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International Conference on Solid State Devices and Materials (SSDM)

An Improved Josephson Interferometer Memory Cell for Nondestructive Read-Out

Masafumi Yamamoto, Akira Ishida

Published Aug 26, 1980• 3 citations
Publisher

Abstract

An improved single flux-quantum Josephson interferometer memory cell which brought stable nondestructive read-out (NDRO) operation was proposed. The cell was constructed with an end-fed asymmetric two-junction interferometer, in which the bit operating current level could be set far above the critical point for stable NDRO operation. Damping condition for proper memory operation was studied by computer simulation. The NDRO operation was experimentally obtained for the McCumber damping parameter βc of 300–400.

DOI
10.7567/ssdm.1980.c-4-4

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An improved single flux-quantum Josephson interferometer memory cell which brought stable nondestructive read-out (NDRO) operation was proposed. The cell was constructed with an end-fed asymmetric two-junction interferometer, in which the bit operating current level could be set far above the critical point for stable NDRO operation. Damping condition for proper memory operation was studied by computer simulation. The NDRO operation was experimentally obtained for the McCumber damping parameter βc of 300–400.
A new Single Flux-Quantum (SFQ) Nondestructive Read-Out (NDRO) random-access asymmetric 2-Josephson-junction interferometer memory cell with only one control line has been investigated and designed. The binary information is stored in the cell nonvolatilely without a bias current. For random access, this is the first known interferometer memory cell without a diagonal line. Therefore, this NDRO SFQ memory cell leads intrinsically to smaller cell area and simpler decoder schemes. A cell design of only 52 lithographic squares is given to demonstrate the potential for high-density memory chips. Wide nominal read and write operating margins are obtained. The NDRO interferometer memory cell proposed is promising for use in a high-performance Josephson computer.
A new vortex transitional NDRO memory cell has been proposed for use as a high-speed cache memory. The memory cell consists of two superconducting loops with one Josephson junction and a two-junction interferometer gate. The superconducting loop stores single-flux quantum. The interferometer gate operates as a sense gate. The memory cell employs a vortex transition in the superconducting loop for the writing and reading of data. The sense gate current margin increases to ±38% since the coupling magnetic flux is amplified by the vortex transition for nominally designed cell parameters. The memory cell operates with a ±33% address signal current margin. This paper describes the memory cell design and the dynamic behavior of computer simulations.
In single flux quantum interferometer memory cells the information can be stored without bias current, if the interferometer inductance L is chosen sufficiently large (\lambda = 2\piLI_{0}/\Phi_{0} = 2\pi) to allow for three stable states (-1-, 0-, 1-mode) at zero control and gate current. In this case the binary informations are represented by the -1- and 0-mode of the interferometer. The nondestructive read out operation of a binary "1" is performed without a transition into another mode or into the voltage state. The binary "1" corresponds to no sense signal. Nondestructive read out of the binary "0" is achieved in switching to the voltage state and in returning to the 0-mode after the end of the drive currents. In this paper the dynamics of switching back into the zero voltage state are investigated by simulations of trajectories in the order parameter phase plane. It is shown that for realistic fabrication tolerances the interferometer settles in the wanted 0-mode and not in the -1- or 1-mode even under severe gate or control current disturbs, if the maximum Josephson currents of the two Josephson junctions are unequal (2:1) and if the McCumber damping factor is sufficiently large (\beta \approx 100).
This paper presents a novel nondestructive memory cell using a two-junction interferometer. Most of single-flux-quantum memory cells have been the destructive ones, which must rewrite the information for logic "1" after a reading operation. This makes the circuitry of the cell complex. To avoid this disadvantage, a novel self-rewriting technique is introduced by setting a coil between the terminals of the interferometer. The principle of the operation of the cell is described and the simulation for the behaviours of the cell are illustrated in detail. The results of the simulation show that the reliable operation of the cell can be achieved with high operating speed.