The leading researcher behind the groundbreaking experiment was Lene Vestergaard Hau (b.1959). Just after her graduation, she received a travel grant from the Carlsberg Foundation, which gave her the opportunity to go to Harvard and pursue her research there.

Even at that early stage, she had a passion for basic research and science driven by curiosity. Later Hau has often pointed out that her Danish school experience, with its open and explorative learning environment, laid the foundation for her scientific approach. In Denmark, she said, she learnt to think for herself and explore the unknown.

Today Hau is internationally acclaimed for her research into the use of so-called Bose–Einstein condensates for manipulating the speed of light. A Bose–Einstein condensate is a particular state of matter that forms when gas is cooled to near absolute zero, -273.15 degrees Celsius. At these low temperatures, the gas particles barely move at all and instead begin to act as a single ‘super particle’, rather than as many individual particles.

In 1997, Hau and her research team generated a Bose–Einstein condensate of ultracold sodium atoms, which they later passed light through. When the beam of light entered this cloud of atoms, the light photons were briefly trapped by the atoms and then released.

This trapping and subsequent release made the light move extremely slowly. This discovery, in 1999, was revolutionary and changed our perception of the interaction of light and matter. It also opened new possibilities for using light in quantum communication and the development of quantum computers.

Hau was not satisfied in slowing down the light to single-carriageway speed. She wanted to stop it entirely. In 2001, she achieved her goal, when her research team discovered and demonstrated that by switching off a control laser in their experimental set-up, they were able to trap the light pulse itself and store it inside the atoms of the condensate. When they reactivated the control laser, the light pulse was released exactly where it had been stopped. The experiment showed for the first time that light can be temporarily stored in atoms, which opened new perspectives for data storage and quantum memory.

In 2007, Hau demonstrated that light can be transformed into matter, moved and then turned back into light. Although this may sound like science fiction, it is in fact solid quantum physics. However, while theoretical quantum physics does explain the phenomenon, researchers were completely taken aback by her thinking and resulting observations.

First she slowed down a light pulse by passing it through a Bose–Einstein condensate. When the control laser was switched off, the light was trapped inside the atoms in the condensate and turned into a ‘matter imprint’ in the form of a small atom pulse. The matter imprint could then be moved, and the control laser was switched back on. Now the light pulse was released in the same form and with the same quantum information as it had at the point of entry – an accomplishment that impressed physicists around the world.

Hau’s research has had a fundamental impact on our understanding of how light can be manipulated. Her results have enabled new technologies, such as quantum computers, ultraprecise measurements, sensors and data communication via light pulses. The ability to slow down, stop and then revive light has proven particularly significant in storing and transporting quantum information, a key task in the development of future quantum computers and secure communications systems.

The chapter is written by Kristian Sjøgren.