AQ Khan YouTube/PakistanTV5 There is nothing much unusual about armed guards outside the houses in my quiet, neat neighbourhood of Islamabad. These are the homes of retired generals, well-connected politicians and international aid workers, all of whom might be targets for Pakistani extremists. But one house stands out. Instead of frail, elderly guards armed with shotguns, one house is ringed by uniformed police officers with AK-47s. Roadblocks control traffic, floodlights illuminate an area of scrub that has been cleared of trees and machine gun positions have been dug in all around.

This is the home of AQ Khan, the metallurgist who did more than anyone to build Pakistan’s atomic bomb and who then went on what can only be described as a proliferation spree, selling nuclear secrets to any rogue state that came calling.

The technology behind today’s test in North Korea can be traced back to the man in the heavily guarded villa. As many as two planes a month arrived in Pakistan from Pyongyang during the late 1990s, bringing the missile technology in exchange for AQ Khan’s secrets, such as how to use centrifuges to enrich enough uranium for a weapon.

Today he is largely a free man. He received a pardon from President Pervez Musharraf in 2004 apparently in return for a televised confession in which he admitted selling the technology but insisted that he acted alone. All very convenient.

So the security outside his house is not so much to keep AQ Khan locked away like a criminal. In fact he can occasionally be spotted at coffee shops and regularly gives interviews to the local media. He has even set up a party to contest elections due this year and has said he is willing to serve as prime minister. His Tehreek-e-Tahafuzz Pakistan party tried to register the image of a missile as its symbol – suggesting a wicked sense of humour.

Nor is the security really to stop the terrorists. As the father of the Islamic bomb he is a source of national pride, revered as a hero and an inspiration to those who want to take on nuclear-armed superpowers on their own terms.

No. The security was tightened in 2011, soon after the American raid to kill Osama bin Laden in Abbottabad. I noticed it on one of my regular strolls to buy ice cream as I passed Dr Khan’s house. The watchman’s hut opposite his home – usually deserted – was buzzing with activity. Fresh razor wire had been laid in the wasteland where baboons and wild boar roamed.

If Navy Seals could fly in undetected to kill the world’s most wanted man barely 30 miles from Islamabad, perhaps they could reach the capital to kidnap a man who has never explained exactly what he sold to whom.

The US still considers him a proliferation risk and would dearly like to question him. How else can we know just what technology North Korea is using and how close Iran might be to a bomb? But for now the man with those secrets remains living in comfort in his fortress home and we can’t be sure how many Pakistani generals and prime ministers knew exactly what he was up to.