A) In the scientists’ apparatus, the atomic ensemble is confined in an optical trap formed by a focused laser beam. This beam is overlapped with counterpropagating “write” and “read” beams. The resulting Stokes and anti-Stokes photons are detected, serving as a useful probe for quantum memory storage. (B) An absorption image of the optically trapped atoms. Image: Thorsten Strassel.Physicists have taken a step closer to realizing long-distance quantum communication, in which a quantum state is transferred from one location to another by becoming entangled with a traveling photon.
The basis for quantum communication is the ability to entangle two photons, A and B, where one photon, B, is sent down a transmission channel. There is also a third photon, C, which is entangled to a quantum state but not to the other two photons. A quantum state may be represented by a group of atoms that shares a superposition between two ground states (in superposition, both ground states exist simultaneously, and there is a certain probability that the atoms are in one ground state or the other).
When physicists perform entanglement swapping by making a Bell state measurement on photons A and C, photon B also becomes immediately entangled to the quantum state, even though it has already traveled down the transmission channel.
The site where the sent photon becomes entangled with the quantum state is called a quantum repeater. Quantum repeaters, which occur throughout the transmission channel, can generate and store entanglement in order to boost the signal, with the aim of getting the entangled state to reach the other end. Entanglement can be stored by some sort of quantum memory device until, ultimately, the quantum state is “read” by being converted into another photon.
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