Bi-SQUID Arrays Research Articles

Critical Current Spread and Thermal Noise in Bi-SQUID Cells and Arrays

We report on the detailed analysis of thermal noise, critical current spread, and parasitic elements influences on the bi-SQUID voltage response and its linearity. These results together with the results of our previous studies serve as a useful guidance in designing high-performance bi-SQUIDs and bi-SQUID arrays implemented in low-temperature superconductor or high-temperature superconductor for many applications such as linear low-noise amplifiers and high-sensitivity antennas.

Development of 2-D Bi-SQUID Arrays With High Linearity

We develop a two-dimensional (2D) Superconducting Quantum Interference Filter (SQIF) array based on recently introduced high-linearity tri-junction bi-SQUIDs. Our bi-SQUID SQIF array design is based on a tight integration of individual bi- SQUID cells sharing inductances with adjacent cells. We provide extensive computer simulations, analysis and experimental measurements, in which we explore the phase dynamics and linearity of the array voltage response. The non-uniformity in inductances of the bi-SQUIDs produces a pronounced zero-field single antipeak in the voltage response. The anti-peak linearity and size can be optimized by varying the critical current of the additional junction of each bi-SQUID. The layout implementation of the tight 2D array integration leads to a distinct geometrical diamond shape formed by the merged dual bi-SQUID cells. Different size 2D arrays are fabricated using standard HYPRES niobium 4.5 kA/cm2 fabrication process. The measured linearity, power gain, and noise properties will be analyzed for different array sizes and the results will be compared with circuit simulations. We will discuss a design approach for the electrically small magnetic field antenna and low-noise amplifiers with high bandwidth based on these 2D bi-SQUID SQIF arrays. The results from this work will be used to design chips densely and completely covered in bi-SQUIDs that has optimized parameters such as linearity and power gain.

DC and RF Measurements of Serial Bi-SQUID Arrays

SQUID arrays are promising candidates for low profile antennas and low noise amplifier applications. We present the integrated circuit designs and results of DC and RF measurements of the wideband serial arrays based on integration of linear bi-SQUID cells forming a Superconducting Quantum Interference Filter (bi-SQUID SQIF). Various configurations of serial arrays designs are described. The measured linearity, power gain, and noise temperature are analyzed and compared. The experimental results are matched to results of mathematical modeling. A serial bi-SQUID SQIF arrays are mounted into a coplanar waveguide (CPW) and symmetrically grounded to corresponding sides of CPW. The RF output comes out from the central common line, which is also used for DC biasing and forms a symmetrical balanced output. The signal and DC flux biasing line is designed as coplanar lines passed in parallel over each bi-SQUID cell in a bidirectional fashion concentrating magnetic flux inside of each cell. Serial bi-SQUID SQIF arrays are fabricated on 5 mm x 5 mm chips using standard HYPRES niobium 4.5 kA/cm2 fabrication process.

Bi-SQUID arrays and parallel SQIF structures for active electrically small antennas

The basic physics of the development of broadband active electrically small superconductive antennas for the subgigahertz and gigahertz frequency ranges, based on series arrays with cells characterized by a highly linear voltage response to the magnetic component B of an electromagnetic signal, are under consideration. As such cells, bi-SQUIDs and cells based on two parallel SQIF structures that are differentially connected are proposed. Series arrays of cells with linear voltage response, including an antenna prototype, are fabricated using standard niobium technology with a critical current density of Josephson junctions of 4.5 kA/cm2 and are studied experimentally. The data obtained allow estimation of the achievable dV/dB conversion factor and the sensitivity δB for an antenna integrated with a magnetic flux converter, placed on an available area of 3.3 × 3.3 mm of a chip 5 × 5 mm in size. These values are found to be 10 μV/nT and 20 fT/Hz1/2, respectively. The conversion factor increases in proportion with the area a2 occupied by the antenna with a square flux converter, and the sensitivity is improved as a−3/2.

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.