A. V. Sharafiev

Active Electrically Small Antenna Based on Superconducting Quantum Array

We introduce Superconductive Quantum Arrays and propose to use these structures as active Electrically Small Antennas (ESA). Several prototypes of the active ESA were fabricated using Nb process with a critical current density of 4.5 kA/cm2 and experimentally evaluated. The magnetic field to voltage transfer function linearity up to 70 dB was measured, and transfer factor dV/dB up to 6.5 mV/μT was observed for the ESA prototype containing 560 cells.

Linear Bi-SQUID Arrays for Electrically Small Antennas

Recently we proposed so-called bi-SQUID based on a 3-junction SQUID circuit capable of providing highly linear voltage response. In this report, we present the experimental evaluation of series arrays of 20 and 128 bi-SQUIDs fabricated with a 4.5 kA/cm2 Nb process as well as a prototype of an active electrically small antenna based on series array of 12 bi-SQUIDs. Both the origins of imperfections of the observed response linearity and the possible ways of the linearity improvement are discussed.

Superconducting Quantum Arrays

Superconducting Quantum Arrays (SQAs) based on integration of quantum cells each consisting of two Josephson-junction parallel arrays [Differential Quantum Cell (DQC)] are analyzed for applications in broadband radio frequency systems. These SQAs are capable of providing highly linear magnetic signal to voltage transfer with high dynamic range. Both detail study of the quantum cells with realistic parameters and analysis of their characteristics, including voltage response linearity, are presented and discussed. A prototype of an active electrically small antenna is implemented based on an SQA containing 560 DQCs. We demonstrated the SQA voltage response swing as high as ~ 100 mV at a transfer factor of ~ 6.5 mV/μT.

Output Power and Loading of Superconducting Quantum Array

Using numerical simulation, we investigate the output power and loading tradeoffs for Superconducting Quantum Arrays (SQAs) suggested for the implementation of broadband radio-frequency systems, including active electrically small antennas capable of providing highly linear transfer characteristics with a high dynamic range. Standard matching loads used for linear microwave networks, including passive antennas, are not applicable for the active SQA-based circuits generally due to the strong shunting effect restricting the attainable linearity. At the same time, the output power of an active device can be increased by a power supply even for the impedance-mismatched circuits. Both the optimum achievable balance of the shunt effects and getting the required output power are analyzed and discussed in detail.

Josephson junctions with nonsinusoidal current-phase relations based on heterostructures with a ferromagnetic spacer and their applications

A detailed review of theories describing the current-phase relations in Josephson junctions based on heterostructures with ferromagnetic layers is presented. The particular attention has been focused on the possibilities of making the so-called ϕ-junctions in which the ground state in the absence of the current is realized when the Josephson phase ϕ is nonzero. The recently popular speculations concerning the possibility of applying the Josephson π- and ϕ-junctions, which are made on the basis of heterostructures with ferromagnetic layers, to the formation of quantum bits (qubits) have been illustrated. An attempt to formulate the requirements for the characteristics of the Josephson heterostructures based on the first principles of the quantum-mechanical description of superconducting interferometers with a low inductance has been made.

Signal and noise characteristics of bi-SQUID

We present an improved analytic theory, numerical simulation, and analysis of noise characteristics of a bi-SQUID in comparison with those of a dc SQUID in an open loop configuration. The analytic theory which had been developed earlier, neglecting a pulse component of the difference of the phases of Josephson junctions, is now completed taking into account the pulse component. In the bi-SQUID, the additional Josephson junction introduces another source of fluctuations and changes its transfer function, nonlinear dynamics, and the noise spectrum transformation. Some increase in the reduced-to-input noise at low values of applied magnetic flux comes from the nonlinear flux to phase difference transformation that was introduced in bi-SQUID as a way to linearize its voltage response.

Bi-SQUIDs with submicron cross-type Josephson tunnel junctions

We present and discuss results for the experimental evaluation of bi-SQUIDs with shunted submicron cross-type Josephson tunnel junctions characterized by low capacitance. Such a technology, meant for SQUID sensors, was developed to enable unshielded operation of the sensors and to avoid unwanted flux trapping. The obtained voltage response swing is as high as 60 µV. The observed differences between the measured and theoretically predicted voltage responses are analyzed and the possible ways to approach a triangular voltage response shape are discussed. An improved design of such bi-SQUIDs with gradiometric topology is suggested to compensate for the influence of the sizable parasitic three-junction loop, which is the main drawback of the implemented bi-SQUID topology.

Microwave Dynamics of Superconducting Quantum Cell

We present an analysis of the microwave dynamics of the Superconducting Quantum Cell (SQC) in the microwave field performed by means of 3-D numerical simulations using a two-plate model. The simulations are necessary for the realization of active electrically small antennas (ESAs) based on Superconducting Quantum Arrays composed of SQCs. It was found that the incident wave mainly affects the SQC through its magnetic component in the same way as a low-frequency magnetic field does. We suggested possible improvements of the cell design by taking into account the current and field distributions in the plate intersection, as well as the size effects.

High Linearity Voltage Response Parallel-Array Cell

We studied in detail a cell consisting of two parallel SQUID arrays or two parallel superconducting interference filters (SQIFs) connected differentially with the goal of achieving highly linear voltage response to magnetic signal. In these different cell designs, we accounted for realistic values of coupling inductances in contrast to limiting case of vanishing inductances considered earlier. We found that a cell based on regular parallel SQUID arrays produces higher linearity as compared to the cell based on SQIFs. This high-linearity cell can be used for realizing Superconducting Quantum Arrays (SQA) capable of providing a broadband, highly-linear magnetic field-to-voltage transfer function and high dynamic range.

Superconducting Quantum Arrays for broadband RF systems

Superconductive Quantum Arrays (SQA) are suggested for the implementation of broadband radio frequency systems capable of providing highly linear magnetic signal to voltage transfer with high dynamic range. Possible basic cells of the arrays and their attainable characteristics are considered. Results of experimental studies and possible applications are presented and discussed.