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Claire de Thoisy-Méchin

Press Relations

Tel. +33 (0)1 44 27 23 34

Email: claire.de_thoisy-mechin@upmc.fr

 

In English:

Katherine Tyrka

International Press Relations

tel. +33 (0)1 44 27 51 05

Email: katherine.tyrka@upmc.fr

Storing a Twisted Photon in a Quantum Memory

Physicists at the Kastler Brossel Laboratory (LKB, CNRS/UPMC/ENS/CdF) have stored in an ensemble of cold atoms and retrieved on demand quantum bits carried by light pulses at the single-photon level. For the first time, the information was encoded in the orbital angular momentum of light. This demonstration, published in Nature Photonics, opens new perspectives for the development of quantum communications.

 

Among the various degrees of freedom of light, the orbital angular momentum, i.e. the possibility of having a helical wavefront, is today exploited to optically manipulate objects, to improve the sensitivity of some measurements or to trap atoms in original configurations. This degree of freedom also offers the possibility of transmitting broadband information by multiplexing it into many spatial modes, meaning as many communication channels. While a photon can only carry one quantum bit with its polarization, which can take only two possible values, the orbital angular momentum, which enables the definition of an infinite dimensional basis, gives the ability to carry a large number of quantum bits on a single photon. So far, this degree of freedom has been used to encode quantum information in particular states, consisting of a superposition of states of different orbital angular momenta.

For the first time, researchers from the Kastler Brossel Laboratory (LKB) in Paris have demonstrated the capability to store these states of light in a quantum memory and then to retrieve the stored information. This ability to store and replay quantum information in the form in which it was transmitted is a key ingredient for the development of communication networks. By demonstrating this type of a quantum memory, this work opens up the prospect of a high-capacity network based on this degree of freedom of light.

Storing light beams while maintaining their quantum properties is an experimental challenge and various strategies have been developed recently. Here, LKB physicists used a technique called electromagnetically induced transparency in a set of cold atoms trapped by lasers: an auxiliary beam illuminating the atoms opens a transparency window in the normally opaque sample. This change is accompanied by a drastic reduction in the propagation speed of light in the medium. Once all the information to be stored has entered the atomic medium, the auxiliary beam is turned off. The light is therefore stopped and fully absorbed by atoms: quantum information is thus transferred to the atoms. When the auxiliary beam is turned back on, the atoms re-emit the light and restore the initial beam.

Using a spatial light modulator, the researchers generated different beams with spatial patterns encoding different qubits. By detecting the light before or after storage, the researchers showed that after a few microseconds of storage, the re-emitted light retained the initially encoded information. In this work, photons prepared in a superposition of two different helicities were recorded and retrieved, and by using only a single atomic ensemble. The next step is to increase the number of helicities used with the aim of storing hundreds of qubits.

 

For more information:

The Kastler Brossel Laboratory (LKB, CNRS/UPMC/ENS/CdF)Nouvelle fenêtre

 

Référence: A quantum memory for orbital angular momentum photonic qubits. Nature Photonics doi:10.1038/nphoton.2013.355Nouvelle fenêtre hal-00906649Nouvelle fenêtre et arXiv:1308.0238Nouvelle fenêtre

 

Research contact:

Julien Laurat, UPMC professor and member of the Institut Universitaire de France Julien.laurat@upmc.fr

 

UPMC press contact:

Claire de Thoisy-Méchin l Tel. +33 (0)1 44 27 23 34/(0)6 74 03 40 19 l claire.de_thoisymechin@upmc.fr