The shares of IonQ Inc. and other companies linked to quantum computing tumbled in premarket trading on Wednesday, after Nvidia Corp. Chief Executive Officer Jensen Huang said that “very useful” quantum computers are likely decades away.“If you kind of said 15 years for very useful quantum computers, that would probably be on the early side. If you said 30, it’s probably on the late side,” Huang said in a question-and-answer session during Nvidia’s analyst day. “If you picked 20, I think a whole...