Long-distance relays for quantum data
Transmission of quantum information is based on the phenomenon of entanglement. Entanglement couples the state of a particle, such as a photon (the quantum of light), to that of another quantum object. The result is a non-classical type of correlation between the two. For instance, a pair of photons can be entangled such that their polarization - the orientation of the plane of oscillation of the electric field – is undetermined. But if the polarization of one is later ascertained, the other is always found to be in the opposite (“orthogonal”) state, even if the measurement is made when the two are far apart.
To build an extensive network based on entanglement, one must be able to measure states of particles that are widely separated. The present limit for photons is some 140 km, because of losses due to absorption on the way. Therefore some sort of "relay station" which is able to store quantum information is necessary. Researchers led by LMU physicist Julian Hoffmann, together with colleagues at the MPI for Quantum Optics, have now achieved a decisive breakthrough. They have managed to entangle two rubidium atoms held in optical traps 20 meters apart. Not only that, the system sends a signal which “announces” that entanglement has been achieved.
Entanglement relays as transmission lines
The “heralded” nature of the process makes it possible to link together many systems like the one just described, finally creating entanglement over a much larger distance. “In the absence of a heralding signal, one would need to implement a much more complicated entanglement protocol,” says Harald Weinfurter, who is affiliated with both LMU and the MPI for Quantum Optics. Thus the method reported by the Munich researchers could enable construction of a “quantum repeater”. By acting as a relay, this effectively sidesteps the problem of signal loss, and allows the propagation of quantum information over much longer distances.
One could also exploit the “spooky action at a distance“, which is how Albert Einstein characterized quantum entanglement, to construct secure channels of communication that are invulnerable to eavesdropping. Any attempt to tap into the channel will disrupt entanglement, automatically reducing the correlation between measurements made at the transmitter and receiver ends of the line. Thus, the new findings bring the dream of a practical quantum cryptography network an important step closer. (Science 5. July) MPI/göd