Microwave rare-earth spin interfaces for quantum information processing – MIRESPIN

The spin degree of freedom of individual dopants trapped in ultra-pure crystals is an outstanding system for quantum information processing (QIP). They can have coherence lifetimes ranging from seconds to several hours at low temperatures, making them ideal candidates for implementing quantum bits in a quantum computing platform and for storing quantum information over long times. However, fully exploiting the quantum potential of solid-state spins requires interfacing them with quantum information carriers (photons) and/or other quantum systems containing quantum information to be stored. Hybrid quantum systems combining circuit QED with spin-doped solids are therefore very attractive for the upcoming quantum technologies. In MIRESPIN, we propose a versatile platform to interface long-lived solid-state electron spins with microwave photons based on coupling the spins with superconducting resonators. Our aim is to reach the single-spin sensitivity limit and the quantum coherent regime of interaction (“strong coupling”) between individual spins and single microwave photons. For this, we will develop quantum-grade custom rare-earth doped crystals and new high-quality factor superconducting resonator architectures which we will couple for the first together. The targeted breakthroughs are expected to be keystones for the development of microwave quantum memories and integrated quantum networks. Recent years have witnessed considerable advances in the detection of microwave photons and the manipulation of their quantum state due to developments in superconducting quantum circuits (SQC), high frequency electronics and cryogenic technology. However, coupling single spins with single photons remains a challenge because of the low low coupling constant that can be achieved with conventional micro-resonator designs. The original strategy pursued in MIRESPIN to increase both coupling strength and spatial resolution towards single spin detection and strong coupling is based on the incorporation of an integrated nanoscale constriction in a high quality factor superconducting resonator. Such a constriction allows a strong confinement of the microwave field, thus a remarkable improvement of the spin-photon interaction. As spin system, we propose rare earth ions in crystals relying on their large g-factors, thus enhanced coupling strength with respect to other spin systems (as for instance NV centers in diamond), and their long coherence lifetimes, which can be further extended by several orders of magnitude by using the so-called ZEFOZ transitions. In addition, rare earths possess the unique characteristic among solid-state spins of having optical transitions with extremely narrow homogeneous broadening, which provides an optical interface allowing spin preparation, reading and reinitialization by all-optical methods as well as the conversion of microwave photons to the optical domain. In our research we will focus on rare-earth ions with an odd number of electrons, such as erbium or ytterbium because they have an electronic spin that is essential for interfacing with superconducting circuits . MIRESPIN is divided in three scientific work packages and one management, dissemination and exploitation work package. All WPs will feature a tight collaboration between the three partners, who share a unique scientific knowledge as well as complementary technical capabilities. For the success of the project we will combine several cutting-edge technological advances, including growth of ultra-pure rare-earth doped crystals, microwave-optical spectroscopy at millikelvin temperatures, quantum-limited microwave amplifiers and nanoscale superconducting resonators