Quantnet testbed: progress towards networking of ion-traps

The Quantnet Testbed aims to connect two ion traps located at LBNL and UCB that emit 854.44 nm photons, whose polarizations are entangled with the ions’ states, through a 10 km fiber link by using Quantum Frequency Conversion (QFC). Here, we discuss the ongoing deployment and integration of QFC nodes to convert these ion-trap photons from 854.44 nm to the telecom C-band using 1900 nm pump light via difference frequency generation (DFG). Firstly, we report on the implementation of polarization-dependent QFC scheme using two fiber-coupled DFG waveguides to convert 853.69 nm photons from two independent, frequency-locked input lasers to 1550.16 nm. These converted photons are then transmitted via single-mode fiber links to a Bell state measurement setup to project them onto a Bell state. We verified temporal indistinguishability of the photons by performing Hong-Ou-Mandel (HOM) interference measurements, achieving a preliminary HOM visibility of >46% for coherent, QFC-converted photons. Secondly, we report the progress towards the deployment of a Mach-Zehnder interferometer node with free space DFG waveguides to perform polarization-preserving QFC of ion trap photons, integrating that system with pump laser locking, multiple stages of filtering, and a scheduling algorithm to coordinate measurements and stabilization procedures. In summary, we develop and use these building blocks for interfacing to the ion traps and later entangling them through the 10 km link.