Lightmatter Inc., a startup building photonic supercomputing products, today announced it has raised $400 million in a late-stage round, raising the company’s valuation to $4.4 billion.

The Series D round was led by T. Rowe Price Associates and joined by existing investors, including Fidelity Management & Research Co. and GV, the venture capital arm of Alphabet Inc. The new round brings the total raised by the company to $850 million to date.

Lightmatter builds full-stack photonic technologies, including networking interconnect for how computer chips communicate and trade data. The company looks to provide higher bandwidth, lower latency connections between chips using optical fiber connections as data centers scale to tackle larger artificial intelligence and high-performance computing workloads.

In less than a decade, graphics processing units have gotten more than 1,000 times faster in terms of operations per second. They have outstripped interconnect networking in terms of sheer bandwidth, Lightmatter Chief Executive Nick Harris told SiliconANGLE in an interview.

Modern-day AI large language models have become so large and complex that they sometimes require thousands or tens of thousands of GPUs working in concert and they spend part of their lifetime idle waiting for data.

“If you’re one of these GPUs, your life is like: OK, I’m waiting for data from memory, crunch, crunch, crunch, OK, waiting for data from another GPU, crunch, crunch, crunch, and just sitting there,” said Harris. “Only 30% of the time you’re doing calculations. So, you’ve got this Ferrari engine, it’s in Manhattan, and there’s stoplights everywhere.”

As frontier AI models expand the training clusters needed to handle them cannot keep up, Harris said. It’s not the chips that are the bottleneck – it’s their ability to communicate with one another. To solve this problem, Lightmatter developed Passage, a next-generation optical networking solution that interposes between data center chips with ultra-dense optical fibers. This can bring bandwidth up to 100 terabits per second, or around 100x current data center interconnect speeds.

In a traditional data center, GPUs interconnect through a large number of networked switches that form a layered hierarchy to connect GPUs together, Harris explained. This generates a great deal of latency because for one GPU to reach another, it must go through several switches first. Much of that idle time GPUs spend is waiting for the data to flow through the network.

“What we’re able to do is flatten that hierarchy,” said Harris. “So instead of six or seven layers of switches, you’ve got two, and each GPU can connect to thousands of GPUs.”

Harris also added that Passage saves power. To sustain one link between two layers of switches, he explained can require up to 300 watts, but with Lightmatter’s solution, it’s only about 50 watts, a sixfold power reduction.

With this financing, Harris said Lightmatter will be preparing Passage for mass deployment in partner data centers and scaling up the company’s team to build on the photonics technology stack.

“We’re innovating across the stack, with photonics,” said Harris. “What we see going forward is that the future is all about interconnect. There’s a limit to how big computer chips can be. It’s wafer-scale, 300 millimeters squared in diameter, so 12 inches. When you get to that point, and I think it’s 10 years away, the only thing that matters is the links between the chips.”

Image: Lightmatter