al-based technology with Nb resonator. The SQN detector was tested in terms of microwave measurements of scattering parameters and two-tone spectroscopy. A substantial shift of the frequency position of the transmission coefficient drop induced by a second tone pump signal was observed, and this effect clearly manifests a nonlinear multiphoton interaction between the second-tone microwave pump signal and an array of qubits

Investigation of the superconducting properties of Nb films covered with PECVD a-Si:H layers for superconducting qubit application

Hydrogenated amorphous silicon (a-Si:H) grown by PECVD has a lower loss tangent (tanδ) among conventional dielectrics(suchasSiO2 andSiNx)andhenceisconsideredasthebestamorphousdielectricmaterialforsuperconducting qubit application. The incorporation of PECVD a-Si:H into the Nb technology requires attention due to the possible degradationofthesuperconductivityoftheNbfilms. Superconductingtransitiontemperature(Tc)andresidualresistivity(ρ0)of20nm,50nmand100nmthickNbfilmsweremeasuredbeforeandaftera-Si:Hdeposition. Thepenetration ofoxygenandhydrogeninsidetheNbfilmswasevaluatedfromthevariationofthelatticeparameterobtainedbyX-ray diffraction. The high process temperature (250◦ C) and the presence of energetic hydrogen ions during the a-Si:H layer growthcausedadecreaseofTc andincreaseofρ0 throughtwophysicalprocesses: 1)oxygendiffusionfromthesurface Nb oxides and 2) hydrogen diffusion inside the Nb films. The degradation of Tc was reduced with the increase of the filmthickness. NitridationofNbfilmsanddepositionofasputteredthinamorphoussiliconlayer(a-Si)ontheNbfilms (in both cases made in situ after the Nb film deposition) were investigated as surface treatments to protect the Nb films during PECVD. It was demonstrated that both methods markedly reduce oxygen and hydrogen diffusion into Nb films during a-Si:H deposition, but the a-Si layer was more effective to protect the Nb films