A single atom and a BEC in two separate laboratories serve as nodes in a basic quantum network. To prepare entanglement between these systems, a laser pulse is used to stimulate the atom to emit a single photon which is entangled with the single atom. The photon is used to transport the entanglement through an optical fibre into a neighbouring laboratory. Here, the photon is stored in the BEC. This procedure establishes entanglement between the single atom and the BEC. After some delay, the photon is retrieved from the BEC and the state of the single atom is mapped onto a second photon. The observation of entanglement between these two photons proves that all steps of the experiment were performed successfully.
Because of its strange consequences the quantum mechanical phenomenon of entanglement has been called “spooky action at a distance” by Albert Einstein. For several years physicists have been developing concepts how to use this phenomenon for practical applications such as absolutely safe data transmission. For this purpose, the entanglement which is generated in a local process has to be distributed among remote quantum systems. A team of scientists around Prof. Gerhard Rempe, Director at the Max Planck Institute of Quantum Optics and head of the Quantum Dynamics Division, has now demonstrated that two remote atomic quantum systems can be prepared in a shared “entangled” state (Physical Review Letters, Advance Online Publication, 26 May 2011): one system is a single atom trapped in an optical resonator, the other one a Bose-Einstein condensate consisting of hundreds of thousands of ultracold atoms. With the hybrid system thus generated, the researchers have realized a fundamental building block of a quantum network.
In the quantum mechanical phenomenon of “entanglement” two quantum systems are coupled in such a way that their properties become strictly correlated. This requires the particles to be in close contact. For many applications in a quantum network, however, it is necessary that entanglement is shared between two remote nodes (“stationary” quantum bits). One way to achieve this is to use photons (“flying” quantum bits) for transporting the entanglement. This is somewhat analogous to classical telecommunication, were light is used to transmit information between computers or telephones. In the case of a quantum network, however, this task is much more difficult as entangled quantum states are extremely fragile and can only survive if the particles are well isolated from their environment.
The team of Professor Rempe has now taken this hurdle by preparing two atomic quantum systems located in two different laboratories in an entangled state: on the one hand a single rubidium atom trapped inside an optical resonator formed by two highly reflective mirrors, on the other hand an ensemble of hundreds of thousands of ultracold rubidium atoms which form a Bose Einstein condensate (BEC). In a BEC, all particles have the same quantum properties so that they all act as a single “superatom”.