16 December 2008—Physicists in Switzerland led by Nicolas Gisin of the University of Geneva reported last week in Nature that they have made a solid-state device capable of storing photons for as long as 1 microsecond. The invention will aid in the development of light-based quantum-cryptography networks, which are theoretically impervious to hacking but are currently limited in range to a few dozen kilometers, primarily because of a lack of a suitable way to store the quantum state of photons.
”Photons are very fragile,” Gisin says. ”We are now able to play with a photon, put it in a quantum fridge, and retrieve it a bit later.”
While similar quantum interactions between light and matter that can be used as rudimentary memory devices have been demonstrated before, those demonstrations have involved mainly gases of atoms chilled to near absolute zero and difficult-to-execute schemes to trap the light. Earlier this month, a group of researchers at the Georgia Institute of Technology announced that they had stored quantum information for a record 32 milliseconds in very cold rubidium atoms confined in an optical trap. Even though it doesn’t store information for as long as the Georgia Tech technique, the Swiss method should be more practical because it relies on a solid-state device and doesn’t need to be cooled as near to absolute zero. Many experts say that the Swiss method has the potential to revolutionize quantum information networks.
Raymond Laflamme, director of the Institute for Quantum Computing at the University of Waterloo, in Canada, called the Swiss work ”an important stepping stone for quantum communication, a critical building block on which to build quantum repeaters, the missing link to make quantum communication global and pervasive.”
The quantum memory chip is kept in a 3 Kelvin chiller [blue] awaiting photons [yellow optical fiber].
Quantum repeaters will be an essential component for long-range quantum information networks because photons degenerate—their quantum state changes—as they travel and need to be regenerated periodically in a way that preserves their original information. No one has been able to make a reliable quantum repeater yet. One of the prerequisites for such a device is a quantum memory that can store photons (and their quantum state) without destroying entanglement. Entanglement, a property important to quantum networks, allows two photons to be linked in such a way that, if someone measures one of the photons, the quantum state of the other becomes known as well. When a photon travels through optical fiber, entanglement degeneration occurs by approximately 300 kilometers.
In October, the city of Vienna switched on the largest quantum information network in existence, spanning roughly 200 kilometers. Anything much beyond that is going to be a challenge without quantum repeaters. The Swiss group thinks its device may be a way forward.
What Gisin and his colleagues—Mikael Afzelius, Hugues de Riedmatten, Christoph Simon, and Matthias Staudt—essentially did was to find a way to trap a photon in a collection of 10 million neodymium atoms embedded in an yttrium orthovanadate crystal, which was cooled to 3 degrees kelvin. When entangled photons from a diode laser were beamed into the crystal, they were stored for up to 1 microsecond, ”and the released photons were entangled as well,” says Gisin.
An immediate goal is to increase the storage time to milliseconds, a period far more useful for quantum networks, says Gisin. The efficiency of the storage is also pretty low, only a few percent of photons are retained, which the Swiss team is trying to increase.
Carl Williams, chief of the atomic physics division at the National Institute of Standards and Technology, in Gaithersburg, Md., points out that the first experiments in quantum communications tend to have low efficiencies, which are typically quickly improved upon.
”This is a very elegant new experiment,” Williams says. With Gisin’s approach, quantum memories ”might be much more easily manufactured.”