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Proceedings of the ... American Control Conference/Proceedings of the American Control Conference

Control of Cryogenic Memory State Transitions in a Josephson Junction Array

Roland Harvey, Zhihua Qu

Published Jun 1, 2018• 6 citations

Abstract

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.

DOI: 10.23919/acc.2018.8430940

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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.

In this chapter, a pulse-based method to facilitate equilibrium transitions in a superconductor memory cell is developed. In a cryogenic environment, non-dissipative storage of non-binary memory states has the potential to drastically increase energy efficiency per operation and decrease the number of memory cells per unit area. Memory cells in consideration are fabricated with Josephson junctions, which are superconducting devices analogous to the classical bit. Dynamics of a memory cell are identical to an array of coupled nonlinear oscillators, which have no known analytic solution. Despite this difficulty, a novel closed-form control technique is developed to design pulses that force stable state transitions in a memory cell. Simulation results are shown to verify the proposed solution.

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.

Abstract The ongoing progress of superconducting logic systems with Josephson junctions as base elements requires the development of compatible cryogenic memory. Long enough junctions subject to magnetic field host quantum phase 2 π -singularities—Josephson vortices. Here, we report the realization of the superconducting memory cell whose state is encoded by the number of present Josephson vortices. By integrating the junction into a coplanar resonator and by applying a microwave excitation well below the critical current, we are able to control the state of the system in an energy-efficient and non-destructive manner. The memory effect arises due to the presence of the natural edge barrier for Josephson vortices. The performance of the device is evaluated, and the routes for creating scalable cryogenic memories directly compatible with superconducting microwave technologies are discussed.

The quantum behavior of Josephson junctions is often exploited to produce superconducting devices of outstanding performance. With respect to information technology applications, Josephson junctions can be used in circuits that perform logic operations in picoseconds, which may result in energy-efficient, high-performance cryogenic computers, provided that memory elements can be developed that can be switched between two stable states by integrated superconducting logic circuits. Here, we show that Josephson junctions based on pseudo-spin-valve barriers could enable such memory elements, by unambiguously differentiating the controlled changes in Josephson coupling from size-dependent remanent-field effects. These changes can be very large and even reversed in magnitude, which is strong evidence of the exchange-field effect on the Cooper pair phase. These devices are the first that demonstrate nonvolatile, size-independent switching of the Josephson coupling in magnitude or phase, and they may allow for the first scalable superconducting memory devices.