Speaker
Description
Large-scale quantum networks provide the basis for many potential quantum communication applications, such as quantum key distribution (QKD) and distributed quantum computing. Due to the widespread availability of telecom fibers used in classical communication, these could serve to implement such networks over potentially several hundred kilometers by encoding and sending quantum information via photons. Using long fiber-based networks also provides challenges such as environmentally induced polarization drifts, high losses due to splices, and the ability to interface different types of quantum memories.
We report on the realization and characterization of a 14 km-long quantum communication fiber testbed as well as the implementation of several quantum network protocols based on a single $^{40}$Ca$^+$ ion as a quantum memory. Due to an exposed above-ground section, a fast and automated polarization drift compensation is implemented that allows us to maintain ~99% process fidelity .
Using the $^{40}$Ca$^+$ ion and an ion-resonant photon-pair source, we demonstrate entanglement distribution with up to ~$99\,\%$ fidelity and atom-to-photon quantum state teleportation [1] with an average fidelity of ~$84\,\%$ over the fiber link [2]. To reduce transmission loss, we utilize quantum frequency conversion from the ionic wavelength of $854\,$nm to $1550\,$nm [3].
We also demonstrate the conversion of atom-photon entanglement from polarization to time-bin encoding in order to enable interfacing dissimilar quantum memories, such as ions and color centers. We use a fiber-based telecom encoding and analyzer setup with active phase and temperature stabilization with a conversion fidelity of $97.8\,\%$. The resulting converted atom-photon entanglement fidelity is $70.4\,\%$ compared to $93\,\%$ without conversion to time-bin. The reduction is mainly due to a low signal-to-background ratio and long-term drifts in the setup.
We also demonstrate a device-independent quantum key distribution protocol [4] based on atom-photon entanglement over an emulated fiber link of $20\,$km length. We also include quantum frequency conversion to telecom and back to the ion wavelength, as well as automated polarization compensation and classical communication to show the viability of the protocol for the actual fiber testbed.
[1] E. Arenskötter et al., Phys. Rev. Research 6, 023061 (2024)
[2] S. Kucera et al., npj Quantum Inf. 10, 88 (2024)
[3] E. Arenskötter et al., npj Quantum Inf 9, 34 (2023)
[4] R. Schwonnek et al., Nat. Commun. 12, 2880 (2021)
| Academic level | PhD student |
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