We investigate the decoherence of a qubit coupled to either a quantum two-level system (TLS) again coupled to an environment, or a classical fluctuator modeled by random telegraph noise. In order to do this we construct a model for the quantum TLS where we can adjust the temperature of its environment, and the decoherence rate independently. The model has a well-defined classical limit at any temperature and this corresponds to the appropriate random telegraph process, which is symmetric at high temperatures and becomes asymmetric at low temperatures. We find that the difference in the qubit decoherence rates predicted by the two models depends on the ratio between the qubit-TLS coupling and the decoherence rate in the pointer basis of the TLS. This is then the relevant parameter which determines whether the TLS has to be treated quantum mechanically or can be replaced by a classical telegraph process. We also compare the mutual information between the qubit and the TLS in the classical and quantum cases.

We study the decoherence of a central spin 1/2 due to a closed environment composed of spin-1/2 particles. It is known that a frustrated spin environment, such as a spin glass, is much more efficient for decoherence of the central spin than a similar-size environment without frustration. We construct a Hamiltonian where the degree of frustration is parametrized by a single parameter κ. By use of this model we find that the environment can be classified by two distinct regimes with respect to the strength of level repulsion. These regimes behave qualitatively different with respect to decoherence of the central spin and might explain the strong enhancement of decoherence observed for frustrated environments.

By use of a generalized Bloch vector construction, we study the decoherence of a system composed of two interacting qubits in a noisy environment. In particular, we investigate the effects of correlations in the noise acting on distinct qubits. Our treatment of the two-qubit system by use of the generalized Bloch vector leads to tractable analytic equations for the dynamics of the 4-level Bloch vector and allows for the application of geometrical concepts from the well known 2-level Bloch sphere. We find that in the presence of correlated or anticorrelated noise, the rate of decoherence is very sensitive to the initial twoqubit state, as well as to the symmetry of the Hamiltonian. In the absence of symmetry in the Hamiltonian, correlations only weakly impact the decoherence rate.

Aim:  The aims of the present study were to retrospectively examine Norwegian patients with osteogenesis imperfecta (OI) and dental implants, and to prospectively study a group of patients with OI in need of such treatment. Our hypothesis was that implant treatment in these patients has approximately the same success rate as in healthy persons. Study group:  Seven patients with a diagnosis of OI and a total of 26 dental implants were invited to participate in the retrospective study. The prospective part of the study included a total of 20 implants in seven patients. Results:  The patients in the retrospective study were seen after an average of 74 months after loading, range 25–135 months. Two implants had been lost, both of them before prosthetic loading and in the same patient. Bone loss of 4 mm or more was seen around three implants in two patients. The patients in the prospective study were followed for a mean observation time of 20 months after loading, range 12–33 months. No implants were lost in that group, and only minimal bone loss was observed (less than 1 mm around two implants in one patient). Conclusion:  In these OI patients, the total implant survival rate was 95.7%, being 93.3% in the retrospective group and 100% in the prospective group, respectively.

We study the effect of external driving on the two level systems (TLSs) assumed to be a major obstacle in increasing the coherence time of solid state Josephson-junction qubits. We find, by use of a Bloch-Redfield approach, that external driving has two major effects on the TLS. The first is increased fluctuations between the two states of the TLS, the significance of this effect compared to thermal fluctuations depend on the energy splitting of the TLS compared to temperature. The second effect is a reduction in the intensity of the noise spectrum at low frequencies, and at the same time an increase in intensity around the renormalized Rabi frequency of the TLS, the driving frequency and at beatings between these two frequencies. Finally we study the ensemble averaged noise spectrum for a typical distribution of TLSs known to give origin to ∝ 1/f noise. We find that strong driving leads to reduced noise at low frequencies, and therefore to an increased dephasing time T Q 2 of the qubit. However this effect is exponentially suppressed when the driving frequency is large compared to temperature, as we typically find for Josephson qubits. We suggest that external driving at frequencies much lower than the qubit frequency might be used in order to enhance the the qubit coherence time.