Shape of things to comeQuantum communication nearer as entanglement swapping realized

The quantum communications field is still in its infancy, but important steps forward are made. We note that researchers from the University of Geneva in Switzerland have, for the first time, realized an “entanglement swapping” experiment with photon pairs emitted continuously by two different sources. You may recall from school that quantum entanglement is that strange phenomenon in which two photons or other quantum bodies behave as one unit, even if they are spatially separate. Entanglement is at the heart of many proposed quantum information and communications schemes, including quantum computing and encryption, so advances on this front are eagerly awaited by security experts.

University of Geneva physicist Matthäus Halder, the experiment’s corresponding scientist, told PhysOrg.com that “Normally, entanglement between two photons is obtained by emitting them simultaneously by the same source. We’ve shown that entanglement can be transferred, or swapped, onto two particles that originated from different sources and were formerly completely independent. This is the first time that two autonomous photons from continuous sources have been entangled.” In the researchers’ scheme, two independent pairs of entangled photons, A1-A2 and B1-B2, are emitted by autonomous sources. By taking a joint measurement on one photon in each pair (say, A1 and B1), these photons fall into an entangled state (which is later verified using detectors), one of the four so-called “Bell states” (named for physicist John Bell, a key contributor to quantum physics). The joint measurement is thus known as a Bell-state measurement (BSM), and it is the foundation of the experiment. As a result of the BSM, the two remaining photons (in our example, A2 and B2) are projected on an entangled state despite being unaware of the other’s presence and never having previously interacted. Hence the entanglement of the initial pairs has been “swapped.”

Now, the key element of a successful BSM is the precise timing of the two photon pairs. Until now this has been obtained by using pulsed sources, which send out photons in discrete bunches. Pulsed sources, however, must be synchronized to emit the photon bunches at an exact time, which a daunting task. Halder and his colleagues show that continuous photon sources can be used. These sources do not require any synchronization, and are therefore likely to be much easier to incorporate into future real-world quantum communications. In this case, photons with the proper timing is obtained not when they are emitted, but when they are later detected by separate detectors. The detectors’ temporal resolution (the precision of its measurements with respect to time) allowed Halder and his group to “post-select” only those photons that were emitted at the right time.

-read more in Matthäus Halder et al., “Entangling Independent Photons by Time Measurement” Nature Phys, 3 (19 August 2007) (doi:10.1038/nphys700): 692-59 (sub. req.)