Trapped ions and quantum frequency conversion for long-distance quantum networks

Abstract

A quantum network based on trapped atomic ions offers a pathway to scale quantum computers, improve precision measurements, and enable quantum-secure communication. Trapped ions are useful local quantum computers, simulators, and sensors because they have long coherence times and can perform high-fidelity gates. They are also able to distribute their entanglement to remote quantum systems via their emitted photons, making them a natural platform for integration into quantum networks. In particular, the longest coherence time and some of the highest-fidelity single- and two-qubit gates have been demonstrated in the ground state qubits of trapped ions. However, ground state ion qubits emit visible or ultraviolet photons that are lossy in optical fibers. Quantum frequency conversion (QFC) allows ground state ion qubits to extend their networking range by frequency down-converting their emitted photons to telecommunications wavelengths. In this dissertation, I discuss testing commercial ion trap systems for integration into long-distance quantum networks. I describe an experiment converting 138Ba+ ion-emitted photons from 493 nm to the telecom O-band and transmitting them through 11 km of deployed fiber in the Mid-Atlantic Region Quantum Internet (MARQI network). The temporal profile of the photons is preserved during the conversion and transmission through the fiber network. Polarization-preserving QFC is required to preserve the entanglement between the ion's spin and the photon's polarization. In this dissertation, I also demonstrate polarization-preserving frequency conversion from 493 nm to 1283 nm in the telecom O-band, with > 97.(0)% process fidelity. The end-to-end conversion efficiency of this process exceeds 9%, inclusive of fiber losses, and the background-subtracted noise photon rate is < 20 cps. The results indicate that our polarization-preserving frequency converter is suitable for preserving entanglement between a ground state barium ion and a telecom photon. This work provides an important advancement towards distributing entanglement in a long-distance quantum network based on ground state ions qubits.