Hybrid rf SQUID qubit based on high kinetic inductance.

We report development and microwave characterization of rf SQUID (Superconducting QUantum Interference Device) qubits, consisting of an aluminium-based Josephson junction embedded in a superconducting loop patterned from a thin film of TiN with high kinetic inductance. Here we demonstrate that the systems can offer small physical size, high anharmonicity, and small scatter of device parameters. The work constitutes a non-tunable prototype realization of an rf SQUID qubit built on the kinetic inductance of a superconducting nanowire, proposed in Phys. Rev. Lett. 104, 027002 (2010). The hybrid devices can be utilized as tools to shed further light onto the origin of film dissipation and decoherence in phase-slip nanowire qubits, patterned entirely from disordered superconducting films.

Thermal Conductance of a Single-Electron Transistor.

We report on combined measurements of heat and charge transport through a single-electron transistor. The device acts as a heat switch actuated by the voltage applied on the gate. The Wiedemann-Franz law for the ratio of heat and charge conductances is found to be systematically violated away from the charge degeneracy points. The observed deviation agrees well with the theoretical expectation. With a large temperature drop between the source and drain, the heat current away from degeneracy deviates from the standard quadratic dependence in the two temperatures.

Noise of a superconducting magnetic flux sensor based on a proximity Josephson junction.

We demonstrate simultaneous measurements of DC transport properties and flux noise of a hybrid superconducting magnetometer based on the proximity effect (superconducting quantum interference proximity transistor, SQUIPT). The noise is probed by a cryogenic amplifier operating in the frequency range of a few MHz. In our non-optimized device, we achieve minimum flux noise ~4 µΦ0/Hz1/2, set by the shot noise of the probe tunnel junction. The flux noise performance can be improved by further optimization of the SQUIPT parameters, primarily minimization of the proximity junction length and cross section. Furthermore, the experiment demonstrates that the setup can be used to investigate shot noise in other nonlinear devices with high impedance. This technique opens the opportunity to measure sensitive magnetometers including SQUIPT devices with very low dissipation.

Thermal conductance of Nb thin films at sub-kelvin temperatures.

We determine the thermal conductance of thin niobium (Nb) wires on a silica substrate in the temperature range of 0.1-0.6 K using electron thermometry based on normal metal-insulator-superconductor tunnel junctions. We find that at 0.6 K, the thermal conductance of Nb is two orders of magnitude lower than that of Al in the superconducting state, and two orders of magnitude below the Wiedemann-Franz conductance calculated with the normal state resistance of the wire. The measured thermal conductance exceeds the prediction of the Bardeen-Cooper-Schrieffer theory, and demonstrates a power law dependence on temperature as T4.5, instead of an exponential one. At the same time, we monitor the temperature profile of the substrate along the Nb wire to observe possible overheating of the phonon bath. We show that Nb can be successfully used for thermal insulation in a nanoscale circuit while simultaneously providing an electrical connection.

Correlated Emission Lasing in Harmonic Oscillators Coupled via a Single Three-Level Artificial Atom.

A single superconducting artificial atom can be used for coupling electromagnetic fields up to the single-photon level due to an easily achieved strong coupling regime. Bringing a pair of harmonic oscillators into resonance with the transitions of a three-level atom converts atomic spontaneous processes into correlated emission dynamics. We present the experimental demonstration of two-mode correlated emission lasing in harmonic oscillators coupled via a fully controllable three-level superconducting quantum system (artificial atom). The correlation of emissions with two different colors reveals itself as equally narrowed linewidths and quenching of their mutual phase diffusion. The mutual linewidth is more than 4 orders of magnitude narrower than the Schawlow-Townes limit. The interference between the different color lasing fields demonstrates that the two-mode fields are strongly correlated.

Thermal conductance by the inverse proximity effect in a superconductor.

We study heat transport in hybrid lateral normal-metal-superconductor-normal-metal structures. We find the thermal conductance of a short superconducting wire to be strongly enhanced beyond the BCS value due to the inverse proximity effect, resulting from contributions of elastic cotunneling and crossed Andreev reflection of quasiparticles. Our measurements agree with a model based on the quasiclassical theory of inhomogeneous superconductivity in the diffusive limit.

Origin of hysteresis in a proximity josephson junction.

We investigate hysteresis in the transport properties of superconductor-normal-metal-superconductor (S-N-S) junctions at low temperatures by measuring directly the electron temperature in the normal metal. Our results demonstrate unambiguously that the hysteresis results from an increase of the normal-metal electron temperature once the junction switches to the resistive state. In our geometry, the electron temperature increase is governed by the thermal resistance of the superconducting electrodes of the junction.

Wideband detection of the third moment of shot noise by a hysteretic Josephson junction.

We use a hysteretic Josephson junction as an on-chip detector of the third moment of shot noise of a tunnel junction. The detectable bandwidth is determined by the plasma frequency of the detector, which is about 50 GHz in the present experiment. The third moment of shot noise results in a measurable change of the switching rate when reversing polarity of the current through the noise source. We analyze the observed asymmetry assuming adiabatic response of the detector.

Extracellular matrix of peripheral nerves in diabetes.

Peripheral nerves are susceptible to develop multiple changes in their morphology and biochemical composition as consequences of diabetes mellitus. This review focuses on diabetes-induced alterations of the extracellular matrix of the peripheral nerves, and on the potential molecular mechanisms causing these changes. The interest towards the extracellular matrix of peripheral nerves of diabetic patients is highlighted by the fact that the extracellular matrix does not only mechanically support the cells which it surrounds, but it also regulates their behavior through specific interactions mediated via molecules on the cell surface, such as integrin receptors and cell surface proteoglycans. Thus, changes in the structure and composition of the extracellular matrix may alter cellular functions in multiple ways. At the ultrastructural level, these changes include e.g. thickening of vascular, perineurial and Schwann cell associated basement membranes; accumulation of microfibrillar material in the vicinity of perineurial cells; and increased diameter of endoneurial collagen fibrils. At the molecular level, the changes may be associated with altered metabolism of various collagen types, such as type I, III, IV and VI collagens.

Expression of laminin and collagen genes by cells cultured from developing human sciatic nerves.

The expression of laminin alpha1, alpha2, beta1, beta2 and gamma1 subunits and proalpha1(I), proalpha1(III), alpha1(IV), alpha1(VI), alpha2(VI) and alpha3(VI) collagen chains was studied by Northern hybridizations, RNase protection assays and indirect immunofluorescence (IIF) labellings in cell cultures initiated from sciatic nerves of 14-27-wk-old human fetuses. The cultures represented mixed populations of Schwann cells, perineurial cells and fibroblasts, as estimated by morphology and S 100 protein immunolabellings. The mRNAs for certain basement membrane (BM) components, laminin beta1 and gamma1 chains and collagen alpha1(IV) chain, were readily detectable by Northern analyses in all cultures. In contrast, laminin alpha1, alpha2 and beta2 chain mRNAs were expressed at markedly lower levels. The expression of laminin alpha1 chain was detectable only by RNase protection assay. RNase protection analysis also demonstrated that the expression of laminin alpha2 chain increased with the developmental stage of the nerve used as the source for cell cultures. The expression of laminin beta2 chain was detected only at the protein level by IIF which demonstrated a faint immunosignal in a small subpopulation of cells. The mRNAs for type I, III and VI collagens were readily detectable in the cultures by Northern hybridizations. In summary, the extracellular matrix genes expressed in fetal human peripheral nerves and corresponding cell cultures display marked similarities. Cell cultures characterized here may prove useful in analyses elucidating potential roles of selected growth factors and cytokines in the induction of e.g. laminin alpha1 and beta2 chain expression by cells of developing peripheral nerves.