An NSF Expeditions Project

Our NSF Expeditions in Computing team aims to invent and demonstrate new experimental approaches to creating CIMs. While our team and others have performed a number of proof-of-principle demonstrations showing that CIMs can heuristically solve Ising problems, we have not yet experimentally shown a cost-effective system that can outperform state-of-the-art conventional methods running on state-of-the-art conventional processors. Two fundamental challenges we are tackling are: 1) how to build CIMs at large scale in a way that the underlying photonics processing is not bottlenecked by electronic processing, and 2) how to build CIMs that operate in a quantum regime that is intractable to simulate classically and delivers a computational advantage.

To address both challenges we are simultaneously working both on new experimental building blocks (at the device/component level) as well as new architectures (at the system level). Our device efforts are focused on miniaturization using nanophotonics. While all our implementations of programmable CIMs so far are based on discrete-component table-top optical systems, one important theme of this project is exploring paths towards on-chip realization of CIMs. Our team is designing and developing new nonlinear devices, such as optical parametric amplifiers, based on thin-film lithium niobate. In addition to reducing the footprint of CIMs and increasing the number of spins, miniaturized components also provide new or enhanced computing resources, which we are exploring both theoretically and experimentally. In the classical regime, miniaturization can lead to orders of magnitude improvements in energy consumption and speed. Nanophotonic devices also have the potential to allow us to explore modules in the quantum regime, and we are pursuing the generation and study of quantum states in nanophotonic CIMs, and their role in quantum-enhanced solution mechanisms in CIMs.

At the level of systems and architecture, we are exploring three different multiplexing strategies for constructing CIMs at large scale: time multiplexing, frequency multiplexing, and space multiplexing. All three involve tradeoffs between ease-of-construction, connectivity, and performance, and we aim to quantify these tradeoffs as well as mitigate the disadvantages of each strategy. Time-multiplexed CIMs, in which each spin is represented by a pulse of light in an optical cavity (typically a fiber loop), have been proven to scale to very large numbers of spins, but there is an open research challenge for how best to programmably couple spins without using electronic measurement-feedback, which limits the system performance. Frequency-multiplexed CIMs, in which each spin is represented by a frequency mode of an optical cavity, allows for very large numbers of spins and long-range coupling between spins, but with research challenges about mitigating the translational symmetry in the couplings, and achieving single-shot readout. Space-multiplexed CIMs, in which each spin is represented by a spatial optical mode, possibly as part of a cavity or possibly as part of a single-pass arrangement, offer a key computational benefit: the ability to construct matrix-vector multipliers where the scalar multiplications of each matrix element with an element of the spin vector all occur in parallel. Research challenges include scaling to large numbers (>1000) of spins and rapidly feeding the output of a matrix-vector multiplication through a nonlinearity and back as the input to the next multiplication. This work is primarily being pursuing with spatially parallel optical matrix-vector multipliers, but there is also an effort to implement a highly parallel electronic (field-programmable gate array) version, which could allow some of the advantages of the CIM strategy for solving Ising problems to be usable by the broader community before "conventional" (photonic) CIMs become broadly available.