The superconducting tunnel effect predicted by Brian Josephson in 1962 is looking increasingly promising for another very practical application—as logic and memory circuits for computers. Josephson-junction circuits are capable of high switching speeds (50–100 picosec have already been obtained). And because such circuits operate at voltages comparable to the superconducting energy gap, the heat generated is typically microwatts, thousands of times lower than high-speed transistor circuits. The diminished heat-removal problem allows one to pack the Josephson circuits much closer than semiconductor circuits. Once high switching speeds are achieved, this packability becomes extremely important because then the major limitation on computer speed is how fast the electrical signal takes to move from one circuit to another.
The superconducting tunnel effect predicted by Brian Josephson in 1962 is looking increasingly promising for another very practical application—as logic and memory circuits for computers. Josephson-junction circuits are capable of high switching speeds (50–100 picosec have already been obtained). And because such circuits operate at voltages comparable to the superconducting energy gap, the heat generated is typically microwatts, thousands of times lower than high-speed transistor circuits. The diminished heat-removal problem allows one to pack the Josephson circuits much closer than semiconductor circuits. Once high switching speeds are achieved, this packability becomes extremely important because then the major limitation on computer speed is how fast the electrical signal takes to move from one circuit to another.
The potential of a new technology, based on the superconducting Josephson tunneling effect, has been assessed for use in high-speed computer systems. Josephson tunneling circuits for memory and logic functions can be switched at sub-nanosecond delay, do not require standby power, and dissipate extremely low energy, typically less than 10-13joule during fast switching operation. High-speed circuits with high packing densities, not limited by power dissipation problems, can therefore be expected. Low thermal noise at cryogenic temperatures ensures reliable operation despite the low energy switching signal. Practically lossless signal transmission via superconducting lines can be exploited for high-speed operation even when sizeable memory and logic modules are assembled. Two random access memory modules have been designed on the basis of single device experiments. Operational characteristics leading to the conclusion that 30 Mbit capacity, less than 1 watt refrigeration requirement (at 3.6° K) and cycle times of 40 ns (in one case) and 15 ns (in the second case), can be achieved are presented.
Experimental superconducting Josephson devices being investigated for use in digital logic and memory circuits have been demonstrated to switch in the 10 to 100 picosecond (1 picosecond = 10-12 sec) range. Projections based on the operation of individual logic circuits indicate that they may surpass semiconductor circuits in very high performance CPU's. This potential is based on the fact that these circuits dissipate extremely little energy, on the order of 10 to 1000 attojoules (attojoule = 10-18 joule), while operating with subnanosecond delays. It should, therefore, be possible to package Josephson devices very densely and interconnect them by properly terminated superconducting transmission lines so that the fast switching speed of individual circuits is retained throughout large logic networks.
Experimental superconducting Josephson devices being investigated for use in digital logic and memory circuits have been demonstrated to switch in the 10 to 100 picosecond (1 picosecond = 10-12 sec) range. Projections based on the operation of individual logic circuits indicate that they may surpass semiconductor circuits in very high performance CPU's. This potential is based on the fact that these circuits dissipate extremely little energy, on the order of 10 to 1000 attojoules (attojoule = 10-18 joule), while operating with subnanosecond delays. It should, therefore, be possible to package Josephson devices very densely and interconnect them by properly terminated superconducting transmission lines so that the fast switching speed of individual circuits is retained throughout large logic networks.
Low energy dissipation and ability to operate at low temperatures provide for Josephson junction circuits a niche as a support for low temperature devices. With high speed operation (Chen W. et al., 1999) capability the Josephson junction circuits make a prime candidate for applications which are difficult to engineer with existing CMOS technology. The development of Josephson junction technology took a major turn for the better with the invention of the Rapid-Single-Flux-Quantum (RSFQ) devices (Likharev K.K. et al., 1991), an improvement over voltage biased Josephson Junctions logic which were plagued with the junction switching and reset problems. The modern applications of SFQ circuits extend to a larger range of temperature operation and the applications vary from low temperature magnetic sensor, to high speed mixed signal circuits, voltage and current standards (Turner C.W. et al., 1998), and auxiliary components for quantum computing circuits. Most of the SFQ circuits are fabricated with Niobium, but Aluminum based circuits are being used for quantum gates (Nielsen M.A. et al., 2000) and qubit operations. SFQ circuits based on Magnesium di-Boride junctions are being developed for higher temperature operations (Tahara S. et al., 2004). Predominantly most of the Josephson junction circuits today are operated at around 4K. All the circuits are optimized usually for liquid helium temperatures, so the circuits operated in helium bath Dewars or cryostat's do not experience any temperature gradients or drift effects which can affect the operating margins. With the improvement in the fabrication technology and soft-wares for SFQ circuit technology, the designing complex circuits have become easier. Complex Circuits with over 20K junctions such digital synthesizer and digital RF Trans-receiver have already been demonstrated (Oleg M.A. et al., 2011). Development of circuits over 100K junctions are actively under progress. However, with enormously large circuits, power requirements also increase. Looking at the range of applications and complexity of the problems of energy minimization, we try to look at the problem in two approaches. One for large complex circuits, we try to reduce the power bias itself, or the overall load of current that is supplied
Flux quantum logic and memory circuits using superconducting Josephson tunnel junctions have high-speed switching times (approximately 1 ps), low power dissipation (< 1 (mu) W per circuit) and low levels of thermally induced electrical noise. Current designs of such circuits employ single trilayer junctions, which impose circuit size and logic threshold limitations. A new design component, the multilayered tunnel junction, consists of a vertically stacked array (a 1D superlattice) of Josephson tunnel junctions. The introduction of multilayered junctions into superconducting electronic circuitry offers a reduction in the current device size, fault tolerances, and new device applications. We present numerical simulations of simple circuits employing multilayered Josephson junctions as design components. Comparison with conventional single flux quantum circuitry is discussed. We also present preliminary measurements of multilayered Josephson junctions fabricated for use in flux quantum devices.
Adiabatic superconducting logic circuits can ensure the practical implementation of operations with the energy dissipation below the Landauer limit. However, applications of the existing solutions are limited because of two contradictory requirements of a high energy efficiency and a sufficiently fast response of devices. Josephson junctions with a negative critical current (π junctions) allow one to obtain a certain form of the potential energy of superconducting circuits and, as a result, a practically required degree of control of dynamic processes in the proposed reversible logic cells. The features of the current transport and balance of Josephson phases in circuits with π junctions make it possible to improve the coupling between the parts of a reversible computer by a factor more than 2. At the same time, the continuous evolution of the state is ensured at higher critical currents and higher characteristic voltages of the main Josephson junctions of adiabatic superconducting logic cells, which allows an increase in the response rate.