PORTLAND, Ore.—Broadband quantum networks inched closer to reality recently when researchers demonstrated the ability to transfer quantum-bits (qubits) from entangled photons to solid-state crystalline memory devices. Using a super-cooled crystal the researchers were able to demonstrate the reversible transfer of entangled qubits from a quantum network waveguide to the solid-state memory and back again.
Researchers at the University of Calgary (Canada) collaborated with the University of Paderborn (Germany) in the reversible transfer of photon-photon entanglement into entanglement between a photon and the solid-state excitation of atoms. The rare-earth (thulium) doped lithium niobate waveguide made use of the photon-echo quantum memory protocol. Separately, another research group at the University of Geneva (Switzerland) demonstrated a similar capability over a 50-meter fiber optical link, paving the way for quantum repeaters that could extend the ultra-secure communications of a quantum network to any distance.
Quantum networks will allow sending information without being afraid of somebody listening in. Photo credit: Riley Brandt
The University of Calgary team demonstrated that their lithium niobate waveguides, which are already widely used for fiber optic communications, can handle signals from five megahertz to five gigahertz, with a memory retention time of seven nanoseconds. Its broadband quantum memory used off-the-shelf lithium-niobate crystals which needed to be supercooled to minus 270 degrees Celsius. Next, the group plans to create a real-time read-write channel using teleportation to transfer the qubits into and out-of its solid-state memory.
"We have already demonstrated entanglement between a photon and the atoms of the crystal. Our next step will be to use interactions with a third photon to teleport its state into our solid-state memory by virtue of that entanglement," said University of Calgary professor Wolfgang Tittel at the Institute for Quantum Information Science. "This teleportation step will enable future quantum networks that provide ultra-secure long-distance communications."
For the future, besides perfecting teleportation as a means of transferring qubits to and from its quantum memories, the researchers are also planning to extend the memory retention time from seven nanoseconds toward a goal of one second—a necessary condition for using repeaters to create large quantum networks.