Researchers at the Georgia Institute of Technology have announced a significant advance in the retention time of multiatom quantum-memory devices—from a previous maximum of 32 μsec to 7 msec. The significance of the advance is that, with retention time of milliseconds, a bit in quantum memory now lasts longer than the time it would take a photon to travel through 100 km of optical fiber from one repeater to another in a long-range quantum network. Quantum networks function by distributing entangled quantum qubits between two repeaters, so modifying the qubit on one end modifies it on the other end, as well.

The Georgia Tech team based its quantum memory on supercooled rubidium-87 atoms. The team arranged the atoms so that arriving light would imprint its phase information on the array. The challenge then was to keep the atoms stable long enough for a laser read-beam to recover that phase information milliseconds later. This task required two approaches.

First, the researchers contained the rubidium atoms within an optical lattice of laser beams. This step significantly reduced the scope of the already-slowed random motion of the atoms by biasing them toward specific positions in the lattice. Second, by pumping the atoms to what is called the clock-transition state, in which the atoms are relatively insensitive to external magnetic fields, the researchers reduced another potential source of disruption. The result was a set of atoms that stayed near their initial position long enough to preserve the phase information they had received from the incoming light.

Researchers emphasized that, although this advance is both necessary and significant, we are still at least a decade away from a quantum network that can operate outside a laboratory environment.