Ronen Sen, India's ambassador to the United States, is an old hand at conducting intense negotiations, having earlier represented the country in Mexico, Russia, Germany and Britain. But it was his experience as secretary to the Atomic Energy Commission of India that served him well over two years and two days of difficult negotiations with the US over a civilian nuclear cooperation agreement. Sen's role in stitching up the deal was crucial. Days before the text of the 123 Agreement was made public, the Indian ambassador sat down with Ashish Kumar Sen in an exclusive face-to-face interaction. Excerpts: