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Quantum Networks

Achieving a quantum internet represents a transformative leap toward next-generation connectivity for applications like secure data transmission, distributed quantum computing, and fundamental physics research. Unlike classical bits and the internet, quantum bits (qubits) require innovative methods to enable efficient information transfer across networks, particularly with large metropolitan scales. For a functional quantum network, scalable and stable nodes that can reliably transmit, process, and manipulate quantum information are essential. In addition, creating reliable links between nodes is also essential.

Our approach uses trapped ions and cavity quantum electrodynamics (QED) to establish robust network nodes, where ions serve as stationary qubits and single photons mediate interactions between nodes. Cavity QED enables controlled interactions between quantised matter (such as ions) and single photons within an optical resonator, creating a reliable interface for information transmission. Specifically, in our experiments, we couple ⁴⁰Ca⁺ ions with the mode of a high-finesse optical cavity, providing an efficient interface between stationary qubits and flying qubits. To enhance entanglement distribution rates between remote ion-cavity nodes, we are also exploring the use of multi-ion super-radiant states to boost coupling strengths and multi-ion decoherence-free subspaces to improve quantum memories. This approach aims to achieve quantum link efficiencies >1, where remote entanglement rates would be higher than decoherence rates.

To develop more compact and efficient quantum network nodes, we experiment with various setups, including a classical lab-based configurations, a fiber-cavity system, and fully rack-mounted designs.

Macro-cavity IoN Trap (MINT)

This lab-based setup consists of an Innsbruck-style linear Paul trap for trapping ⁴⁰Ca⁺ ions inside a 20 mm long optical cavity. By implementing a cavity-mediated Raman process, we achieve a regime where the rate of coherent coupling between the ions and the cavity mode is comparable to the rates of system decoherence, namely cavity and atomic decay.

We are able to precisely control each ion’s individual coupling to the cavity mode by adjusting the position of the cavity relative to the ions and by varying the separation between the ions themselves. This allows us to fine-tune the interaction dynamics in the system. The ions’ electronic state is detected using fluorescence measurements with a photomultiplier tube and an EMCCD camera, while cavity photons are detected on avalanche photodiodes. The electronic states of the ions are monitored using fluorescence detection methods through a photomultiplier tube and an EMCCD camera. Additionally, photons from the cavity are detected with avalanche photodiodes, providing a means to observe the ion–cavity interaction. The ion-photon entanglement created locally here sets the basis for remote ion-ion entanglement.   

Fiber Cavity

The purpose of this project is to integrate a fiber-based Fabry-Pérot cavity into a linear Paul trap. By coupling such a microscopic optical cavity to a trapped 40Ca+ ion, we can achieve an ion-cavity coupling strength that exceeds both the cavity-field decay rate and the ion’s spontaneous emission rate. This approach is one way to enhance the bandwidth of quantum networks using trapped ions.

A significant challenge in fiber-based ion-cavity systems arises from the accumulation of surface charges on the dielectric mirror coatings, which exert forces on the ion. To mitigate this issue, we integrate the dielectric fiber mirrors into the hollow endcap electrodes of the Paul trap. By applying a DC voltage to the endcap electrodes, we effectively counteract these forces. The two mirrors forming the cavity are mounted on translation stages to align the cavity with the ion. The length of the fiber cavity is actively stabilized using the Pound-Drever-Hall method to maintain resonance between the cavity and the ion.

By employing a laser-controlled, cavity-mediated Raman process, we generate photons within the cavity, where they are inherently fiber-coupled.

Rack-Mounted Deployable Node

Within the Quantum Internet Alliance (QIA) Consortium of the EU's Quantum Flagship, this project focuses on developing and characterising a compact and portable trapped-ion processing node (end node) with the capability of preparing, manipulating, and detecting quantum states. In this rack-mounted setup, trapped ⁴⁰Ca⁺ ions will be coupled to a high-finesse optical cavity, serving as the interface between stationary and flying qubits. Subsequently, outside the rack, these photons are converted to 1550 nm telecom wavelength photons, facilitating integration into a metropolitan-scale network. The long-term vision is to expand to a pan-European quantum network with these end nodes deployed to data centres. This is a collaboration project with the Innsbruck Distributed Quantum Systems group and Alpine Quantum Technologies (AQT).

Funding

Funding for this project is provided by

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Past funding for this project was provided by

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