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  • Quantum networks receive memory boost

Photonics Spectra
Dec 2010
Marie Freebody,

ATLANTA – Long-distance quantum communication could be a step closer, thanks to a suite of optical technologies developed by researchers at Georgia Institute of Technology. The group has demonstrated a low-noise system for converting photons carrying quantum information to telecom wavelengths suitable for long-distance transmission and for increasing quantum memory storage times by a factor of 30.

Quantum networking applications such as long-distance communication and distributed computing require quantum memory elements, or nodes, that can store information for at least 1 s. This is long enough to transmit information to the next node in the network.

To make the task even more challenging, long-distance transmission via optical fibers is most efficient at telecom wavelengths, whereas storage protocols are most compatible with wavelengths in the near-infrared.

Graduate assistant Alexander G. Radnaev and colleagues addressed these problems and published their findings Sept. 26, 2010, in Nature Physics. They reported quantum memory storage of up to 0.1 s, an important step toward the quantum memory goal. They also successfully demonstrated wavelength conversion back and forth between 795 and 1367 nm.

“The motivation is to develop a set of capabilities for future distributed quantum communication and information processing systems,” Radnaev said. “We have improved the neutral atom quantum memory lifetime by more than an order of magnitude, to about 0.1 s, and made the memory compatible with fiber telecommunication networks by converting quantum states of light, efficiently and noiselessly, into and out of the telecommunication band.”

In this vacuum chamber, cold rubidium clouds used for memory are suspended. The optics around it are used to illuminate the atoms with various light fields. Courtesy of Gary W. Meek, Gary W. Meek Photography Inc.;

In the setup, the investigators prepared a sample of ultracold rubidium atoms in an optical lattice – which served as a long-lived quantum memory – then illuminated the sample with a laser to generate a signal photon at 795 nm. The polarization of the signal photon entangled with a matter qubit, or quantum bit, creating the same generation process. The matter qubit was “stored” in the quantum memory.

In a separate cold atomic sample, they converted the wavelength of light from 795 nm to 1.4 μm, preserving the light polarization. They passed the light through 100 m of standard telecom fiber, but Radnaev believes that this could be extended to larger distances.

“Light in the telecom band maximizes communication distances due to reduced absorption in optical fibers, while near-infrared light generates the long-lived matter qubits stored in quantum memories,” he said. “Thus, wavelength conversion represents a quantum interface between the qubit storage and transmission stages of a quantum network.”

The low-noise detection and high-conversion efficiency into and out of the telecom band has allowed the researchers to demonstrate for the first time a neutral atom quantum memory compatible with telecom fiber networks.

Although there is still a ways to go before intercontinental distance networks are realized, graduate assistant Yaroslav O. Dudin, co-author of the paper, hopes to improve the quantum memory toward lifetimes of a few seconds.

“For intercontinental distance, it is necessary to store quantum information for periods on the order of one second, possibly even longer,” Dudin said. “Enhancing the quantum memory time is therefore a major step on the way toward a practical quantum repeater.”

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