BEIJING (Reuters) – The new U.S. ambassador to China, Nicholas Burns, has arrived in the country and will be in quarantine for three weeks, the U.S. embassy said in a tweet on Saturday. The United States has been without an ambassador in Beijing since Terry Branstad stepped down in October 2020. Burns, a veteran diplomat and former U.S. ambassador to NATO, arrives at a time when relations between the world’s two biggest economies are at their worst in decades. Recent points of friction include the U.S.-led diplomatic boycott of last month’s Beijing Winter Olympics, and China’s diplomatic backi…