Matter-to-Light Quantum State Transfer Demonstrated

Daniel S. Burgess

Physicists Alexander M. Kuzmich and Dzmitry N. Matsukevich of Georgia Institute of Technology in Atlanta have reported the transfer of quantum state information from clouds of ultracold atoms to single photons. The work may have applications in the development of quantum repeaters to mitigate the natural decoherence of entangled states over distance in a communications channel, enabling the construction of arbitrarily large quantum communications networks.

Physicists Alexander M. Kuzmich (left) and Dzmitry N. Matsukevich have transferred quantum state information from a two-component ensemble of ultracold rubidium-85 atoms to a photon, suggesting the possibility of arbitrarily large quantum networks. Courtesy of Georgia Institute of Technology. Photo by Gary Meek.

Quantum networking is the subject of research worldwide, in part because it promises perfectly secure information transfer for sectors such as banking and defense. To become viable, however, it must be possible to "entanglement swap" quantum states along shorter segments of the optical path, extending the distance over which communication fidelity could be preserved. One way to do this might involve the transfer of the quantum states back and forth between matter and optical radiation along the communications channel.

The Georgia Tech researchers have achieved matter-to-light quantum state transfer using their methodology. First, they prepared two ensembles of ultracold rubidium-85 atoms in the desired quantum state by illuminating them with a 140-ns-long, 780-nm laser pulse and detecting the resulting shorter-wavelength photon generated by spontaneous Raman scattering, called the signal. Next, after a delay of 100 to 200 ns, they interrogated the ensembles with a 115-ns-long, 795-nm pulse, producing a single idler photon that contained information about the spatial states of the atom clouds in its polarization. After passing through a polarization state transformer and a polarizing beamsplitter, the idler photon was measured by single-photon detectors, recovering the quantum state.

In their analysis of the technique, Kuzmich and Matsukevich found that the fidelity of the reconstruction of the intended quantum state and of the entanglement between the signal and idler photons exceeded classical limits, indicating successful state preparation and readout. The calculated efficiency of the quantum state transfer was only ~3 percent, which they suggest can be improved by employing ensembles of a greater number of atoms.

The physicists are working to link two of their experimental systems to form a basic repeater. To be compatible with standard optical networks, a version of the setup that operates in the C-band, around 1550 nm, must be developed.