Storage technique brings quantum networks boost

 

Gallery

By Stewart Mitchell

Posted on 13 Dec 2008 at 11:59

Physicists at the Georgia Institute of Technology have brought usable quantum networks a step closer by setting a new record for the length of time that quantum information can be stored in and retrieved from a cluster of cold atoms.

Though the information remains usable for just milliseconds, the scientists claim that even this short lifespan should allow data transmission from one quantum repeater to another in an optical network.

The new record - 7 milliseconds for rubidium atoms stored in an optical trap - smashed the existing mark of 32 microseconds and means the information is available for 200 times longer than in previous methods.

"This is a really significant step for us, because conceptually it allows long memory times necessary for long-distance quantum networking," says Alex Kuzmich, associate professor at Georgia's School of Physics. "For multiple architectures with many memory elements, several milliseconds would allow the movement of light across a thousand kilometres."

The keys to extending the storage time include using a one-dimensional optical lattice to confine the atoms and an atomic phase that is insensitive to magnetic effects.

The purpose of quantum networking is to distribute entangled qubits over long distances across existing optical networks. But losses in the optical fibre mean that repeaters must be installed at regular intervals - about every 100 kilometres - to boost the signal.

Those repeaters will need quantum memory to receive the photonic signal, store it briefly and then produce a photonic signal that will carry the information to the next node, and on to its final destination.

In simple terms, each atom in the cluster "sees" the incoming signal slightly differently to store phase information that can later be "read" from the ensemble with another laser.

Other research teams have previously stored quantum information well in single atoms or ions, but single atoms have limitations.

"The advantage of using these ensembles as opposed to single atoms is that if we shine into them a 'read' laser field, because these atoms have a particular phase imprinted on them, we know with a high degree of probability that we are going to get a second photon - the idler photon - coming out in a particular direction," says Stewart Jenkins, another researcher on the project. "That allows us to put a detector in the right location to read the photon."

Though the work significantly advances quantum memories, practical quantum networks probably are at least a decade away, Kuzmich believes.

"In practice, you will need to make robust repeater nodes with hundreds of memory elements that can be quickly manipulated and coupled to the fibre," he says. "Eventually, they will get good enough so we can make a jump to having systems that can work outside the laboratory environment."