Celestial AI, one of several Silicon Valley chip startups aiming to crack a key speed constraint in artificial intelligence, said on Tuesday it has raised an additional $250 million in venture capital, bringing its total raised to date to $515 million. Celestial AI is tapping photonics – a technology that uses light, rather than electrical signals – to create speedy links between AI computing chips and memory chips. The speed of that connection, a measure called memory bandwidth, has become so central to advancing AI systems that it is one of the factors that determine whether a chip is subject to U.S. government export controls such as those designed to limit China’s AI advances.