Researchers discover a way to avoid decoherence in a quantum system

A team of physicists in Israel has used the scattering of a photon when it strikes an atom to better understand the process of decoherence. In a paper the team has published in the journal Science, the group describe how, as part of their research, they found that the spin of an atom prior to being shot with a single photon determined whether decoherence took place or not.

Decoherence is the process that comes about when a quantum state transitions to a classical world state. Scientists are studying the way it comes about (and ways to prevent it from happening) to help in designing atomic clocks and hopefully one day, a quantum computer.

In this new effort, the researchers fired single photons at atoms and then studied the results using a detector. When the photons struck the atoms, they were deflected, a process called scattering. In so doing, they discovered that if the photon struck an atom whose spin was not aligned in the same direction as its path, than the photon and atom became entangled—where two particles behave as if one, even at a distance. If the photon and atom's spin were aligned, however, entanglement did not occur.

This experiment suggests a way to prevent decoherence—if the photon and atom became entangled, they experienced decoherence the moment the photon struck the detector and was measured—one of the basic rules of quantum mechanics. If the two didn't become entangled though, then decoherence never occurred because there never was a superposition state (a scenario defined by quantum mechanics whereby systems can exist simultaneously in more than one state) in the first place. It also shows that decoherence can perhaps be controlled in a quantum system by taking advantage of an atom's spin state.