First "quantum repeater" realized

New progress in quantum communication

In a quantum network, there are two neighboring atomic ensembles, which serve as communication nodes and are each entangled with individual photons. The ultracold atoms, which serve as quantum stores, are provided by laser light cooling through magneto-optical traps in ultra-high vacuum glass cells. The two ensembles are converted into a common entangled state by a joint Bell state measurement takes place at the two single photons. The entangled state of the atomic ensembles can be read out for further connections. The brightness of the red light guide reflects the photon loss. The glass cube is a so-called polarizing beam splitter (PBS), which is needed for the collective Bell measurement. © Julia Gless
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An international team of scientists has made another big step towards realizing quantum communication over long distances. In experiments, they were able to realize a stable "quantum repeater" or quantum converter, which has the potential to serve as a central building block in future quantum communication networks.

If you want to transmit data over long distances, you have to counteract the unavoidable attenuation by amplifying the transmitted signal. This regeneration of the transmitted signal is performed in classical communication in so-called repeater stations. When quantum information is to be transmitted, the same fundamental principles of quantum physics that make quantum communication absolutely secure prevent such amplification without losing the transmitted information.

Transfer quantum states to photons

In "Nature", scientists from the University of Heidelberg, the University of Science and Technology of China and the Vienna University of Technology around Professor Jian-Wei Pan now present a stable quantum repeater and demonstrate for the first time an interrelation exchange with the storage and readout of light.

In the experiment, the researchers generated quantum entanglement, an essential component of quantum information processing, between two spatially separated atomic ensembles connected by a 300 meter long fiber optic cable. The stored entangled quantum states were transferred to photons after a specified storage time and thereby verified.

Secure exchange of information more important

The secure exchange of information is an important cornerstone of our society today. Quantum communication, the transmission of data encoded in quantum bits, is based on the laws of quantum mechanics and provides an efficient and completely secure way to exchange information in a network. display

At present, unavoidable losses - absorption of photons in the communication channel - limit the range of quantum communication. As a result, the number of resources required increases exponentially with distance.

Communication channel divided into several segments

To solve this problem, Briegel, D r, Cirac, and Zoller (BDCZ) proposed the construction of quantum repeaters in 1998. The basic idea is to divide the communication channel into several short segments. The restriction is now being built up first in high quality in the short segments.

Then they are connected by a so-called restriction exchange. The required resources of this quantum communication protocol grow much slower with increasing communication distance than with previous protocols and are thus practically feasible. This implies that the constraint generated in the intermediate stages can be conserved in a quantum memory.

The key challenge lies in linking the BDCZ protocol with quantum storage. This was successfully demonstrated within the new study by the realization of a functional BDCZ quantum repeater.

Ultracold atoms generated

In the experiment, two ensembles of one million ultra cold atoms with a temperature of 100 micro Kelvin - 273.15 C - are first generated in two magneto-optical traps. In each ensemble, a common quantum state of the atoms, each with a photon, is then condensed by a Raman scattering process.

In the following, the researchers transform the ensembles into a constrained state by performing a joint Bell state measurement on the two individual photons, the so-called change of confinement. For this purpose, these photons are previously passed through a 300-meter-long glass fiber. The restriction thus created is now stored in the atoms and can later be read out, verified and further used by transferring the atomic quantum state back to new photons.

On the way to the quantum network

The method demonstrated here, to generate confusion by a common Bell measurement on the photons, is intrinsically robust according to the scientists. In particular, it is independent of its phase, and therefore hardly sensitive to length changes in the communication channel. This is essential to enable confinement and confinement between quantum storage over long distances, both central elements of a functional quantum repeater with stationary atomic qubits as quantum storage and flying photonic qubits as quantum Nachrichtentrger.

The researchers assume that the experimentally demonstrated elements can be extended to a quantum network. For a robust application, however, the quality of the quantum memory and the atom-photon barrier must be significantly improved.

(idw - University of Heidelberg, 29.08.2008 - DLO)