‘Superradiant’ laser could boost performance of atomic clocks

A new ‘superradiant’ laser design could be 100 to 1,000 times more stable than the best conventional visible lasers, according to its inventors.

Scientists from the Joint Institute for Laboratory Astrophysics (JILA) in Colorado, US, say the laser could boost the performance of the most advanced atomic clocks and related technologies including communications, navigation systems and space telescopes.

The laser prototype uses a technique called phased arrays, where light waves from a large group of identical atomic ‘antennas’ made from rubidium are synchronised to build a combined wave with unusual properties.

Currently the light produced is much weaker than that from conventional lasers but is much less sensitive to environmental vibrations.

‘It’s like what happens in the classical world but with quantum objects,’ said James Thompson, a physicist at JILA, which is a joint institute of the National Institute of Standards and Technology (NIST) and the University of Colorado Boulder (CU).

‘If you line up lots of radio antennas that each emit an oscillating electric field, you can get all their electric fields to add up to make a really good directional antenna,’ he said in a statement.

‘In the same way, the individual atoms spontaneously form something like a phased array of antennas to give you a very directional laser beam.’

Traditional lasers use mirrors to bounce photons back and forth through a gain medium that amplifies their power to build them into an intense beam of light. The laser frequency wobbles slightly because of vibration in the mirrors caused by outside activity.

In the new laser, the synchronised rubidium atoms are energised and emit photons that emerge so quickly from the device that they do not have time to take on the mirrors’ vibrations and so there is no frequency wobble.

A very small number of photons do remain, which is just enough to maintain the right oscillating electric field that keeps the atoms synchronised.

The atoms are tuned with other low-power lasers to increase the speed at which they emit photons from one to 10,000 per second, making the light superradiant.

‘This superradiant laser is really, really dim — about a million times weaker than a laser pointer,’ said Thompson. ‘But it is much brighter than one would expect from the ordinary unco-ordinated emissions from individual atoms.’

Thompson’s measurements show that the stability of the laser beam frequency is less than 1/10,000th as sensitive to mirror motion as in a normal optical laser. This suggests the new approach might be used in the future to improve the best lasers by a factor of 1,000.