Long-distance single photon transmission from a trapped ion via quantum frequency conversion

Trapped atomic ions are ideal single photon emitters with long lived internal states which can be entangled with emitted photons. Coupling the ion to an optical cavity enables efficient emission of single photons into a single... [ view full abstract ]

Trapped atomic ions are ideal single photon emitters with long lived internal states which can be entangled with emitted photons. Coupling the ion to an optical cavity enables efficient emission of single photons into a single spatial mode and grants control over their temporal shape. These features are key for quantum information processing and quantum communication. However, the photons emitted by these systems are unsuitable for long-distance transmission due to their wavelengths. Here we report the transmission of single photons from a single ⁴⁰Ca⁺ ion coupled to an optical cavity over a 10 km optical fibre via quantum frequency conversion (QFC) from 866 nm to the telecom C-band at 1,530 nm. We observe non-classical photon statistics of the photons, as well as the preservation of their temporal shape throughout. This telecommunication ready system can be a key component for long-distance quantum communication as well as future cloud quantum computation.

A single calcium ion is confined in a Paul trap and coupled to an optical cavity at 866 nm. Single photons are generated via a laser driven cavity-assisted Raman transition. The photons are then coupled into a PPLN waveguide along with 1,995 nm light to convert them to 1530 nm with an overall efficiency of 3%, and transmitted along a 10 km fibre. A Hanbury-Brown-Twiss setup allows the second-order correlation function (g⁽²⁾) to be measured.

For the direct cavity emission, a g⁽²⁾of 0.0017 ± 0.0012 was measured. For the frequency converted photons, g⁽²⁾= 0.67 ± 0.07 and 0.59 ± 0.07 was obtained before and after the 10 km fibre respectively. These results are all below the classical limit in agreement with the expected values for an ideal single-photon source given the signal-background ratio.

The techniques and results in this present a wide range of opportunities. QFC could provide a way to establish entanglement between disparate quantum systems over long distances. With ion-cavity systems providing excellentcontrol over the photon emission process, making it the prime candidate for networked quantum information systems, the results presented here are a major step towards this application.