Physicists in Austria have devised a new technique for entangling photons using the property of "orbital angular momentum". The researchers say that the large amount of orbital momentum they have imparted to the photons paves the way for the entanglement of macroscopic objects and could also find applications in remote sensing and quantum computing.
Entanglement is a connectedness between two (or more) particles that does not exist in classical physics. It means that determining the quantum state of one of the particles automatically and instantaneously fixes the quantum state of the second particle, no matter how far apart those particles are – a phenomenon that Einstein famously called "spooky action at a distance". Often this is achieved using the polarization of photons – the direction of vibration of a light wave's electric field – such that pairs of entangled photons are constrained to vibrate at right angles to one another even though each of the photons is randomly polarized.
In the latest research, Anton Zeilinger, Robert Fickler and colleagues at the University of Vienna entangled photons in orbital angular momentum (OAM). Giving photons OAM means twisting a beam's wavefront so that as the beam travels forward its wavefront rotates around the propagation axis. This property has been well studied using laser beams and is exploited in so-called optical spanners, which use lasers to trap and rotate small objects. But Zeilinger's group was interested specifically in entangling twisted photons; in other words, producing pairs of photons with opposite directions of twistedness. That twistedness is represented by the quantum number l – the number of times the wavefront rotates around the propagation axis in the space of one wavelength. "The goal of our experiment was to see how high we could get.