A new study into using quantum mechanics for communication and memory devices could provide a breakthrough in the transmission of information using quantum networks. Such networks would allow information to be sent quickly and securely through the use of photons.
The scientists from Canada, Germany and Switzerland, whose paper is published in Nature [Saglamyurek et al. Nature (2011) doi: 10.1038/nature09719], have recently described all the key properties of a quantum memory system that could be useful in a quantum repeater or network; these include the potential to store entangled photons, broadband storage, robustness, efficiency, and greater storage time. The team is now working towards a demonstration where all these properties can occur simultaneously.
Wolfgang Tittel from the University of Calgary, who co-authored the paper, commented “We have demonstrated, for the first time, that a crystal can store information encoded into what’s called in quantum mechanics, entangled quantum states of photons.”
Quantum networks work very differently to conventional communication networks, where pulses of light moving through optical fibers carry information, which is then stored on a computer hard disk for future use. Instead, this study used quantum communication to encode information into entangled states of photons. The researchers used a crystal doped with rare-earth ions and cooled to -270 oC to ensure that the entangled photons were still able to communicate with each other and without the quantum link becoming broken.
Tittel explains “The results show that entanglement, a quantum physical property that has puzzled philosophers and physicists for almost one hundred years, is not as fragile as is generally believed.” It is this robustness that allows entanglement to be mapped in and out of a solid state device.
The team is confident that the similarity of their memory to the normal building blocks of telecommunication devices will permit ease of integration with existing information technology. Their storage device, a thulium-doped lithium niobate crystal, is further enhanced by the possibility of storing photons as short as 100 ps, much shorter than has previously been achieved.
The research could offer a couple of important applications, such as a quantum network (or, more specifically, a quantum repeater) where quantum memory constitutes nodes where the information encoded into photons is stored until it is needed, and also for a photon-based quantum computer, where quantum memories are also required to synchronize quantum information.
The next stage is to improve the efficiency of sources of entangled photons; the scientists hope to develop a useful quantum repeater within the next five to ten years, so there is still much to do.

Laurie Donaldson