As President Trump flew to South Korea on Wednesday to prepare for a summit with the Chinese leader, Xi Jinping, he made some remarks that set off alarm bells among Washington officials concerned about America's rivalry with China. "We'll be speaking about Blackwell," Mr. Trump said of his meeting with Mr. Xi, referring to the most advanced artificial intelligence chip from the U.S. chipmaker Nvidia. Mr. Trump called the technology a "super duper chip"; complimented Nvidia's chief executive, Jensen Huang; and declared, "We're about 10 years ahead of anybody else in chips." Mr. Trump's comments signaled a major potential change for U.S. policy that many Washington officials warn poses a national security risk. Selling such advanced A.I. chips to China is currently banned, and U.S. officials have worked for years to restrain Beijing's access to the cutting-edge technology.