Rapid-fire single photons for quantum information processing

May 17, 2012

Georgia Tech researchers adjust optics as part of research into the production of single photons for use in optical quantum information processing and the study of certain physical systems (credit: John Toon)

Researchers at Georgia Tech have used lasers to reliably and individually produce single photons from individual atoms in a cloud of ultra-cooled rubidium gas.

Single photons are an essential element for guaranteed secure communications in quantum cryptography — where an attacker can use extra or stray photons to eavesdrop on a message — and to address individual qubits in quantum memory architectures, where single atoms may serve as the storage unit.

This reliable, high-speed method of creating single photons can also be used to distribute entanglement to remote locations, as used in quantum teleportation, thus making the method a potential enabler for quantum repeaters for long-distance quantum communications, says Georgia Tech prof. Alex Kuzmich, lead investigator for the research.

“We are able to convert single-atom excitations into single photons with very substantial efficiency, for investigating entangled states of atoms, spin waves and photons,” he explained . “This new photon source is about 1,000 times faster than any existing system.”

Creating single photons: (A) A cold dense sample of atomic Rubidium gas is prepared as an optical lattice. Two light fields (Ω1 and Ω2) excite a single atom from the ground state to produce a spin wave. A readout laser pulse (Ω3) converts the spin wave into a single photon. Two detectors, D1 and D2, measure the photon. (B) Relevant quantum energy levels for the Rubidium gas; and electronic, hyperfine, and Zeeman quantum numbers. (Credit: Y. O. Dudin et al/Science)

“The next goal may to develop a quantum gate between light fields to deterministically create complex entangled states of atoms and light, which would add valuable capabilities [such as scalable, complex quantum memories] to the fields of quantum networks and computing,” explained Kuzmich. “With further increases in efficiency and generation rate — and integration with long-lived quantum memories — such a single-photon source may make optical quantum information processing possible.”

“Our results also hold promise for studies of dynamics and disorder in many-body systems with tunable interactions,” Kuzmich explained. “In particular, translational symmetry breaking, phase transitions and non-equilibrium many-body physics could be investigated in the future, using strongly-coupled Rydberg excitations of an atomic gas.”

A primary feature of these systems is that the interaction between the atoms can can be tuned — from very strong, to none at all. This tunable interaction may enable physicists to understand the behavior of complex materials, and to create entirely new, exotic quantum phases of matter that don’t yet exist in nature.

Ref.: Y. O. Dudin, A. Kuzmich, “Strongly Interacting Rydberg Excitations of a Cold Atomic Gas,” Science, April 2012 DOI:10.1126/science.1217901