Researchers at the Kastler Brossel Laboratory in Paris have showed that a cloud of cold atoms surrounding an ultrathin fiber can store light propagating along the fiber. The technique makes use of an effect called electromagnetically induced transparency (EIT), which normally occurs in clouds of atomic gases.
Julien Laurat at the Université Pierre et Marie Curie explains that using fibres would make the memories more compatible with existing optical-information technology. He also points out that because the light is contained in fibres, there is no need for mirrors, lenses and other components that can make free-space optical systems unwieldy.
The researchers began by elongating an optical fibre until its diameter was less than half a micron. They insert this thinned portion inside a vacuum chamber, which is filled with a cloud of approximately 2000 laser-cooled caesium atoms.
The dimensions of the fibre are crucial to the experiment’s success – because it is thinner than the wavelength of the guided light, about 40% of the light’s energy propagates outside the fibre, in a so-called evanescent field. By interacting with this evanescent field, the atoms can slow and stop the guided light for several microseconds, even though none of the atoms are actually inside the fibre.
Using the electromagnetically induced transparency (EIT) technique, which is well-known in free space but combined here for the first time with a fiber, the researchers slowed down the light pulse by 3,000-fold and then halted it completely.The information conveyed by the laser light is transferred to the atoms in the form of a collective excitation, a large quantum superposition. Later, after a programmable period, the light was released into the fiber, reconstituting the initial encoded information that can once again travel. Storage times of up to 5 micro seconds were demonstrated, corresponding to a traveling distance of 1 km if the light had not been halted.
The experiment by the Paris team also showed that even light pulses containing only one photon can be stored, with a very large signal-to-noise ratio. This feature will enable the use of this device as a quantum memory, an essential ingredient for the creation of future quantum networks.
Mikhail Lukin, a physicist at Harvard University in Massachusetts, US, who studies slow light, is impressed with the research. He told physicsworld.com that “It combines several previously demonstrated phenomena and techniques to take another step towards making quantum-memory techniques robust and practical.”
The research is published in Physical Review Letters with the title “Demonstration of a Memory for Tightly Guided Light in an Optical Nanofiber”.
