Programmable electrical coupling between stochastic magnetic tunnel junctions

TL;DR

This paper demonstrates a scalable analog unit cell with programmable electrical coupling between stochastic magnetic tunnel junctions, enabling flexible and tunable correlations for probabilistic computing applications like simulated annealing.

Contribution

It introduces a practical, tunable coupling circuit for $p$-bits that overcomes previous limitations in tunability and energy efficiency, advancing probabilistic computing hardware.

Findings

Achieved programmable coupling with a 1us time constant, faster than SMTJ dwell times.

Enabled both positive and negative correlations over a wide range.

Demonstrated coupling across devices with varying timescales.

Abstract

Superparamagnetic tunnel junctions (SMTJs) are promising sources of randomness for compact and energy efficient implementations of probabilistic computing techniques. Augmenting an SMTJ with electronic circuits, to convert the random telegraph fluctuations of its resistance state to stochastic digital signals, gives a basic building block known as a probabilistic bit or $p$-bit. Though scalable probabilistic computing methods connecting $p$-bits have been proposed, practical implementations are limited by either minimal tunability or energy inefficient microprocessors-in-the-loop. In this work, we experimentally demonstrate the functionality of a scalable analog unit cell, namely a pair of $p$-bits with programmable electrical coupling. This tunable coupling is implemented with operational amplifier circuits that have a time constant of approximately 1us, which is faster than the mean dwell times of the SMTJs over most of the operating range. Programmability enables flexibility, allowing both positive and negative couplings, as well as coupling devices with widely varying device properties. These tunable coupling circuits can achieve the whole range of correlations from $-1$ to $1$, for both devices with similar timescales, and devices whose time scales vary by an order of magnitude. This range of correlation allows such circuits to be used for scalable implementations of simulated annealing with probabilistic computing.