Parity effect and single-electron injection for Josephson-junction chains deep in the insulating state

We have made a systematic investigation of charge transport in 1D chains of Josephson junctions where the characteristic Josephson energy is much less than the single-island Cooper-pair charging energy, $E_\mathrm{J}\ll E_{CP}$. Such chains are deep in the insulating state, where superconducting phase coherence across the chain is absent, and a voltage threshold for conduction is observed at the lowest temperatures. We find that Cooper-pair tunneling in such chains is completely suppressed. Instead, charge transport is dominated by tunneling of single electrons, which is very sensitive to the presence of BCS quasiparticles on the superconducting islands of the chain. Consequently we observe a strong parity effect, where the threshold voltage vanishes sharply at a characteristic parity temperature $T^*$, which is significantly lower than the the critical temperature, $T_c$. A measurable and thermally-activated zero-bias conductance appears above $T^*$, with an activation energy equal to the superconducting gap, confirming the role of thermally-excited quasiparticles. Conduction below $T^*$ and above the voltage threshold occurs via injection of single electrons/holes into the Cooper-pair insulator, forming a non-equilibrium steady state with a significantly enhanced effective temperature. Our results explicitly show that single-electron transport dominates deep in the insulating state of Josephson-junction arrays. This conduction process has mostly been ignored in previous studies of both superconducting junction arrays and granular superconducting films below the superconductor-insulator quantum phase transition.Comment: 8 pages, 6 figure

Charge noise in single-electron transistors and charge qubits may be caused by metallic grains

We report on measurements of low-frequency noise in a single-electron transistor (SET) from a few hertz up to 10 MHz. Measurements were done for different bias and gate voltages, which allow us to separate noise contributions from different noise sources. We find a 1/f noise spectrum with two Lorentzians superimposed. The cut-off frequency of one of the Lorentzians varies systematically with the potential of the SET island. Our data is consistent with two single-charge fluctuators situated close to the tunnel barrier. We suggest that these are due to random charging of aluminum grains, each acting as a single-electron box with tunnel coupling to one of the leads and capacitively coupled to the SET island. We are able to fit the data to our model and extract parameters for the fluctuators

A planar Al-Si Schottky Barrier MOSFET operated at cryogenic temperatures

Schottky Barrier (SB)-MOSFET technology offers intriguing possibilities for cryogenic nano-scale devices, such as Si quantum devices and superconducting devices. We present experimental results on a novel device architecture where the gate electrode is self-aligned with the device channel and overlaps the source and drain electrodes. This facilitates a sub-5 nm gap between the source/drain and channel, and no spacers are required. At cryogenic temperatures, such devices function as p-MOS Tunnel FETs, as determined by the Schottky barrier at the Al-Si interface, and as a further advantage, fabrication processes are compatible with both CMOS and superconducting logic technology.Comment: 6 pages, 4 figures, minor changes from the previous version

Crossover from time-correlated single-electron tunneling to that of Cooper pairs

We have studied charge transport in a one-dimensional chain of small Josephson junctions using a single-electron transistor. We observe a crossover from time-correlated tunneling of single electrons to that of Cooper pairs as a function of both magnetic field and current. At relatively high magnetic field, single-electron transport dominates and the tunneling frequency is given by f=I/e, where I is the current through the chain and e is the electron's charge. As the magnetic field is lowered, the frequency gradually shifts to f=I/2e for I>200 fA, indicating Cooper-pair transport. For the parameters of the measured sample, we expect the Cooper-pair transport to be incoherent.Comment: 5 pages, 4 figures; v2: minor changes, clarifications, addition

High resolution measurements of the switching current in a Josephson tunnel junction: Thermal activation and macroscopic quantum tunneling

We have developed a scheme for a high resolution measurement of the switching current distribution of a current biased Josephson tunnel junction using a timing technique. The measurement setup is implemented such that the digital control and read-out electronics are optically decoupled from the analog bias electronics attached to the sample. We have successfully used this technique to measure the thermal activation and the macroscopic quantum tunneling of the phase in a small Josephson tunnel junction with a high experimental resolution. This technique may be employed to characterize current-biased Josephson tunnel junctions for applications in quantum information processing.Comment: 10 pages, 8 figures, 1 tabl

Arrays of Cooper Pair Boxes Coupled to a Superconducting Reservoir: `Superradiance' and `Revival.'

We consider an array of Cooper Pair Boxes, each of which is coupled to a superconducting reservoir by a capacitive tunnel junction. We discuss two effects that probe not just the quantum nature of the islands, but also of the superconducting reservoir coupled to them. These are analogues to the well-known quantum optical effects `superradiance,' and `revival.' When revival is extended to multiple systems, we find that `entanglement revival' can also be observed. In order to study the above effects, we utilise a highly simplified model for these systems in which all the single-electron energy eigenvalues are set to be the same (the strong coupling limit), as are the charging energies of the Cooper Pair Boxes, allowing the whole system to be represented by two large coupled quantum spins. Although this simplification is drastic, the model retains the main features necessary to capture the phenomena of interest. Given the progress in superconducting box experiments over recent years, it is possible that experiments to investigate both of these interesting quantum coherent phenomena could be performed in the forseeable future.Comment: 23 pages, 5 figures Clarifications made as recommended by refere

Single-shot measurement of the Josephson charge qubit

We demonstrate single-shot readout of quantum states of the Josephson charge qubit. The quantum bits are transformed into and stored as classical bits (charge quanta) in a dynamic memory cell - a superconducting island. The transformation of state |1> (differing form state |0> by an extra Cooper pair) is a result of a controllable quasiparticle tunneling to the island. The charge is then detected by a conventional single-electron transistor, electrostatically decoupled from the qubit. We study relaxation dynamics in the system and obtain the readout efficiency of 87% and 93% for |1> and |0> states, respectively.Comment: submitted to Rapid Communications of Phys. Rev. B (february 2004

Osteopontin Facilitates West Nile Nirus Neuroinvasion via Neutrophil Trojan Horse Transport

West Nile virus (WNV) can cause severe human neurological diseases including encephalitis and meningitis. The mechanisms by which WNV enters the central nervous system (CNS) and host-factors that are involved in WNV neuroinvasion are not completely understood. The proinflammatory chemokine osteopontin (OPN) is induced in multiple neuroinflammatory diseases and is responsible for leukocyte recruitment to sites of its expression. In this study, we found that WNV infection induced OPN expression in both human and mouse cells. Interestingly, WNV-infected OPN deficient (Opn −/−) mice exhibited a higher survival rate (70%) than wild type (WT) control mice (30%), suggesting OPN plays a deleterious role in WNV infection. Despite comparable levels of viral load in circulating blood cells and peripheral organs in the two groups, WNV-infected polymorphonuclear neutrophil (PMN) infiltration and viral burden in brain of Opn −/− mice were significantly lower than in WT mice. Importantly, intracerebral administration of recombinant OPN into the brains of Opn −/− mice resulted in increased WNV-infected PMN infiltration and viral burden in the brain, which was coupled to increased mortality. The overall results suggest that OPN facilitates WNV neuroinvasion by recruiting WNV-infected PMNs into the brain

Osteopontin Facilitates West Nile Virus Neuroinvasion via Neutrophil “Trojan Horse” Transport

West Nile virus (WNV) can cause severe human neurological diseases including encephalitis and meningitis. The mechanisms by which WNV enters the central nervous system (CNS) and host factors that are involved in WNV neuroinvasion are not completely understood. The proinflammatory chemokine osteopontin (OPN) is induced in multiple neuroinflammatory diseases and is responsible for leukocyte recruitment to sites of its expression. In this study, we found that WNV infection induced OPN expression in both human and mouse cells. Interestingly, WNV-infected OPN deficient (Opn−/−) mice exhibited a higher survival rate (70%) than wild type (WT) control mice (30%), suggesting OPN plays a deleterious role in WNV infection. Despite comparable levels of viral load in circulating blood cells and peripheral organs in the two groups, WNV-infected polymorphonuclear neutrophil (PMN) infiltration and viral burden in brain of Opn−/− mice were significantly lower than in WT mice. Importantly, intracerebral administration of recombinant OPN into the brains of Opn−/− mice resulted in increased WNV-infected PMN infiltration and viral burden in the brain, which was coupled to increased mortality. The overall results suggest that OPN facilitates WNV neuroinvasion by recruiting WNV-infected PMNs into the brain

Broadband electrically detected magnetic resonance of phosphorus donors in a silicon field-effect transistor

We report electrically detected magnetic resonance of phosphorus donors in a silicon field-effect transistor. An on-chip transmission line is used to generate the oscillating magnetic field allowing broadband operation. At milli-kelvin temperatures, continuous wave spectra were obtained up to 40 GHz, using both magnetic field and microwave frequency modulation. The spectra reveal the hyperfine-split electron spin resonances characteristic for Si:P and a central feature which displays the fingerprint of spin-spin scattering in the two-dimensional electron gas.Comment: 4 pages, 4 figures, submitted to AP