Dec. 15, 2020 5:30 am ET High-frequency traders are using an experimental type of cable to speed up their systems by billionths of a second, the latest move in a technological arms race to execute stock trades as quickly as possible. The cable, called hollow-core fiber, is a next-generation version of the fiber-optic cable used to deliver broadband internet to homes and businesses. Made of glass, such cables carry data encoded as beams of light. But instead of being solid, hollow-core fiber is empty inside, with dozens of parallel, air-filled channels narrower than a human hair. Because light travels nearly 50% faster through air than glass, it takes about one-third less time to send data through hollow-core fiber than through the same length of standard fiber.