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Superconductor Science and Technology

A ternary memory cell using small Josephson junction arrays

Niketh Nair, Yehuda Y. Braiman

Published Sep 19, 2018• 10 citations
Publisher

Abstract

Abstract In this paper we present a ternary cryogenic memory cell paradigm that is based on an array of inductively coupled Josephson junctions. We show how reading, writing and resetting are implemented using single flux quantum current pulse inputs to the circuit and reading voltage pulse outputs from the circuit. We further show how both destructive readout and non-destructive readout can be implemented.

DOI
10.1088/1361-6668/aae2a9

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Abstract In this paper we present a ternary cryogenic memory cell paradigm that is based on an array of inductively coupled Josephson junctions. We show how reading, writing and resetting are implemented using single flux quantum current pulse inputs to the circuit and reading voltage pulse outputs from the circuit. We further show how both destructive readout and non-destructive readout can be implemented.
Abstract In this paper we analyze a cryogenic memory cell circuit based on a small coupled array of Josephson junctions. All the basic memory operations (e.g., write, read, and reset) are implemented on the same circuit and different junctions in the array can in principle be utilized for these operations. The presented memory operation paradigm is fundamentally different from conventional single quantum flux operation logics (SFQ). As an example, we demonstrate memory operation driven by a SFQ pulse employing an inductively coupled array of three Josephson junctions. We have chosen realistic Josephson junction parameters based on state-of-the-art fabrication capabilities and have calculated access times and access energies for basic memory cell operations. We also implemented an optimization procedure based on the simulated annealing algorithm to calculate the optimized and typical values of access times and access energies.
Abstract In this paper, experimental verification for a cryogenic memory cell consisting of three inductively coupled Josephson junctions is presented. Design and operational logic of this type of cell was recently introduced. The basic memory cell operations (read, write, reset) can be implemented on the same simple circuit and this type of memory cell is fundamentally different from the existing single flux quantum-based memory cells. Here, we present design principles and experimental validation for write operations and readout operation for four versions of the design, each with different parameters. Our experimental results show excellent resemblance with the previously presented memory cell operational logic and also show an excellent fit with simulations of the entire memory cell circuit that includes the memory cell and all the peripheral circuitry.
The talk is based on an overview which was published recently (1). For this reason, only an abstract is given here, which includes Later results not contained in the aforementioned overview. The presently-known Josephson memory cells store information as persistent currents in superconducting loops, or what is equivalent as a magnetic flux in the area of the ring. This method of storage is nonvolatile and has no power consumption in the storing state. In a superconducting ring system, the magnetic flux is quantized, and cells have been built which operate either with only one flux quantum, or with many of them. For the Latter cells with many flux quanta (ring cells), quite a number of designs exist. They have one or two Josephson junctions in the ring, which allow writing. An external junction which is controlled by the ring current serves for non-destructive reading of the information. The smallest cell realized had an area of 900 ?m2 with a 2 ?m minimum line width. Switching speed was ? 80 ps. Cells which store information as a single-flux quantum (single flux-quantum cells = SFQ cells) in the ring, usually contain two Josephson junctions used for reading and writing. Read-out in most of these cells is destructive, but methods of reading non-destructively have also been proposed. The smallest device made had an area of ? 150 ?m2 with a minimum line width of 2 ?m. The read signal had a rise time ? 100 ps.
The design and experimental results for a novel type of Josephson memory cell are presented. The cell has a simpler construction than the conventional Henkels-type, using a one-junction SQUID (superconducting quantum interference device) capacitively coupled to a single junction. Binary data are stored as single flux quanta and are destructively read out by sensing a pulse caused by a vortex-mode transition. Computer simulations suggest that the read operation could be performed with a bias margin for the sense junction of +or-28% and that the rise time of the sense current in a 16-bit memory cell would be as fast as 30 ps, despite having to sense a vortex-mode transition. Results are presented for a memory cell fabricated with Nb-AlO/sub x/-Nb junctions.>
Energy efficient cryogenic memory and Josephson junction arrays have the potential of revolutionizing high-performance computing systems based on single flux quantum logic. In this paper, a pulse control is designed to switch among stable equilibrium states of a Josephson junction array. Josephson junction arrays have the ability to store state information at low temperatures and energies, which naturally is useful for cryogenic memory applications. Although typical memory applications involve binary states, as a memory cell, the Josephson junction array has the ability to occupy nonbinary states given the nonlinear nature of the dynamics, which can lead to a reduced number of cells required in practical applications among other advantages. Transitions between states can be achieved through experimental testing for specific transitions, however, there are no known closed-form solutions to arbitrarily transition from one stable state to another. The system dynamics are similar to an array of coupled nonlinear oscillators, for which there is no known analytical solution. By providing an energy-based analysis for the junction array, a lower bound is determined and system trajectories are closely approximated such that the array is capable of transitioning between any desired stable equilibrium states under a Gaussian pulse input.