Quantum science is playing an ever-increasing role in our societies. More than just an academic field confined to dark laboratories, it enabled the development of novel technologies now massively produced and part of our everyday life. Two iconic examples are lasers and transistors, stemming from what is now regarded as the “first quantum revolution”, which took advantage of one particular feature of the quantum theory, allowing one to treat a particle as a wave and reversely. Physicists are now dealing with the “second quantum revolution”, which harnesses two other properties: the “superposition principle”, allowing a quantum object to be in two states at the same time; and “entanglement”, or correlations between two separate quantum systems that classical physics cannot account for. It has been realized in the 1980’s that these peculiarities could be used to envision a new and powerful way of processing information: replacing classical bits by quantum bits (or qubits), the field of “quantum information” was born. An emblematic goal of this field is to build quantum computers, promising an exponential speedup compared to their classical counterparts. If such a machine remains elusive, other less complex devices based on the same principles are now emerging, sometimes even commercially. They perform tasks such as quantum cryptography, simulation or metrology. Technologies arising from this second quantum revolution are actually on the verge of becoming relevant economically. This fact was recently acknowledged by the European Union, who launched in 2016 the “Quantum Technology Flagship”, a 1 billion Euros program to support the development of new quantum devices, from basic science to the market.
The MSCA ION-QNET aimed at investigating such a novel quantum technology at the fundamental level. More specifically, the goal was to build an elementary quantum network based on trapped ions and optical cavities. The previous sentence contained three important concepts:
- a “quantum network” is an ensemble of distant stationary quantum systems (one or several qubits) which can communicate with each other thanks to propagating quantum objects (optical photons). A long-term vision is a “quantum internet” interconnecting quantum computers. For now, the community investigates how to implement quantum links between simpler systems.
- a “trapped ion” is one type of qubit among others, including superconducting qubits, color centers in diamond, neutral atoms, or quantum dots. It consists of a charged atom, trapped using electric fields and slowed down using lasers. Other lasers can manipulate or read-out the ion’s internal state, and couple two neighboring ions. Trapped ions hold record values for one- and two-qubit gates fidelities, making them one of the most promising qubits for quantum computers.
- an “optical cavity” is a trap storing light. The most common cavity, and that used in the project ION-QNET, is the Fabry-Perot cavity, consisting of two mirrors facing each other. One quantum of light (or photon) entering the cavity bounces on the mirrors many times before leaving. Due to these many return trips, a qubit placed in a cavity can interact with light much more efficiently than would be the case in free space. This property has provided researchers with an invaluable tool to investigate light-matter interactions at the most fundamental level, a single photon coupled to a single atom, in a field known as cavity quantum electrodynamics (cavity-QED).
The goal of ION-QNET was to take advantage of cavity-QED to make ions and photons interact and build a quantum network made of two “nodes”, each consisting of an ion trap and an optical cavity. At the start of the project, one node was existing and working; however, its relatively large size put an upper bound on the available ion-photon coupling strength. Therefore, for the second node, we aimed at a stronger coupling by miniaturizing the system. A key ingredient is the “fiber-based Fabry-Perot cavity”, or simply “fiber cavity”, whose mirrors are fabricated directly on the end facets of optical fibers. Fiber cavities had been developed recently for experiments with neutral atoms, but their integration with trapped ions was still at an early stage. The key objective of the project ION-QNET was to adapt fiber cavities to the needs of trapped ions, in order to enter the yet unobserved strong coupling regime, where the ion-photon coupling dominates over unavoidable dissipative processes. The next objective was to allow an ion and a photon to exchange quantum information, in order to, finally, perform proof-of-principle experiments involving both network nodes.
