A system for measuring absolute frequencies of up to 4.25 THz using a Josephson point contact

A system for measuring the absolute frequency of a far-infrared (FIR) laser is described. Josephson point contacts have been utilized in the system as a frequency harmonic mixer connecting microwaves and optically pumped CH₃OH laser lines. The Josephson point contacts are capable of generating beat signals of 90 GHz microwaves and FIR waves of up to 4.25 THz. To measure the frequency of the beat signals from the Josephson junction with a frequency counter, tracking oscillators have been developed, which tracks the beat signals by phase locking and regenerate clean signals for frequency counting. It is shown that the absolute frequency can be measured to an accuracy of about 100 Hz by using the tracking oscillators.     

General energy relations for Josephson junctions

The Josephson junction acts as a nonlinear lossless inductor and can be applied for mixing and parametric amplification in the microwave region. Since an alternating current can be generated in the junction by an applied dc voltage, a dc power flow must be taken into consideration. Energy relations for the Josephson junction are derived which are similar to the Manley-Rowe equations but with an additional term for the dc power. These equations show the possibility of realizing dc-pumped parametric amplifiers.     

Nonlinear mixer gain calculations for Josephson junctions

We have numerically solved the steady-state solutions of the initial value problem associated with a current-driven Josephson weak-link junction shunted by an ohmic resistance. The nonlinear mixing action of the junction leads to Shapiro steps in the DC response with step height in units of the mixing frequency. Mixer gains have been calculated with a wide range of parameter values and intrinsic chaos is observed whenever Shapiro steps are prevalent     

High-reliability Pb-alloy Josephson junctions for integrated circuits

The process developed and recently used at IBM for fabricating experimental Pb-alloy Josephson tunnel-junction devices, and the factors which influence the stability of such devices during repeated cycling between 300 and 4.2 K are reviewed. A new, fine-grained Pb0.84In0.12Au0.04alloy base electrode material has been developed that has excellent thermal cycling stability. In an experiment carried out to evaluate the cyclability of devices prepared with this material, excellent results were obtained: the cyclability of large-area junctions was improved by ∼100× compared to that of similar junctions prepared with the recently used, larger-grained Pb0.84In0.12Au0.04base electrodes. In the best cases, populations of 2600 large junctions and 2350 interferometers were found to withstand 400 and 700 thermal cycles to 4.2 K, respectively, before the first failures were observed. These results indicate that with the use of fine-grained electrodes, Pb-alloy Josephson devices have good potential for meeting the cycling requirements of computer systems.     

Modeling of HTS Josephson junctions

We discuss recent developments for HTS Josephson junctions: (i) isolated (S/I) and resonantly coupled (S/I/S) Andreev interface states, (ii) d-wave models with resonantly coupled midgap states for ab-plane junctions and (iii) resonant tunneling models for c-axis junctions.     

Picosecond applications of Josephson junctions

The behavior of simple superconducting circuits in the picosecond regime is described in a comprehensive way, with primary emphasis being given to the step function and pulse responses of these circuits. Topics receiving detailed discussion include Josephson-junction modeling with both the microscopic and shunted-junction models. Limitations of the shunted-junction model are explored by comparing it with experimental results and with the microscopic model. An approximate evaluation is given of the important dynamical properties of junctions made with the dominant fabrication technology (Pb-alloy systems), as a function of tunneling barrier thickness. Rounding out the device aspects of the discussion, we describe in detail the properties of superconducting microstrip transmission lines, with an emphasis on their high-speed behavior. Turning to simple circuits we review experimental results on the measurement of picosecond regime transient signals. The concept of turn-on delay is analyzed anew, providing simplified and extended results. Details of concepts for pulse height and pulsewidth measurements are explored, leading to the conclusion that the time resolution of superconducting circuits is limited to approximately the period of one plasma oscillation. With present Pb-alloy fabrication technology this limit is 2 ps.     

Superconducting memory device using Josephson junctions

A superconducting storage device is proposed in which a Josephson tunnelling junction, switched by a control film strip, replaces the cryotron in a one-cryotron-per-bit storage cell. The device should be fast, work at 4.2° K and require a simple technology, which must, however, yield reproducible thin Josephson tunnelling layers.     

Logic gates with shaped Josephson junctions

Long (L//spl lambda//SUB j/>5) in-line Josephson junctions, with varying width along the length L of the device, are investigated as logic gates (/spl lambda//SUB j/ being the Josephson penetration depth). The devices realized have an asymmetric threshold characteristic with almost suppressed sidelobes, providing good logic gain and permitting logic fan-in with multiple control lines. Optimum conditions are found for junctions with width varying approximately sinusoidally along the device length. The so-called shaped junctions are incorporated in various flip-flop circuits to evaluate the transfer time and transfer efficiency of loop circuits, and in a self-resetting inverter circuit to demonstrate the feasibility of self-resetting logic. The principle of current steering and the relatively large operating currents (I/SUB G//spl sime/6 mA) make the circuits suitable for medium-speed applications such as in the decode and control logic of a main-memory chip. For a fan-out of four, the minimum circuit delay is 300 ps, resulting in a power-delay product in the order of 3/spl times/10/SUP -15/ J.     

Interface-engineered Josephson junctions optimized for high JC

High-temperature superconducting interface-engineered junctions were fabricated using YBa₂Cu₃O_7-δ on LaAlO₃ (LAO) and sapphire substrates. We report on the improvements in the electrical characteristics by junction narrowing. Originally, the 2-μm-wide junctions had high critical current densities of 1.2 × 10⁶ A/cm at 4.0 K and showed wide junction effects. Narrowing the junctions to below a micrometer reduced the wide junction effect over a large range of temperatures and the junctions had characteristic voltages of 5.16 mV at 4.0 K. The magnetic-field modulation of the critical current was also more ideal after narrowing. Furthermore, we show that interface engineered junctions on sapphire substrates have similar characteristics to those on LAO     

Ion beam oxidation of Nb-based Josephson junctions

Ion beam oxidation and structuring of Nb-based planar Josephson junctions are briefly described and preliminary results are presented.     

Magnetically enhanced terahertz response of Josephson junctions

We investigate the magnetic field effects on the terahertz (THz) response of grain boundary Josephson junctions. First, we show some experimental results of the THz response enhanced by a dc magnetic field. The experimental results and device configuration indicate that the THz RF magnetic field plays a role in enhancing the response. Second, we numerically simulate the current-voltage characteristics and obtain the power dependence of Shapiro steps. Since the junctions are wide compared to the inferred Josephson penetration depth, multiple current paths within a junction are possible and superconducting quantum interference device (SQUID) models should be used. Two kinds of SQUID models are used: an RF-current drive model and an RF-field activation model. Shapiro step enhancement by a dc magnetic field can be reproduced with the use of the RF-field activation model. Finally, we discuss step-height dependence on SQUID parameters as well as give a qualitative explanation for the different predictions of two SQUID models     

Microwave match of low-impedance thin-film Josephson junctions

It is demonstrated how a thin-film low-impedance Josephson junction can be matched in a rectangular waveguide at 9 GHz, thus reducing the necessary power in voltage standard applications by a factor of 250. The method is also applicable to higher frequencies and to microwave devices such as detectors and mixers.     

A content addressable memory using Josephson junctions

A content-addressable memory circuit using Josephson nondestructive readout (NDRO) memory cells is described. The memory circuit proposed performs searching functions, such as coincidence, incoincidence, and don't-care functions, in addition to the conventional memory function of writing and reading. This memory circuit is able to achieve the `less than' function in addition to the three functions listed above. Computer simulation of a 3-word by 3-b memory was used to investigate how high-performance operation can be achieved. The simulation results show that the four operations for all combinations of binary inputs have been achieved with a cycle time of less than 80 ps and a 0.28-μW/cell dissipation. The simulation results also show the design tolerances of the gate currents of four superconducting quantum interference device (SQUID) gates used in the memory circuit to range from 25% to 17%     

Current/voltage characteristics of Josephson junctions

The I/V characteristic and voltage waveform are investigated for a current-driven Josephson junction, assuming a triangular current/phase-difference relationship.     

Microwave receiver system with quantum superconducting Josephson junctions based on LTSC and HTSC

The paper is concerned with the methods for frequency measurement in millimeter and submillimeter wave ranges using AC Josephson effect. Basing on the method the Microwave Superconductive Receiver System was designed. The main results obtained for niobium Josephson junctions and bridges made of HTSC, as a measurement element are presented.     

Harmonic and intermodulation performance of Josephson junctions

In this paper a simple mathematical model is presented for the inherent resistive nonlinearity of a Josephson junction. By incorporating this model in the RSJ model of the Josephson junction, series expressions are obtained for the harmonic and intermodulation performance of Josephson junctions irradiated by multisinusoidal signals. Special cases are considered where the series expressions can be reduced to simple analytical expressions.     

Terahertz spectroscopy based on high-T c Josephson junctions

A review of the authors’ studies on the spectroscopy employing the ac Josephson effect in high-temperature superconducting junctions is presented. It is demonstrated that Hilbert spectroscopy and admittance spectroscopy can be successfully implemented in the terahertz frequency range with the use of high-temperature superconductor bicrystal Josephson junctions.     

Fluxon modes in stacked HTSC intrinsic Josephson junctions

Fluxon modes in stacked Josephson junctions (SJJ's) are studied both theoretically and experimentally. It is shown that the critical current J_c across the stack is multiple valued due to a possibility of having different fluxon configurations (modes), which is in good agreement with experiment on Bi₂Sr₂CaCu₂O_8+x, the temperature and magnetic field dependencies of the critical current were calculated for a stack with nonidentical junctions.     

Josephson junctions as self-oscillating mixers at 34 GHz

Adjustable point-contact Josephson junctions have been used as self-oscillating mixers at 34 GHz, with an intermediate frequency of 300 MHz. The average conversion loss was about 18 dB, but mismatch losses and waveguide losses may amount to 10 dB at present.     

A parallel full adder circuit using Josephson junctions

Detailed investigations have been carried out on a Josephson parallel full adder as an example of a functional circuit using Josephson junctions. This circuit can be constructed with fewer devices as compared with conventional Josephson adders. Two-junction interferometers (d.c.-SQUIDs) are utilized as switching elements of the circuit. Only (2N+1) d.c.-SQUIDs are required for construction of an N bit circuit. Discussions focus on design theory of the full adder circuit. An experimental 4 bit circuit operation is also demonstrated.