Quantum bits (qubits) are the basic building blocks of any quantum computer. Superconducting qubits have been created with a 'top-down' approach that integrates superconducting devices into macroscopic electrical circuits [1-3], whereas electron-spin qubits have been demonstrated in quantum dots [4-6]. The phase coherence time (Tau2) and the single qubit figure of merit (QM) of superconducting and electron-spin qubits are similar -- Tau2 ~ microseconds and QM ~10-1000 below 100mK -- and it should be possible to scale-up these systems, which is essential for the development of any useful quantum computer. Bottom-up approaches based on dilute ensembles of spins have achieved much larger values of tau2 (up to tens of ms) [7, 8], but these systems cannot be scaled up, although some proposals for qubits based on 2D nanostructures should be scalable [9-11]. Here we report that a new family of spin qubits based on rare-earth ions demonstrates values of Tau2 (~ 50microseconds) and QM (~1400) at 2.5 K, which suggests that rare-earth qubits may, in principle, be suitable for scalable quantum information processing at 4He temperatures

No abstract

Static magnetic susceptibility, crystal field and exchange interactions in rare earth titanate pyrochlores Malkin, B. Z.; Lummen, T. T. A.; van Loosdrecht, P. H. M.; Dhalenne, G.; Zakirov, A. R. Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. AbstractThe experimental temperature dependence (T = 2-300 K) of single crystal bulk and site susceptibilities of rare earth titanate pyrochlores R 2 Ti 2 O 7 (R = Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb) is analyzed in the framework of crystal field theory and a mean field approximation. Analytical expressions for the site and bulk susceptibilities of the pyrochlore lattice are derived taking into account long range dipole-dipole interactions and anisotropic exchange interactions between the nearest neighbor rare earth ions. The sets of crystal field parameters and anisotropic exchange coupling constants have been determined and their variations along the lanthanide series are discussed.

No abstract

Optical absorption spectra of the Yb 2 Ti 2 O 7 single crystals and luminescence spectra of the Y 2 Ti 2 O 7 :Yb (1%) polycrystalline samples were registered at temperatures 4.2-300 K. These spectra and earlier published data on magnetic properties of Yb 3+ ions and on the temperature dependence of the electric field gradient at Yb nuclei in Yb 2 Ti 2 O 7 were used to analyze the crystal-field parameters in rare-earth titanates with the pyrochlore structure. The self-consistent sets of crystal-field parameters for rare-earth ions in the 16d sites with the D 3d symmetry that describe satisfactory all known single-ion magnetic properties and low-energy excitations in R 2 Ti 2 O 7 crystals (R = Tb, Ho, Er, Tm, Yb) are presented.

The electron paramagnetic resonance (EPR) spectra of impurity Ho 3+ ions in monocrystals LiYF.~:Ho 3 § (0.1 and 1%) with the natural abundance of 6Li (7.42%) and 7Li (92.58%) isotopes, and in the sample 7LiYF4:Ho3+ (0.1%) isotopically pure in 7Li were taken at the temperature 4.2 K in the frequency range of 165-285 GHz. Resonance transitions between crystal field sublevels (the ground non-Kramers doublet and the nearest excited singlet) of the 518 term were detected. The refined set of crysta] field parameters and the effective constant of the magnetic hyperfine interaction were determined from the detailed analysis of the recorded spectra at frequencies varied by 0.05 GHz. The fine structure of EPR lines with intervals of about 300 MHz observed in the sample LiYF4:Ho 3+ (0.1%) can be interpreted as a result of the isotopic disorder in the Li sublattices. Direct information about energy gaps at the anticrossing points of the electron-nuclear sublevels of the ground doublet was obtained. These gaps are induced by the hyperfine interaction that mixes doublet and singlet states and by random crystal fields. Weak EPR signals from distorted single ion and pair centers of impurity Ho 3+ ions were resolved. From a comparison of the measured and simulated spectra, estimates of spectral parameters of the dimer centers have been obtained.

We report high-resolution optical absorption spectra for NdFe 3 (BO 3 ) 4 trigonal single crystal which is known to exhibit a giant magnetoelectric effect below the temperature of magnetic ordering T N = 33 K. The analysis of the temperature-dependent polarized spectra reveals the energies and, in some cases, symmetries and exchange splittings of Nd 3+ 84 Kramers doublets. We perform crystalfield calculations starting from the exchange-charge model, obtain a set of six real crystal-field parameters, and calculate wave functions and magnetic g-factors. In particular, the values g ⊥ = 2.385, g || = 1.376 were found for the Nd 3+ ground-state doublet. We obtain B loc =7.88 T and |J FN |= 0.48 K for the values of the local effective magnetic field at liquid helium temperatures at the Nd 3+ site and the Nd -Fe exchange integral, respectively, using the experimentally measured Nd 3+ ground-state splitting of 8.8 cm -1 . To check reliability of our set of crystal field parameters we model the magnetic susceptibility data from literature. A dimer containing two nearest-neighbor iron ions in the spiral chain is considered to partly account for quasi-one-dimensional properties of iron borates, and then the mean-field approximation is used. The results of calculations with the exchange parameters for Fe 3+ ions J nn = -6.25 K (intrachain interactions) and J nnn = -1.92 K (inter-chain interactions) obtained from fitting agree well with the experimental data.

No abstract