Flux Quantum Devices Research Articles

Verification of stable operation of rapid single flux quantum devices with frequency-dependent dissipation

It has been suggested that rapid single flux quantum (RSFQ) devices could be used as the classical interface of superconducting qubit systems. One problem is that the interface acts as a dissipative environment for a qubit. Recently, ways to modify the RSFQ damping to reduce the dissipation have been introduced. One of the solutions is to damp the Josephson junctions by a frequency-dependent linear circuit instead of the plain resistor. The approach has previously been experimentally tested with a simple SFQ comparator. In this paper we perform experiments with a full RSFQ circuit, and thus conclude that in terms of stable operation the approach is applicable to scalable RSFQ circuits. Realization and optimization issues are also discussed.

High quality pulling crystals for electronic device applications

We fabricated a high quality Y123 single crystal for electronic device applications by the modified pulling method. A high quality 13 mm-squared single crystal was obtained along 〈0 0 1〉 pulling. An extremely high quality 8 mm squared single crystal was obtained along 〈1 0 0〉 pulling. The value of the full width at half maximum of the X-ray rocking curve of the (0 0 1) face pulled along 〈1 0 0〉 direction was about 0.09°. This value was about a half of that grown along 〈0 0 1〉 direction. Additionally, we compared the etch pit density of the 〈0 0 1〉 pulled single crystal with that of a liquid phase epitaxy (LPE) film. The etch pit density of the LPE films is about 100 times higher than that of the pulled crystal. The substrates made of the pulled crystals are applicable especially to single flux quantum devices.

Superconducting digital electronics

Superconducting devices have intrinsically superior characteristics to those of semiconductor devices. Presently, we can fabricate more than twenty thousand junctions on one chip using niobium technology. We have demonstrated the operation of a network system with a superconducting interconnection chip using voltage-state logic. Single flux quantum devices are promising for future superconducting applications because the clock frequency of SFQ logic is higher than that of voltage-state. We have proposed a high-end switch based on hybrid architecture using optical devices, semiconductors and SFQ devices. To demonstrate the high-speed operation of SFQ circuits, we developed an arbiter circuit that uses SFQ components and the arbiter circuit operates at 60 GHz. We also have developed a high Tc superconducting (HTS) SFQ sampler system for observing ultra-fast signal waveforms. In addition, we will discuss the prospects of future superconducting devices based on the fabrication technologies we developed.

Multilayer Josephson junction flux quantum devices

We describe the properties of flux quantum circuitry employing the relatively young technology of multilayer Josephson junctions with n superconductor-insulator (SI) layers. Multilayer junctions can be employed as both passive and active devices to increase circuit integration density, allow for new logic/voltage thresholds and higher impedances, and improve thermal noise stability. We present the results from numerical simulations of a conventional RSFQ circuit and two novel circuits with multilayer junction designs. Neural circuitry is a focus of our novel multilayer designs. We also discuss layout and fabrication issues, considering the recent progress in the fabrication of Nb multilayer junctions with AlN tunnel barriers, which exhibit intrinsic overdamping at the level of each SI layer. Included in this discussion is a long term assessment of a multilayer approach in view of deep sub-micron and high T/sub c/ technologies.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Experimental investigation of a high frequency sampling system based on shunted Josephson junctions

The authors propose a new type of sampler design based on shunted junctions ( beta c=1). The sampler is to be used for the investigation of single flux quantum devices (RSFQ) employing similar junctions (Kaplunenko et al. 1989, Filippenko et al. 1991). They measure the time resolution of the sampler and delay time of the shunted Josephson junction array. The circuit was manufactured on a single chip and was DC powered. They employed Nb-AlOx-Nb tunnel junctions of 300 A m-2 critical current density and observed Josephson oscillations of the output junction at frequencies from 6 to 50 GHz. Despite a long plasma period of 20 ps the time resolution was 5 ps. The transmission line of 27 Josephson junctions (critical current 165 mu A each) demonstrated time delay of 0.5 ps mu A-1.

Dynamics of some single flux quantum devices: I. Parametric quantron

In this paper and the following one, the properties of two systems operating with single flux quanta are calculated. The first system is a quantron (a one-junction interferometer) with a Josephson gate as a junction. By a proper change of its critical current in time, the switching of quantron stable states can be controlled by a small external flux much less than Φ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</inf> . This "Parametric Quantron" can be used as the basic cell of a Single Quantum Logic System. The most wonderful property of this device is that the energy dissipation during one logical step (PτT factor) can be much less than the thermal energy K <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</inf> T and is limited by quantum effects only: Pτ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> \simg10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> h.

Dynamics of some single flux quantum devices: II. Inhomogeneous flux shuttle

In this report we discuss the properties of a flux shuttle, similar to that described recently but with its parameters depending periodically on the coordinate along the length of the structure. In the case of a proper choice of the parameters a one-directional shift of vortices (flux quanta) appears to be possible along the structure, induced by ac current flowing directly in the superconducting electrodes. The shift for one structure period takes typically one psec and does not pertrub the relative distance between vortices of any sign (orientation).