FET: Small: Collaborative Research: A Probability Correlator for All-Magnetic Probabilistic Computing: Theory and Experiment

概率计算是一种计算范式，它可以比传统的数字计算更有效地解决某些问题。数字计算处理的是确定性二进制位，要么是0，要么是1，而概率计算处理的是概率位（p位），有时是0，有时是1。这与处理量子比特（q-bit）的量子计算不同，量子比特是0和1的叠加（因此一直是0和1的混合物）。量子计算通常需要最多的硬件资源，而数字计算需要最少的硬件资源，概率计算介于两者之间。概率计算中的大多数硬件资源都致力于在两个或多个p位流之间生成特定的相关性。本项目将研究并演示一个系统，该系统将大大减少与生成相关性相关的硬件负担。结果将使概率计算比现在更有效率。该项目将对该领域的K-12、本科生和研究生进行教育，以增加熟练的科学家和工程师的数量，同时推进计算机领域的发展。概率计算的主要挑战之一是在概率比特流之间生成所需的相关性所需的复杂硬件。这种硬件通常由微控制器、模数转换器、移位寄存器等组成，它们消耗大量功率，并极大地扩展了系统在芯片上的占地面积。在这个项目中，一个超紧凑和极其节能的相关器或反相关器将被研究和证明，可以在两个p位流之间产生可调程度的反相关。该方法是通过两个磁隧道结（MTJs）实现的，其软层非常接近，因此偶极子耦合。比特状态在mtj的电阻状态（高或低）中编码。当自旋极化电流产生自旋传递转矩时，一个MTJ产生p位。电流以其大小决定的概率将电阻状态设置为高或低，而另一个MTJ的位状态由与第一个MTJ的偶极子耦合决定。极强的偶极子耦合会产生完全的反相关，而极弱的偶极子耦合则不会产生相关。偶极子耦合效应将通过对第二个MTJ施加（电产生的）局部应变来控制，该应变调节其内部能垒，从而将p位元之间的反相关程度从0%调制到100%。这个项目将导致对下一代计算中使用新兴纳米磁物理的设备的新理解。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Probabilistic computing is a computing paradigm that can solve certain problems more efficiently than traditional digital computing. While digital computing deals with deterministic binary bits that are either 0 or 1, probabilistic computing deals with probabilistic bits (p- bits) that are sometimes 0 and sometimes 1. This is distinct from quantum computing that deals with quantum bits (q-bits) which are a superposition of 0 and 1 (and hence a mixture of both 0 and 1 all the time). Quantum computing usually requires the most hardware resources and digital computing the least, with probabilistic computing between the two. Most of the hardware resources in probabilistic computing are devoted to generating specific correlations between two or more p-bit streams. This project will study and demonstrate a system that will greatly reduce the hardware burden associated with generating correlations. The results will make probabilistic computing much more efficient than it currently is. The project will educate K-12, undergraduate, and graduate students in this field to increase the pool of skilled scientists and engineers while advancing the field of computing.One of the major challenges in probabilistic computing is the complex hardware needed to generate required correlations between probabilistic bit streams. This hardware usually consists of microcontrollers, analog-to-digital converters, shift registers, etc., that consume significant power and vastly expand the system’s footprint on a chip. In this project, an ultra-compact and extremely energy-efficient correlator or anti-correlator will be studied and demonstrated that can generate tunable degrees of anti-correlation between two p-bit streams. The approach is implemented with two magnetic tunnel junctions (MTJs) whose soft layers are in close proximity and hence dipole-coupled. Bit states are encoded in the resistance states (high or low) of the MTJs. One MTJ generates a p-bit when driven by a spin-polarized current delivering a spin-transfer-torque. The current sets the resistance state high or low with a probability determined by its magnitude, while the bit state of the other MTJ is determined by dipole coupling with the first. Very strong dipole coupling will result in perfect anti-correlation, while very weak dipole coupling will result in no correlation. The effect of dipole coupling will be controlled by applying (electrically generated) local strain to the second MTJ, which modulates its internal energy barrier, thereby modulating the degree of anti-correlation between the p-bits from 0% to 100%. This project will result in new understanding of devices that use emerging nanomagnetic physics for the next generation of computing.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.