We present cavity QED experiments with an Er 3+ :Y2SiO5 (Er:YSO) crystal magnetically coupled to a 3D cylindrical sapphire loaded copper resonator. Such waveguide cavities are promising for the realization of a superconducting quantum processor. Here, we demonstrate the coherent integration of a rare-earth spin ensemble with the 3D architecture. The collective coupling strength of the Er 3+ spins to the 3D cavity is 21 MHz. The cylindrical sapphire loaded resonator allowed us to explore the anisotropic collective coupling between the rare-earth doped crystal and the cavity. This work shows the potential of spin doped solids in 3D quantum circuits for application as microwave quantum memories as well as for prospective microwave to optical interfaces.Today, the field of quantum information science is looking for the possible physical and technological realization of future quantum processors. A considerable attention is focused on the study of isolated quantum systems such as trapped ions, electronic and nuclear spins, optical photons and superconducting (SC) quantum circuits. A promising route towards the realization of a feasible technology lies in the coherent integration of different systems resulting in hybrid quantum system [1]. Such a hybrid system will benefit from the best physical features of its isolated parts, as for instance, scalability and rapid manipulation of SC qubits, and long coherence time of atoms [2, 3].One way of implementing a hybrid quantum system, is to couple atomic ensembles magnetically to a planar superconducting quantum circuit [3]. Here, the strong confinement of a resonator mode along a coplanar microwave line mediates a strong collective coupling between the spins of the trapped atoms and the SC resonator. In spite of the persisting development of experiments on coupling trapped rubidium atoms to planar SC circuits, the SC hybrid circuits based on trapped atoms are still challenging to realize in practice [4,5]. In that respect, crystals doped with magnetic ions (nitrogen vacancy centers in diamond or rare-earth ion doped solids) are an appealing alternative atomic system [6][7][8][9][10][11][12]. Such solid states spin systems can easily be integrated with various planar SC quantum circuits.In contrast to the long coherence times of spin systems [13,14], the coherence of modern SC planar circuits is still limited by few microseconds, due to uncontrollable coupling to the environment [15]. The drastic improvement in coherence is possible by introducing a new architecture for SC quantum circuits based on threedimensional resonators, which has been recently proposed and successfully implemented [16,17]. Two-qubit gate operations have been demonstrated [18], and multiqubit entanglement schemes in 3D circuit QED have been proposed [19].From the perspective of electron spin resonance (ESR) spectroscopy, 3D cavities are used since the beginning of the field. It is also known, that a paramagnetic material with a very narrow inhomogeneous spin linewidth Γ 2 /2π (∼ 100 kHz at the microwav...

Quantum memories are integral parts of both quantum computers and quantum communication networks. Naturally, such a memory is embedded into a hybrid quantum architecture, which has to meet the requirements of fast gates, long coherence times and long distance communication. Erbium doped crystals are well suited as a microwave quantum memory for superconducting circuits with additional access to the optical telecom C-band around 1.55 µm. Here, we report on circuit QED experiments with an Er 3+ :YAlO3 crystal and demonstrate strong coupling to a superconducting lumped element resonator. The low magnetic anisotropy of the host crystal allows for attaining the strong coupling regime at relatively low magnetic fields, which are compatible with superconducting circuits. In addition, Ce 3+ impurities were detected in the crystal, which showed strong coupling as well.PACS numbers: 42.50. Fx, 76.30.Kg, 03.67.Hk, 03.67.Lx, Reliable operation of quantum information and communication protocols requires a quantum memory (QM), i.e. a system, which allows for storage and on-demand retrieval of a quantum bit [1,2]. This can be realized by a great variety of physical systems such as single trapped ions [3], atoms [4], single spins [5], two-level defects [6] and spin-ensembles [7], which differ by their frequency band, coherence time and operating conditions. Rare-earth (RE) ions doped into a solid represent one of most promising systems suitable for quantum memories, because their inner shell 4f optical electronic transitions possess very long coherence times [8]. The excellent optical properties of RE doped crystals are confirmed and harvested by the world wide research in quantum optics. This includes a light-matter interface at the single photon level [9], an efficient and broadband quantum memory for light [10,11], a quantum memory at the telecom Cband [12], an atomic frequency comb memory [13], storage of entanglement in a RE doped crystal [14] and generation of entanglement between two crystals [15]. Yet, in contrast to the single atom approach, a quantum memory based on RE doped solids allows for the implementation of multimode storage protocols [16,17].There are seven RE's ions (Ce 3+ , Nd 3+ , Sm 3+ , Gd 3+ , Dy 3+ , Er 3+ , Yb 3+ ), which are suited for a microwave quantum memory due to the presence of a large electronic spin associated with an unquenched orbital moment [18]. Most of them have access to the nuclear spin degrees of freedom, which allow for long term storage [19]. These RE ions can be doped into a variety of host crystals, and therefore, can potentially be integrated with superconducting (SC) quantum circuits [20]. The resulting hybrid quantum system can consist of a SC qubit, a transmission line or a resonator magnetically coupled to the spin ensemble [21]. The exclusive feature of some RE ions (Nd 3+ , Er 3+ , Yb 3+ ) is the presence of optical transitions inside standard telecommunication bands. A quantum memory based on these RE elements can be very attractive for quantum communication between qubits...

We report on hybrid circuit QED experiments with focused ion beam implanted Er 3+ ions in Y 2 SiO 5 coupled to an array of superconducting lumped element microwave resonators. The Y 2 SiO 5 crystal is divided into several areas with distinct erbium doping concentrations, each coupled to a separate resonator. The coupling strength is varied from 5 MHz to 18.7 MHz, while the linewidth ranges between 50 MHz and 130 MHz. We confirm the paramagnetic properties of the implanted spin ensemble by evaluating the temperature dependence of the coupling. The efficiency of the implantation process is analyzed and the results are compared to a bulk doped Er:Y 2 SiO 5 sample. We demonstrate the successful integration of these engineered erbium spin ensembles with superconducting circuits.PACS numbers: 42.50. Pq, 76.30Kg A future quantum information technology will most probably rely on employing different quantum systems, where each subsystem is specialized on fulfilling distinct tasks 1,2 . For instance, modern superconducting (SC) quantum circuits are well suited for implementing scalable and fast quantum processors 3 . However, these SC qubits suffer from relatively short coherence times 4 . In contrast, spin doped solids possess long coherence times of up to a second 5 such that they can serve as a quantum memory. Hybrid circuit QED offers a promising way for implementing a complete quantum computer, i.e. a processor interfaced with a memory unit 6 .In the recent years, Y 2 SiO 5 (YSO) crystals doped with rare-earth (RE) ions have moved into the focus of quantum information science 7-9 . Additionally, strong coherent coupling of Er:YSO to a SC lumped element microwave resonator has been demonstrated 10 . Most of the current research activity relies on RE:YSO crystals which are typically grown using the Czochralski method, where the RE doping takes place during the growth process 11-13 .In this article, we focus on the practical implementation of a hybrid quantum system. In a practical circuit, memory elements need to be placed at specific positions, where they can fulfill their tasks without interfering with the rest of the quantum circuitry. One possible approach is to locally implant spins into an empty crystal or directly into the substrate where the circuit is fabricated on 14 . Recently, weak coupling (∼ 1 MHz) of a superconducting resonator to Gd 3+ ions implanted into a sapphire substrate has been reported 15 . Here, we employ YSO as a host material for RE ions, which promises long optical and spin coherence times [16][17][18] . Moreover, the optical transition of Er 3+ lies within the standard telecom C-band, which allows for the implementation of a reversible coherent microwave to optical interface for quantum communication 19,20 .In our work, we use a focused ion beam (FIB) to implant Er 3+ ions into an undoped Y 2 SiO 5 (YSO) crystal with high spatial resolution 21 . We then perform circuit QED experiments on these crystals and confirm the successful implantation of erbium ions by studying the electron spin r...