Future quantum computers could dramatically outperform computers following the classical computation paradigm on some classes of calculations, promising solutions to previously intractable problems. Likewise, quantum communication promises data transfer that is by nature impervious to eavesdropping and manipulation. Any such technology requires carriers of quantum information. Photons, individual quanta of light, are a natural choice: they are easy to route and widely used to communicate classical information. At the same time, they can also carry quantum information with low loss because they do not interact with each other.
While this is an advantage for relaying information, it is a disadvantage for information processing where photon-photon interactions are required to perform computations on the quantum information bits (qubits) the photons represent. These interactions have to be mediated via the nonlinear optical response of matter, for example individual atoms. Physical systems that have light interact with atoms in a controlled manner are called light-matter interfaces. Beyond their technological use, photon-photon interactions in light-matter interfaces can lead to nontrivial quantum manybody states of photons that are scientifically interesting in their own right. Challenges faced by any design for a light-matter interface include reliably exchanging qubits between photons and atoms, and maximizing the photon-photon interactions they mediate.
A particularly promising interface consists of individual atoms trapped in a regular array close to an optical nanofiber. Nanofibers (NF) can be fabricated from off-the-shelf glass fibers via a heating and pulling technique, without the need for complex nanofabrication. Light fields guided by the NF leak into the surrounding vacuum and interact with atoms placed there. The NF conveniently serves a double purpose: First, it helps pinning individual atoms in place by trapping them using far-detuned laser light. Second, it provides optical access to interface the atoms with near-resonant photons: photons sent through the fiber can interact with the trapped atoms, and photons subsequently re-emitted by the atoms can again be collected in the fiber.
Beyond that, NF-coupled atom arrays possess features that set them apart, including spatial order of the atoms and the promise to realize arrays with a spacing of the atoms smaller than the wavelength of resonant light. Due to the spatial order, interference effects can drastically influence photon absorption and emission. The small separation of atoms in subwavelength arrays means that the electric dipoles between neighboring atoms can directly influence each other. Atoms then do no longer act as independent scatterers, but respond as a collective to photons passing through the NF. Previous theoretical studies predicted that the combination of collective optical response and interference can result in a drastically reduced error rate for some quantum information processing tasks, and result in interesting, complex states involving multiple photons at once.
Any experimental demonstration of these effects has to come to term with real-world imperfections: at present, not every trap site in a NF-based atom array is actually filled with an atom during the loading procedure, and atoms are not perfectly pinned but can move in their traps. These imperfections are likely to negatively impact both the interference effects due to order, and the collective optical response due to a close spacing of the atoms. The aim of this project was to investigate the impact of these imperfections and to mitigate and potentially exploit them as a resource.
