UniQuE - Unidirectional Quantum Engineering

Circuits based on superconductors metals that lose all of their resistance when cooled down to temperatures close to absolute zero have proven to be a promising platform for the development of quantum technologies, but an on-demand unidirectional emitter has not yet been experimentally realised. A project recently funded by the FWF aims to implement such an element which is important for future quantum networks and modular quantum computing. The project is hosted by the Austrian Academy of sciences at the Institute for Quantum Optics and Quantum Information (IQOQI) in Innsbruck. In a modular quantum computing approach quantum information is routed via flying qubits, usually photons along a waveguide, and processed in local nodes. To create a fully interconnected quantum network, in which quantum information can be exchanged between any node in the network, the nodes need to be capable of absorbing or transmitting an incoming photon on-demand. This requirement can be achieved by engineering chiral-light matter interactions, meaning that the interaction depends on the propagation direction of the photons in the waveguide or transmission line. Within the recently funded project entitled Unidirectional Quantum Engineering of Light- Matter Interactions with Superconducting Qubits (UniQuE). Teresa Hönigl-Decrinis aims to experimentally realise such a quantum node and investigate its further use in the future. As theoretically proposed by collaborators at the IQOQI, she will engineer effective chiral-light matter interactions by exploiting destructive interference between two superconducting qubits coupled to a common transmission line at a particular distance apart. As a result, the quantum node is expected to emit a photon only in one direction determined by the propagation direction of the photons along the transmission line. The realisation of this project requires careful design, sophisticated nanofabrication and state of the art low temperature measurement techniques. Her pre-acquired expertise in the field, as well as support from Prof. Gerhard Kirchmair and collaborators at IQOQI will provide an excellent foundation for the UniQuE project. Once the UniQuE goals are realised, they will extend current capabilities, significantly advance the field of quantum computing and open new research directions using itinerant microwave photons in a waveguide.

In the UniQuE project, funded by the Lise Meitner programme of the FWF, we worked towards building directional photonic interfaces with superconducting circuits for future quantum networks. Circuits based on superconductors - metals that lose all of their resistance when cooled down to temperatures close to absolute zero - have proven to be a promising platform for the development of quantum technologies. Building large-scale quantum computers requires a modular approach, in which quantum information is distributed via "flying qubits", usually photons along a waveguide and processed in local nodes. To create such a quantum network, each node needs to be capable of mediating the transfer of information between a quantum processing unit and a microwave waveguide. Here, we aimed to build such a photonic interface by placing two superconducting qubits coupled to a common waveguide a specific distance apart such that the node only emits into one direction due to destructive interference. Since Prof. William Oliver's group at MIT achieved the experimental demonstration of this idea while we were still in the development phase, we have shifted our focus on the control of collective dark states in waveguides. These states are of particular interest because they are protected from decoherence as they decouple from the waveguide environment and thus exhibit long lifetimes. This makes them promising candidates for photon storage, excitation transfer and photon-photon gates and thus fit well into the projects overall goal of building photonic interfaces for quantum networks.