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In a nonlinear crystal illuminated by a strong laser, the photon wavelength is converted to the optimal value for long-distance travel. [Image: IQOQI Innsbruck / Harald Ritsch]

For the first time, scientists have observed a quantum entanglement of light and matter over a 50-km length of optical fiber (Npj Quantum Inf., doi:10.1038/s41534-019-0186-3). The experiments at two Austrian laboratories could lead to better synchronization of atomic clocks and, eventually, quantum networks for applications such as distributed sensing.

Past challenges

The team at the Institute for Quantum Optics and Quantum Information and the University of Innsbruck, Austria, faced several challenges. Some previous entanglement-transmission experiments involved photons at wavelengths that are strongly absorbed by optical fiber. Several processes, such  as added noise from frequency conversions, can lead to decoherence of the entanglements. Finally, the speed of light sets a hard upper limit on the timing of attempts to repeat transmissions—it takes all of 500 μs for photons to make a 100-km round trip.

Entanglement, conversion and transmission

To solve these issues, Ben Lanyon, a physicist at the two Innsbruck institutions, and his colleagues first created a Paul trap containing a calcium-40 ion and then hit the ion with a 393-nm-wavelength Raman laser pulse. That pulse, in turn, triggered the release of an 854-nm photon into the trap’s cavity.

The photon was entangled with two electronic qubit states of the ion, and then two waveguide crystals and a pump laser operating at a wavelength of 1902 nm converted it to the standard telecommunications frequency of 1550 nm. Next, the experimental apparatus directed the entangled photon bundle down a 50-km length of single-mode fiber, where the photon’s polarization was analyzed and the photon was registered on a single-photon counting module.

Counting down until decoherence

Lanyon’s group also studied how long the ion–photon entanglement could be stored before decoherence set in. The entanglement was still in place after 20 ms, the longest time measured in the experiments—good news for future experiments involving transmission over even longer distances.

The researchers say that their work could be easily extended to a 100-km pipeline of quantum entanglement transmission and eventually to multi-mode quantum networking.