From New Scientist:
"Clocks that gain or lose no more than a fraction of a second over the lifetime of the universe could be on the way, thanks to a technique for cutting through the "heat haze" that compromises the accuracy of today's instruments.
The most accurate atomic clock we have now is regulated by the electrons of a single aluminium ion as they move between two different orbits with sharply defined energy levels. When an electron goes from the higher energy level to the lower it emits radiation of a precise frequency. That frequency is used to mark out time to an accuracy of better than 1 part in 1017, or 1 second in 3 billion years.
That's pretty good, but it could be better. Infrared photons emanating from the background cause the two energy levels to shift by slightly different amounts..."