Quantum interference filters based on oxide superconductor junctions for microwave applications

We have studied superconducting quantum interference filters (SQIFs) based on bicrystal neodymium gallate substrates, which can be used in the microwave frequency range. The characteristics of a serial SQIF have been compared for the first time with those of a single superconducting quantum interference device (SQUID) and a chain of serially connected SQUIDs with equal areas of superconducting loops. The regime of SQIF operation with a voltage-flux (V-Φ) characteristic determined by the magnetic-field dependence of the critical current in the Josephson junction has been analyzed. It is shown that the output noise of a SQIF measured with a cooled amplifier in the 1–2 GHz range is determined by the slope of the V-Φ characteristic. The influence of a spread in the parameters of Josephson junctions in the SQIF on the integral V-Φ characteristic of the whole structure is considered.     

Experiments on a Superconducting Qubit Manipulated by Fast Flux Pulses: The Issue of Non-adiabaticity

The double SQUID qubit is a superconducting interferometer (SQUID) made of two Josephson junctions and two superconducting loops. Its energy potential can be greatly modified in shape and symmetry by using two magnetic control fluxes that can change the potential from a double well to an almost harmonic single well: This feature is exploited for manipulating the qubit without resorting to the usual NMR-like techniques with microwave irradiation. The qubit machinery relies on these operations being performed non-adiabatically, realizing a transition between the two lowest-lying energy levels, at the same time avoiding excitation of upper levels, a condition that can be satisfied by using control pulses with proper risetime. We show experimental results referring to manipulation of the qubit at different rates and make a qualitative comparison with theoretical expectations.     

1988 Applied Superconductivity Conference

The following topics are dealt with: superconducting electronics; superconducting quantum interference devices (SQUIDs); magnetometers; Josephson device memories; thin-film superconducting materials; tunnel junctions; Josephson device logic circuits; high-T_c (critical temperature) superconductors; YBaCuO superconductors: ceramic superconductor memories; millimeter-wave detectors; Josephson device mixers; superconducting transmission-line structure; superconducting microwave cavities; tunnel spectroscopy; laser-induced switching of superconductors; gradiometers; harmonic mixing; SIS (superconductor-insulator-superconductor) mixers; superconducting bolometers; superconductor device fabrication; SSC; (Superconductor Super Collider); magnets; superconducting magnets; chaos in Josephson junction systems; superconducting coils; superconducting material preparation; MHD; (magnetohydrodynamics) magnets; magnetic resonance imaging (MRI) magnets; and niobium materials devices     

A fast Josephson SFQ shift register

A Josephson SFQ (single flux quantum) shift register circuit operating under a two-phase power was designed and tested. The test cell was fabricated using the Nb/AlO_x/Nb junction SNEAP process. The main data latch is DC biased; the data storage is accomplished by the transfer of a single flux quantum in or out of a SQUID (superconducting quantum interference device) superconducting loop. A two-phase sinusoidal clock with offset is used to operate the circuit. Simulations verify that the operating frequency can go beyond 70 GHz, with operating margins exceeding 20%. Preliminary measurements indicate that the circuit operates as intended     

On the transfer function of a weak link ultra-low capacity RF-SQUID

In the investigation of the transfer function (field sensitivity) of a weak link ultra-low capacity RF-SQUID, there is a problem which concerns the response of the quantum nonlinear oscillator formed by the geometrical inductance of the superconducting ring, the Josephson inductance and the weak link capacity, to the external magnetic flux. In order to account for the influence of the small quantum charge fluctuation on the Josephson potential, a phenomenological approach is proposed, which leads to a phase shift consisting of two additives. The first additive is a constant depending on the weak link parameters, and the second is linear in the external magnetic flux. The influence of the quantum fluctuation is effectively reduced to increasing the period of the current-phase relation for the RF-SQUID in the external magnetic flux, which leads to a reduction of the response of such a quantum SQUID in comparison with the usual RF SQUID.     

High-speed Josephson integrated circuit technology

The author describes recent progress in high-speed integrated circuits using niobium junctions. He briefly describes the circuit fabrication process and then introduces the modified variable threshold logic (MVTL) gate family. The lowest experimentally obtained MVTL OR-gate delay was only 2.5 ps with a power consumption of 17 μW/gate. This gate family is used in various high-speed logic circuits, such as 8-bit shift registers, 16-bit ALUs (arithmetic logic units), and 4-bit microprocessors. The author confirmed the high-speed operation of less than 10 ps per gate on average for these circuits. A novel high-sensitivity magnetic sensor using a SQUID (superconducting quantum interference device) was also developed. It is called a single-chip SQUID magnetometer because the feedback circuit, which is operated at room temperature is a conventional SQUID system, has been integrated on the same chip as the SQUID sensor itself     

Superconducting Electronics at mK Temperatures

The interest in superconducting electronics working at millikelvin temperatures has increased during the past few years, and several novel devices and amplifiers utilizing mesoscopic Josephson junctions have been developed. We review the present status of a few of these devices, foremost the inductively-read superconducting Cooper pair transistor and the Bloch oscillating transistor. As a comparison, we review the status of dc SQUID devices which provide the traditional amplifier choice when approaching the standard quantum limit in ultra-sensitive measurements. In addition, we discuss a new type of current pump, the Sluice, in which modulation of Josephson energy is employed to produce large currents at high accuracy. These new developments continue the flourishing “Otaniemi tradition” in ultra-low-temperature physics and SQUID magnetometry for which Academician Olli V. Lounasmaa acted as the primus motor over three decades.     

SQUID operation with ferromagnetic core superconducting dc transformers

We successfully used ferromagnetic core (Cryoperm 10) superconducting transformers to enhance the transfer of external magnetic flux to a symmetric two-holes rf biased quantum interference device (SQUID). The flux transfer ratio has been enhanced up to a factor of ten in comparison with air core superconducting transformers. It has been observed that the presence of ferromagnetic cores in the holes of the SQUID does not introduce additional noise, while it introduces a reduction in the Q of the 19 MHz tank circuit.The experimental results are discussed in the framework of a detailed analysis of the behaviour of the complete SQUID plus tank circuit plus ferromagnetic core superconducting transformer circuit.     

A flash Josephson A/D converter constructed with one-junctionSQUIDs

A novel approach to the construction of a flash-type Josephson A/D (analog/digital) converter is presented. Simulations show that one-junction SQUID (superconducting quantum interference device) comparators can have a greater than fivefold advantage in bandwidth over the two- or three-junction SQUIDs in an A/D circuit. Assuming a Nb junction technology, the simulations show that a 6-bit A/D converter using one-junction SQUID comparators could have a sampling rate of ~20 GHz with ~5 bits of resolution for a 5-GHz input signal. Detailed analysis and simulations of an A/D converter constructed with one-junction SQUIDs are presented. Further improvement can be made by using a coding algorithm which requires 2N-1 comparators, instead of N, for an N-bit A/D converter     

Attachment of magnetic molecules on a nanoSQUID

A novel procedure combining monolayer self-assembly with electron beam lithography has been developed for attaching ferritin nanoparticles to a submicron thin-film SQUID (superconducting quantum interference device). After opening a window in the PMMA (polymethylmethacrylate) resist, organic linker molecules are used to attach ferritin to the exposed parts of the gold overlayer of a Nb nanoSQUID. This allows the magnetic nanoparticles to be located optimally as far as magnetic coupling to the nanoSQUID is concerned.     

Very low-power Stirling cryocoolers using plastic and composite materials

An experimental investigations of several concepts for very-low-power cryocoolers for operating highly-sensitive superconducting devices has been undertaken. The devices to be cooled are those using Josephson junctions, such as SQUID magnetometers, voltage standards and A/D converters. The common basic feature of the concepts is that very low levels of magnetic, electric, and mechanical interference are potentially realizable. Nylon and epoxy-glass composites have been used for cold cylinders, pistons, and displacers.

A multi-stage Stirling machine of these materials reliably maintains a temperature below 10 K, with a few mW heat load and an input drive power of 50 W.A single-stage machine maintains a temperature in the neighbourhood of 4K with the hot end an at 10K and a drive power of the order of 5W. Specific heats and thermal conductivities of the materials have been measured between 5 and 10 K, for purposes of analysis of regeneration inefficiency.     

Cryogenic Voltage Comparator System for 2e/h Measurements

The design and operation of a cryogenic voltage comparator system for precision 2e/h measurements is described. Major improvements embodied in the new 2e/h system include the use of 1) a single microstripline-coupled Josephson tunnel junction to obtain usable step voltages up to 10 mV at 10.0 GHz, 2) a cryogenic voltage divider comprised of two resistors whose ratio is calibrated with a low-temperature dc current comparator, 3) a superconducting quantum interference device (SQUID) null detector, and 4) superconducting switching. The accuracy of the present 196:1 divider system is estimated to be about 2 parts in 108 on the basis of preliminary tests and is limited by resistor self-heating during calibration.     

Structural and Magnetic Analyses of Magnetic Nanoparticles Coated with Oleate Molecules

Physical and chemical properties of the magnetic nanoparticles coated with oleate have been investigated by means of transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Mossbauer spectroscopy, superconducting quantum interference device (SQUID) magnetometry, infrared spectra (IR) and the contact angle device. The results show that doped Al ions in Fe3O4 nanoparticles are located on the octahedral sites of the spinel structure. Oleate is coated on the magnetic nanoparticles with two layers by chemical absorbing, the outer layer can be washed away. The wetting of the surface of magnetic nanoparticles coated with monolayer has been changed from hydrophilicity to hydrophobicity, and the nanoparticles can be dispersed very well in some organic solutions.     

Design and fabrication of thin-film resistive SQUIDs

A partly resistive superconducting quantum interference device, or, RSQUID, is a device utilizing the nonstationary Josephson effect. The resistive part of the device makes it possible to apply a voltage to the Josephson junctions and, in this way, to convert it into frequency. The transconductance ratio of this conversion is defined by the physical constants: 1/ø₀ = 2e/h (ø₀ is the flux quantum). Five different types of thin-film RSQUIDs are described in this paper. They are: an rf RSQUID with an rf pump and one Josephson junction, a dc RSQUID with dc bias and two Josephson junctions, and three topologically different double RSQUIDs with an rf pump, two Josephson junctions and two quantization loops. All five types were successfully fabricated. The resistive part of all devices was an S-N-S sandwich with Nb as the superconductor (S) and Cu as the normal metal (N). Such a design of the resistive part sets its resistance on the order of a few tens of .On leave from the Omsk State Technical University, Mir Prospect 11, 644050, Omsk, Russia.     

Quantum Computation and Quantum Coherence in Mesoscopic Josephson Junctions

Macroscopic quantum dynamics of Josephson junctions hasattracted recently renewed interest in the context of quantumcomputation. Quantum coherence of the charge states in asingle-Cooper-pair box or the flux states in an rf SQUID canbe used to create charge and flux qubits for quantumcomputation. The aim of this paper is to give a briefintroduction to the basic concepts of quantum computation withJosephson junctions and to discuss the theory of quantumcoherence in the rf-stimulated macroscopic resonanttunneling of flux in SQUID's. The results for resonant fluxtunneling show that the quantum coherent flux oscillationsshould lead to splitting of resonant tunneling peaks,indicating that the resonant tunneling can be a convenienttool for studying macroscopic quantum coherence of flux.     

An Operation Circuit of a Micro-SQUID Magnetometer below 1?K

We have been developing a micro superconducting quantum interference device (??-SQUID) magnetometer for the study of quantum effects in ??m- or nm-sized magnetic materials. In the ??-SQUID magnetometer, the sample is placed on the SQUID loop directly and the flux is detected by the loop itself. The close proximity between the sample and the SQUID loop is of great advantage for highly sensitive detection of magnetization from the tiny magnetic materials. However, the heat release during the operation of the SQUID could cause a great problem in experiments below 1?K because of the close proximity of the SQUID and sample. Here we report a digital circuit for the ??-SQUID magnetometer which can reduce the heat release in the low temperature operation and whose parts are commercially available. The circuit consists of a FPGA-digital board with analog-to-digital and digital-to-analog converters, and an independent pre-amplifier. By using this circuit we succeeded in measuring quantum tunneling of magnetization in the single molecule magnet, Mn12Ph at 0.7?K.     

Possible Pitfalls in SQUID Magnetometry of Superconducting Samples: The Case of RuSr2GdCu2O8

Inhomogeneities of the field in the superconducting magnet of a superconducting quantum interference device (SQUID) magnetometer can create serious artefacts in the d.c. magnetization measurements of superconducting samples. We discuss the problem focusing on the procedure of calculating the magnetic moment of a sample from the measured SQUID output signal. The (weakly) ferromagnetic superconductor RuSr₂GdCu₂O₈ has been reported sensitive to inhomogeneities of the SQUID's superconducting magnet, which create artefacts in its measured magnetization. Indeed, we show that, because of the small values of the magnetic hysteresis width, field changes less than 100 T over the scanning length inside the magnet are enough to create spurious signals in the measured magnetic moment of this compound.     

DC SQUIDs based on YBCO step edge junctions

We describe the fabrication and testing of dc SQUIDs (Superconducting QUantum Interference Devices) obtained by photolithographic patterning of YBa₂Cu₃O_7–x thin films deposited both on SrTiO₃ and MgO substrates. The Josephson junctions in the superconducting loop are of the step-edge type, where the weak link is obtained through the growth of grains with different orientations across suitably prepared steps previously etched on the substrate surface. TheI–V characteristics of the devices tested show multiple branches and instabilities having a weak dependence on the external magnetic field, probably due to formation of junction clusters on the substrate step. The SQUID devices showed quantum interference behavior and an easily detectable voltage modulation with the applied magnetic field at a temperature of 77 K.     

Flux flow and resonant modes in multi-junction Josephson stacks

Magnetic field dependent current-voltage characteristics of stackled Nb/(Al−AlO_x/Nb)_n long Josephson junctions are investigated experimentally. The thickness of their common superconducting electrodes provides the magnetic coupling between the junctions. For stacks of n=7 Josephson junctions the current-voltage characteristics display collective flux-flow behaviour of Josephson vortices. In the interior layers Josephson vortices move simultaneously under the influence of the bias current. The flux-flow behaviour is modulated by a complicated structure of cavity-like resonances which show broad range of characteristic frequencies. The measurements can be qualitatively explained by the Kleiner model for the resonances in stacks. Mutual locking of junctions in the stack is indicated by pronounced cavity resonances with large voltage spacing.