For the first time, quantum bit snared, stored, and read

Jan. 25, 2008

A team of scientists from the University of Heidelberg, Germany, the Technical University of Vienna, Austria, and the University of Science and Technology of China for the first time has succeeded in buffering a quantum bit during its transmission. The achievement could be used for the construction of quantum repeaters and perhaps, eventually, to build a memory for a quantum computer.

The team succeeded in storing the quantum bit while performing an experimental transmittal of an unknown quantum state, a spokesperson of the group explained. Hitherto, it was not possible to store and read out a quantum state.

During the experiment, the scientists transferred the state of a photon to what they called an atomic quantum store. In this atomic ensemble, the state was stored for 8 μs before it was read out again and transferred to a photon.

In order to achieve this goal, the atomic ensemble formed an interface that enables the transfer from quantum states to matter and back without destroying the information contained. In future quantum technology applications, such an interface could play a significant role, the scientists say. Hitherto, quantum information could not be read without destroying it.

In the experiment, the scientists used photonic qubits as information media; the quantum information was coded by the degree of freedom of the polarization. The quantum store was formed through the collective spin state of an ultra-cold ensemble of about 1 million rubidium atoms.

Initially during the experiment, the polarization state of a photon was entangled with the state of the quantum store. This entanglement between an atom and a photon serves as a resource for the teleportation of an unknown photonic qubit to an atomic one, the group described the process. The 'teleported' state now can be stored in the collective state of the atomic ensemble and later be read.

The main problem is the short storage duration for the information, the spokesperson said. "The atomic ensemble is very susceptible to interfering fields such as the earth magnetic field," the spokesperson said. "Even after massive magnetic shielding it can interfere with the information, and even at the very low temperature, the atoms move and collide, which leads to decoherencies and to the deletion of the information."

The principle developed by the scientists could be used to build repeater stations for long-range quantum data transmission, the spokesperson said. Basically it also could be used to design memory blocks for quantum computers. However, quantum encryption is much closer to technical feasibility that quantum computers. "Quantum cryptography could be reality in some five years", the spokesperson said, "while it would be overbold to predict when quantum computers could be implemented."

SOURCE: EETimes Europe