Transporting spatially entangled photons through an optical fiber

June 30, 2011 by Miranda Marquit feature
Two photons can be entangled in their mode structure. This has the advantage that more information can be encoded than by using polarization. Wolfgang Löffler and colleagues from the Quantum Optics group were able to demonstrate that this so-called 'spatial' entanglement can survive fiber transport. Credit: Wolfgang Löffler.

( -- "Spatially entangled photons is a hot topic in quantum information science, and optical fibers are the cornerstone of our communication society," Wolfgang Loffler tells "So far, though, no one that we know of has demonstrated that you can use a fiber to transport a photon entangled in its spatial degree of freedom."

Löffler is a scientist at Leiden University in The Netherlands. Löffler worked with E.R. Eliel and J.P Woerdman at Leiden, as well as T.G. Euser, M. Scharrer and P. St. J. Russell at the Max Planck Institute for the Science of Light in Erlangen, Germany, to demonstrate that a spatially entangled photon can successfully travel through an optic fiber. The team presents its findings in : “Fiber Transport of Spatially Entangled Photons.”

Right now, many photons are entangled using only their polarization degree of freedom. This limits the amount of information that can be encoded, and also limits the amount of entanglement possible per photon. However, by using the spatial degree of freedom, there are many more options for information encoding. “Polarization-based entanglement is strictly two-dimensional,” Löffler explains. “When you deal with spatially quantum-entangled photons, you have entanglement in higher dimensions, and you can encode more data.” He also points out that, “The spatial structure of a single photon field has basically infinite dimensions.”

Such high-dimensional entanglement could lead to better security in quantum cryptography, among other applications. “More quantum entanglement per photon can be used to enhance security or increase the resilience against noise,” says Löffler. “The prospects are great.”

However, not just any will do; in most conventional fibers, the spatial information is quickly lost, and only the spectral or temporal information is preserved. In order to take advantage of the ability to encode information in spatial degree of freedom, a special fiber is needed. “Working with Russell, who is a pioneer in photonic crystal fibers, we were able to find a suitable multimode fiber.”

Löffler says that this special fiber is constructed in such a way that light can be guided, mainly through air, in a hollow core. “This has a less detrimental effect on the photons that we want to get through the fiber,” he says. Löffler and his colleagues tested the multi-mode fiber, and were able to get a positive result in a rudimentary quantum cryptography setup.

Even with the positive result, Löffler cautions that there are still some issues that need to be resolved. “We need to figure out how to extend this scheme to longer distances, so we will need to study the fiber transport mechanisms closer to understand them better.” He also points out that the team was able to demonstrate transport of 3D entangled photons through the fiber, and the next step is to add more dimensions.

It is also important to remember that the team has tested only a limited number of fibers. “We have excluded many different fiber types, and we have found this one that works so far,” Löffler says. “There are additional fiber structures to test before we can say which would truly work best for transport of the spatially .”

Although there is still work to do, Löffler is optimistic about the future. “We showed that you can combine spatial entanglement with transport, and that is promising for what could be next.”

Explore further: Mass weddings -- NIST's new efficient 2-photon source

More information: W. Löffler, T.G. Eliel, M. Scharrer, P. St. J. Russell, and J.P. Woerdman, “Fiber Transport of Spatially Entangled Photons,” Physical Review Letters (2011). Available online:


Related Stories

Mass weddings -- NIST's new efficient 2-photon source

April 12, 2007

For a variety of applications in physics and technology, ranging from quantum information theory to telecommunications, it’s handy to have access to pairs of photons created simultaneously, with a chosen energy. In a significant ...

Entanglement can help in classical communication

March 30, 2011

( -- When most of us think of entanglement, our minds jump immediately to quantum communication. "Entanglement has become very well known and useful in quantum communication," Robert Prevedel tells ...

Recommended for you

'Material universe' yields surprising new particle

November 25, 2015

An international team of researchers has predicted the existence of a new type of particle called the type-II Weyl fermion in metallic materials. When subjected to a magnetic field, the materials containing the particle act ...

CERN collides heavy nuclei at new record high energy

November 25, 2015

The world's most powerful accelerator, the 27 km long Large Hadron Collider (LHC) operating at CERN in Geneva established collisions between lead nuclei, this morning, at the highest energies ever. The LHC has been colliding ...

Exploring the physics of a chocolate fountain

November 24, 2015

A mathematics student has worked out the secrets of how chocolate behaves in a chocolate fountain, answering the age-old question of why the falling 'curtain' of chocolate surprisingly pulls inwards rather than going straight ...

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

not rated yet Jun 30, 2011
Go Netherlands :)

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.