A question about national debt

I have an honest question that I cannot find an answer too on the internet, without running into blogs, posts, etc. My question is: If , in a perfect world, the U.S. borrows say 1 billion form China (and theoretically we owed them nothing, essentially a blank slate). When we made the billion plus that was required under the contract, HOW is it actually transferred? Is it electronic, bullion, bonds? I honestly do not know I am asking out of true ignorance.

No one? Bueller? Bueller?