Physicists Develop New Method For Entangling Photons

Image Source: Robert Fickler, University of Vienna

 Quantum Physics

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.

In Austria, physisists 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.

The work is published in the journal Science.

Entanglement is a behavior that is consistent with quantum-mechanical theory, has been demonstrated experimentally, and it is accepted by the physics community. However there is some debate about whether a possible classical underlying mechanism could explain why this correlation occurs instantaneously even when the separation distance is large. The difference in opinion derives from espousal of various interpretations of quantum mechanics – 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.
Entangled pairs.

In the image above a laser beam exhibiting a superposition of 10 right-handed and 10 left-handed quanta of orbital angular momenta, producing 10 + 10 = 20 bright spots on the inner ring. Photons in such modes rotate simultaneously clockwise and counter-clockwise.

False-color image of a laser beam exhibiting a superposition of 100 right-handed and 100 left-handed quanta of orbital angular momenta, resulting in 100+100=200 bright spots on the inner ring. Image Source: Robert Fickler/University of Vienna

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.

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 this number," says team member Radek Lapkiewicz.

Other groups have previously entangled photons with OAM by firing laser beams into "nonlinear" crystals and then siphoning off the very small fraction of photons that spontaneously split inside the crystal to produce two lower-energy entangled photons. Those entangled photons carry a broad spectrum of OAM. But, says Lapkiewicz, this approach, being "limited by what nature gives", yields l values only up to about 20.
Twisting around

The Austrian group also used a nonlinear crystal to generate photons. In this case, however, the photons were entangled in polarization and this entanglement served only as a first step. The next step was to send the photons within each pair down separate optical fibres and then impart them with OAM. The researchers did this by bouncing the photons off a tiny screen known as a spatial light modulator, which is a device that alters the phase of the reflected light from point to point, so changing the shape of the beam's wavefront. This wavefront deformation depended on the photons' polarization, so that photons polarized in one direction received a kick of positive OAM whereas those polarized at right angles got a negative kick. The net result was to change the photons' entanglement from one of polarization to one of OAM.

Using this technique, Zeilinger and co-workers found they could obtain differences in quantum number as high as 600 (in other words l = +300 on one photon and l = –300 on the other). Lapkiewicz points out that there is, in theory, no upper limit to a photon's l value, which suggests that a photon – a quantum object – could acquire as much OAM as a macroscopic object, leading to what he calls a "tension between the quantum and classical worlds".

He cautions that the current result is still "many orders of magnitude" too small to rotate even tiny objects. He speculates that such manipulation might one day be possible by combining the momentum of many photons entangled together.

Zeilinger's group also says that its technique could be useful for carrying out remote sensing, particularly in low-light biological-imaging experiments. The idea would be to measure tiny rotations by attaching the rotating object to a circular mask with regularly spaced radial slits. One photon in each entangled pair would be given a high OAM value, while the other would keep its polarization. With the mask placed in the path of the OAM photons and rotated very slightly, the rate of simultaneous detection of the two sets of photons would change. The trick is that a polarizer placed in the path of the polarized photons would need to rotate through l times as large an angle to register the same change, so multiplying the sensitivity of the measurement by l times.

According to Lapkiewicz, the work might also be applied to quantum information. For example, he says, it might allow quantum processors that rely on polarization entanglement to be connected to those that instead exploit OAM.



SOURCE  Physics World

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