Quantum Frequency Conversion

The ability to control the optical frequency of quantum state carriers (i.e. photons) is an important functionality for future quantum networks. It allows all matter quantum systems – nodes of the network – to be compatible with the telecommunication C-band, where fiber losses are the lowest, therefore enabling long distance fiber quantum communication between them. It also allows dissimilar nodes to be connected with each other, thus resulting in heterogeneous networks that can take advantage of the different capabilities offered by the diversity of its constituents.

In our group two promising quantum memories are investigated: a Rubidium based cold atomic cloud – emitting photons at 780 nm – and a solid state Praseodymium doped crystal, absorbing or emitting photons at 606 nm. Here we investigate the quantum frequency conversion of single photons emitted or absorbed by these quantum memories, using three-wave mixing processes in non-linear waveguides. The main challenge of quantum frequency conversion is to achieve high conversion efficiency in a coherent and noiseless manner, key to preserving the quantum properties of the converted photons.

The first and main objective of this study is to show the compatibility of the visible quantum memories with the telecom C-band. We first demonstrated the conversion of single photons emitted by the laser-cooled Rubidium cloud to the telecom C-band at 1552 nm, using difference frequency generation in a PPLN waveguide. Thanks to the high conversion efficiency and narrowband spectral filtering of the pump-induced noise, the converted photons were detected with high signal-to-noise ratio and preserved their non-classical properties through the conversion process to a high degree [1]. In second experiment quantum correlations between a converted telecom photon and a spin wave stored in the atomic cloud were measured [2]. In a different conversion process, between the telecom wavelength and 606 nm, we also demonstrated the storage of up-converted telecom photons in the Praseodymium based quantum memory [3]. We are now currently studying the possibility of converting single photons emitted by this memory to the telecom wavelengths [4].

The second interest of quantum frequency conversion is its ability to bridge the frequency gap between disparate quantum systems. In a recent study [5] we showed the quantum state transfer between the cold atomic cloud and the solid state memory. To connect the two systems we used a cascaded frequency conversion (first from 780nm to 1552nm, and then back to visible at 606 nm) using two PPLN waveguides. In this experiment we showed that a single excitation stored in the atomic ensemble can be mapped into a single photon at 780 nm, converted to telecom, transferred to different laboratory via an optical fiber, up-converted to 606 nm, stored in the solid state memory and finally retrieved with high signal-to-noise ratio. We also showed that qubits (based on time-bin encoding) can be faithfully transferred between the two different systems. This first demonstration of a photonic interconnection between two heterogeneous quantum systems paves the way towards the realization of hybrid quantum networks.

References:

People in this Project:

Funding: