Rare-earth-doped crystals are excellent hardware for quantum storage of photons. Additional functionality of these materials is added by their waveguiding properties allowing for on-chip photonic networks. However, detection and coherent properties of rare-earth single-spin qubits have not been demonstrated so far. Here we present experimental results on high-fidelity optical initialization, effcient coherent manipulation and optical readout of a single-electron spin of Ce 3 þ ion in a yttrium aluminium garnet crystal. Under dynamic decoupling, spin coherence lifetime reaches T 2 ¼ 2 ms and is almost limited by the measured spin-lattice relaxation time T 1 ¼ 4.5 ms. Strong hyperfine coupling to aluminium nuclear spins suggests that cerium electron spins can be exploited as an interface between photons and long-lived nuclear spin memory. Combined with high brightness of Ce 3 þ emission and a possibility of creating photonic circuits out of the host material, this makes cerium spins an interesting option for integrated quantum photonics.

The silicon-vacancy center in diamond offers attractive opportunities in quantum photonics due to its favorable optical properties and optically addressable electronic spin. Here, we combine both to achieve all-optical coherent control of its spin states. We utilize this method to explore spin dephasing effects in an impurity-rich sample beyond the limit of phonon-induced decoherence: Employing Ramsey and Hahn-echo techniques at temperatures down to 40 mK we identify resonant coupling to a substitutional nitrogen spin bath as limiting decoherence source for the electron spin.

Recent progress in observing and manipulating mechanical oscillators at quantum regime provides new opportunities of studying fundamental physics, for example to search for low energy signatures of quantum gravity. For example, it was recently proposed that such devices can be used to test quantum gravity effects, by detecting the change in the [x,p] commutation relation that could result from quantum gravity corrections. We show that such a correction results in a dependence of a resonant frequency of a mechanical oscillator on its amplitude, which is known as amplitudefrequency effect. By implementing of this new method we measure amplitude-frequency effect for 0.3 kg ultra high-Q sapphire split-bar mechanical resonator and for ∼ 10 −5 kg quartz bulk acoustic wave resonator. Our experiments with sapphire resonator have established the upper limit on quantum gravity correction constant of β0 to not exceed 5.2 × 10 6 , which is factor of 6 better than previously measured. The reasonable estimates of β0 from experiments with quartz resonators yields β0 < 4×10 4 . The data sets of 1936 measurement of physical pendulum period by Atkinson [1] could potentially lead to significantly stronger limitations on β01. Yet, due to the lack of proper pendulum frequency stability measurement in these experiments the exact upper bound on β0 can not be reliably established. Moreover, pendulum based systems only allow to test a specific form of the modified commutator that depends on the mean value of momentum. The electro-mechanical oscillators to the contrary enable testing of any form of generalized uncertainty principle directly due to a much higher stability and higher degree of control.

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 explore optical coherence and spin dynamics of an isotopically purified 166 Er: 7 LiYF 4 crystal below 1 K and at weak magnetic fields < 0.3T. Crystals were grown in our lab and demonstrate narrow inhomogeneous optical broadening down to 16MHz. Solid-state atomic ensembles with such narrow linewidths are very attractive for implementing of off-resonant Raman quantum memory and for the interfacing of superconducting quantum circuits and telecom C-band optical photons. Both applications require a low magnetic field of ∼10mT. However, at conventional experimental temperatures T>1.5K, optical coherence of Er:LYF crystal attains m 10 s time scale only at strong magnetic fields above 1.5 T. In the present work, we demonstrate that the deep freezing of Er:LYF crystal below 1 K results in the increase of optical coherence time to m 100 s at weak fields.