Reversible tractor beam moves objects 100 times farther

Concept of photophoretic light–particle interaction in gases.

Credit: Nature Photonics (2014) doi:10.1038/nphoton.2014.242

A team of researchers working in Australia has built a tractor beam that bests the distance of other efforts a hundred fold, and it can both push and pull objects.

In their paper published in the journal Nature Photonics, the team describes their tractor beam project, how it works and what purpose it might serve.

A tractor beam is of course a beam emitted by a source that is used to hold and/or move another object.

The idea was popularized by the Star Trek series and other science fiction works, and was used typically by one starship to trap and move another starship.

In real life, physicists have made some inroads into developing a tractor beam (mostly based on the movement of photons to propel particles) but thus far, the objects trapped and moved have been extremely small and the distance moved even smaller.

In this new effort the researchers used a different technique to move an object that was bigger and to move it much farther, a hundred times as far.

Dr. Vladlen Shvedov (L) and Dr. Cyril Hnatovsky adjust the hollow laser beam in their lab at the Australian National University. 

Credit: Stuart Hay, ANU

The new tractor beam is based on heat, a laser that shines a doughnut-shaped beam (it has a cold center) was fired at a gold covered tiny (0.2mm diameter) glass bead that was small enough to just fit inside the beam, where it was cold.

The heat from the surrounding beam caused the surface of the bead to heat, creating hotspots.

When the hotspots came into contact with air particles, those particles were repelled, which in turn caused an opposing force against the glass bead, pushing it (up to a distance of 20cm).

The researchers found they could change the movement of the beads by adjusting the polarisation of the laser, causing changes in the hotspots on the beads.

That meant the beads could be pushed forward, stopped, pulled back, or held in place.

The team suggests their tractor beam could be useful in real world applications because of its versatility and because it requires just a single beam.

They believe it might be used for removing pollutants from the air, or for pulling undesirable particles from samples of materials.

They also note that it could very easily be made much larger, noting they were prevented from doing so by the small size of their lab.

More information: 
Economic tools for evaluating liabilities in environmental justice struggles, Nature Photonics (2014) DOI: 10.1038/nphoton.2014.242

Satellite quantum-communication circles

Communications protected by quantum encryption systems offer unconditional security – if you know which way is up.

A new quantum protocol is the first that promises to work independently of orientation, which will prove vital if quantum communications are ever to be sent via satellites.

Many quantum encryption protocols work by measuring the "up" or "down" spins on pairs of entangled photons shared between a sender, conventionally called Alice, and a receiver called Bob.


The two members of an entangled pair of photons always have an opposite spin from one another. If an eavesdropper were to intercept one, the very act of reading it would affect the entangled pair in a detectable way.

The distance record for quantum encrypted communications between two sites on Earth is 144 kilometres. If quantum encryption is to go global the data must be sent via satellite links, and here the conventional method hits a snag: a spinning satellite's sense of up and down changes over time, making it harder to interpret a photon's spin and establish a key.

Clockwise corkscrew
A team at the University of Bristol in the UK has invented a protocol independent of orientation that exploits the fact that photons can have an entangled circular polarisation as well as entangled spin.

Circularly polarised light can be imagined to corkscrew either clockwise or anticlockwise along its axis of travel. The two forms are readily identified regardless of the receiver's orientation.

Some modern 3D-movie projector systems already polarise light in this way to differentiate the two images used to form the 3D illusion. Doing so ensures that a cinemagoer wearing polarised glasses sees the 3D effect even if they tilt their head.

A 3D system that uses horizontally and vertically polarised light to differentiate the two images only works if the viewer's glasses are orientated in the same up-and-down direction as the theatre projector – in other words, only if the glasses and the projector share the same physical frame of reference.