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March 01, 2010

Reversible light storage Improved Over Ten Times to 9% And Could Enable Quantum Communication Ranges of Thousands of Kilometers


Quantum encrypted communication is currently limited to about 100 kilometers (60 mile) range. Improving the light storage efficiency will enable better quantum repeaters. Improving quantum repeaters by over ten times will allow for quantum encrypted communication with ranges of thousand of kilometers. Improvements in materials used can increase the effiency of light storage even more that what has already been achieved.

Arxiv - Efficient light storage in a crystal using an Atomic Frequency Comb (12 page pdf)

We demonstrate efficient and reversible mapping of a light field onto a thulium-doped crystal using an atomic frequency comb (AFC). Thanks to an accurate spectral preparation of the sample, we reach an efficiency of 9%. Our interpretation of the data is based on an original spectral analysis of the AFC. By independently measuring the absorption spectrum, we show that the efficiency is both limited by the available optical thickness and the preparation procedure at large absorption depth for a given bandwidth. The experiment is repeated with less than one photon per pulse and single photon counting detectors. We clearly observe that the AFC protocol is compatible with the noise level required for weak quantum field storage.



Atomic Frequency Comb AFC)

* the AFC represents an extreme situation: high narrow absorbing peaks are separated by fully transparent region. Such a comb can absorb close to 100% of the light and reduces intrinsic dephasing during the storage time 1/T.
* the retrieval efficiency should be high in the forward direction (54%) and ideally perfect (100%) in the backward configuration.
* In order to achieve high efficiency, it is thus crucial to achieve a precise spectral shaping of the inhomogeneous absorption profile.
* AFC has been shown to have the highest temporal multimode capacity in principle, which is a critical figure of merit in the prospect of quantum
communication
* In this paper we show how the AFC protocol can be efficiently implemented in an appropriate crystal namely Tm3+:YAG
* Thulium has an interaction wavelength (793nm) easily accessible with laser diode as compared to other rare-earth ions (praseodynium and europium).
* we apply a 210G magnetic field along the [001] crystalline axis. This splits the ground and excited levels into a nuclear spin doublet


To conclude, we have successfully applied the AFC protocol to a Tm3+:YAG crystal whose properties are of particular interest in the prospect of quantum repeaters. We have observed efficiencies up to 9.13±0.10%. Our measurements correspond to more than an order of magnitude improvement as compared to previous realizations in Nd3+:YVO4. Since the material properties gives the ultimate width of the spectral selection, they strongly influence the efficiency. This comparison shows that further improvements will demand material developments. This is especially true for the future backward configuration since Tm3+:YAG and Nd3+:YVO4 have only two ground states. This configuration does not allow, at the time, optical pumping in an auxiliary level and the application of control pulses

New Journal of Physics - Efficient light storage in a crystal using an atomic frequency comb

We demonstrate efficient and reversible mapping of a light field on to a thulium-doped crystal using an atomic frequency comb (AFC). Owing to an accurate spectral preparation of the sample, we reach an efficiency of nine per cent. Our interpretation of the data is based on an original spectral analysis of the AFC. By independently measuring the absorption spectrum, we show that the efficiency is limited by both the available optical thickness and the preparation procedure at large absorption depth for a given bandwidth. The experiment is repeated with less than one photon per pulse and single-photon counting detectors. We clearly observe that the AFC protocol is compatible with the noise level required for weak quantum field storage.

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