Physicists at Georgia Institute of Technology have set a new record for the length of time quantum information can be stored and retrieved from an array of atoms. Though the information remains usable for just a few milliseconds, seven to be exact, that should be enough for the data to pass from one quantum repeater to another on an optical network. (The previous record was 32 μsec.)
The Georgia researchers used an array of rubidium-87 atoms cooled to almost absolute zero to minimize atomic motion. To retain information, the array is bathed in the light of a laser carrying the signal, which lets each atom share in storing the data as part of a collective excitation. Each atom sees the incoming signal, a rapidly oscillating electromagnetic field, slightly differently. Each atom is also imprinted with this phase information that can be read later from the array with another laser. To keep the array in its excited state longer (i.e., extend the memory time), researchers confined the atoms using an optical lattice of laser beams. The laser’s frequency was tailored to attract atoms to specific locations, though they are not held tightly at those places. The scientists also pumped the atoms to the so-called “clock-transition state,” a state in which they are insensitive to magnetic fields. This helps keep the atoms confined and oscillating at the proper frequencies.
Quantum computing sends qubits (two correlated data bits that are either 0 or 1), over long distances. They would travel as photons across optical networks that form a global telecom system. But loss in optical fibers makes repeaters necessary every 100 km or so to boost the signal. And these repeaters will need quantum memory to store a photonics signal while a new uncorrupted signal is fashioned to carry data to the next node.