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Saturday, February 2, 2008

Quantum Teleportation and Memory in Tandem Achieved


Brainiac 5 & Superman © DC Comics
In research that may be a key step toward real-life quantum communication—the transmission of information using atoms, photons, or other quantum objects—researchers created an experiment in which a quantum bit of information is transported across a distance of seven meters and briefly stored in memory. This is the first time that both quantum memory and teleportation, as the information transfer is known, have been demonstrated in a single experiment.

In the present research an unknown quantum state of a photonic qubit is transferred into quantum memory via teleportation and is stored by two clusters of rubidium atoms. Each cluster contains approximately one million atoms, collected by a magneto-optical trap. The teleported photonic qubit can be stored in memory and read out up to eight microseconds (millionths of a second) before the state is lost.

This setup does have some serious problems. The quantum memory duration is very short and the probability that the photon will be teleported is low. Therefore, the researchers say that “significant improvements” need to be made before the scheme could be used in practical applications. link

Memory-built-in quantum teleportation with photonic and atomic qubits. 2008. Yu-Ao Chen, et al. Nature Physics advance online publication, 20 January 2008.

The top-left diagram shows the structure and the initial populations of atomic levels for the two ensembles. At Bob's site, the anti-Stokes fields emitted from U and D are collected and combined at PBS1, selecting perpendicular polarizations. Then the photon travels 7 m through the fibres to Alice's site to overlap with the initial unknown photon on a beam splitter (BS) to carry out the BSM. The results of the BSM are sent to Bob through a classical channel. Bob then carries out the verification of the teleported state in the U and D ensembles by converting the atomic excitation to a photonic state. If the state | + is registered, Bob directly carries out a polarization analysis on the converted photon to measure the teleportation fidelity. On the other hand, if the state | - is detected, the converted photon is sent through a half-wave plate via the first-order diffraction of an AOM (not shown). The half-wave plate is set at 0° serving as the unitary transformation of . Then the photon is sent through the polarization analyser to obtain the teleportation fidelity.