Measurements of Phase Dynamics in Planar Josephson Junctions and SQUIDs.

We experimentally investigate the stochastic phase dynamics of planar Josephson junctions (JJs) and superconducting quantum interference devices (SQUIDs) defined in epitaxial InAs/Al heterostructures, and characterized by a large ratio of Josephson energy to charging energy. We observe a crossover from a regime of macroscopic quantum tunneling to one of phase diffusion as a function of temperature, where the transition temperature T^{*} is gate-tunable. The switching probability distributions are shown to be consistent with a small shunt capacitance and moderate damping, resulting in a switching current which is a small fraction of the critical current. Phase locking between two JJs leads to a difference in switching current between that of a JJ measured in isolation and that of the same JJ measured in an asymmetric SQUID loop. In the case of the loop, T^{*} is also tuned by a magnetic flux.

Out-of-equilibrium phonons in gated superconducting switches.

Recent experiments have suggested that superconductivity in metallic nanowires can be suppressed by the application of modest gate voltages. The source of this gate action has been debated and either attributed to an electric-field effect or to small leakage currents. Here we show that the suppression of superconductivity in titanium nitride nanowires on silicon substrates does not depend on the presence or absence of an electric field at the nanowire, but requires a current of high-energy electrons. The suppression is most efficient when electrons are injected into the nanowire, but similar results are obtained when electrons are passed between two remote electrodes. This is explained by the decay of high-energy electrons into phonons, which propagate through the substrate and affect superconductivity in the nanowire by generating quasiparticles. By studying the switching probability distribution of the nanowire, we also show that high-energy electron emission leads to a much broader phonon energy distribution compared with the case where superconductivity is suppressed by Joule heating near the nanowire.

Majorana fermions from Landau quantization in a superconductor and topological-insulator hybrid structure.

We show that the interplay of cyclotron motion and Andreev reflection experienced by massless-Dirac-like charge carriers in topological-insulator surface states generates a Majorana-particle excitation. On the basis of an envelope-function description of the Dirac-Andreev edge states, we discuss the kinematic properties of the Majorana mode and find them to be tunable by changing the superconductor's chemical potential and/or the magnitude of the perpendicular magnetic field. Our proposal opens up new possibilities for studying Majorana fermions in a controllable setup.

Measuring ultrasmall time delays of light by joint weak measurements.

We propose to use weak measurements away from the weak-value amplification regime to carry out precision measurements of time delays of light. Our scheme is robust to several sources of noise that are shown to only limit the relative precision of the measurement. Thus, they do not set a limit on the smallest measurable phase shift, contrary to standard interferometry and weak-value-based measurement techniques. Our idea is not restricted to phase-shift measurements and could be used to measure other small effects using a similar protocol.

Electron-phonon coupling in crystalline organic semiconductors: microscopic evidence for nonpolaronic charge carriers.

We consider electron-phonon coupling in crystalline organic semiconductors, using naphthalene for our case study. Employing a first-principles approach, we compute the changes in the selfconsistent Kohn-Sham potential corresponding to different phonon modes and go on to obtain the carrier-phonon coupling matrix elements (vertex functions). We then evaluate perturbatively the quasiparticle spectral residues for electrons at the bottom of the lowest unoccupied (LUMO), and holes at the top of the highest occupied (HOMO), band, obtaining Z(e) ≈ 0.74 and Z(h) ≈ 0.78, respectively. Along with the widely accepted notion that the carrier-phonon coupling strengths in polyacenes decrease with increasing molecular size, our results provide strong microscopic evidence for the previously conjectured nonpolaronic nature of bandlike carriers in these systems.

Quantum simulation of small-polaron formation with trapped ions.

We propose an analog quantum simulation of small-polaron physics using a one-dimensional system of trapped ions acted upon by off-resonant standing waves. This system, envisioned as an array of microtraps, in the single-excitation case allows the realization of the antiadiabatic regime of the Holstein model. We show that the strong excitation-phonon coupling regime, characterized by the formation of small polarons, can be reached using realistic values of the relevant system parameters. Finally, we propose measurements of the quasiparticle residue and the average number of phonons in the ground state, experimental probes validating the polaronic character of the phonon-dressed excitation.

Interferometric and noise signatures of Majorana fermion edge states in transport experiments.

Domain walls between superconducting and magnetic regions placed on top of a topological insulator support transport channels for Majorana fermions. We propose to study noise correlations in a Hanbury Brown-Twiss type interferometer and find three signatures of the Majorana nature of the channels. First, the average charge current in the outgoing leads vanishes. Furthermore, we predict an anomalously large shot noise in the output ports for a vanishing average current signal. Adding a quantum point contact to the setup, we find a surprising absence of partition noise which can be traced back to the Majorana nature of the carriers.

Measuring the momentum of a nanomechanical oscillator through the use of two tunnel junctions.

We propose a way to measure the momentum p of a nanomechanical oscillator. The p detector is based on two tunnel junctions in an Aharonov-Bohm-type setup. One of the tunneling amplitudes depends on the motion of the oscillator, the other one not. Although the coupling between the detector and the oscillator is assumed to be linear in the position x of the oscillator, it turns out that the finite-frequency noise output of the detector will in general contain a term proportional to the momentum spectrum of the oscillator. This is a true quantum phenomenon, which can be realized in practice if the phase of the tunneling amplitude of the detector is tuned by the Aharonov-Bohm flux Phi to a p-sensitive value.

Generalized Stern-Gerlach effect for chiral molecules.

The Stern-Gerlach effect is well known as spin-dependent splitting of a beam of atoms with magnetic moments by a magnetic-field gradient. Here, we show that an induced gauge potential may lead to a similar effect for chiral molecules. In the presence of three inhomogeneous light fields, the center of mass of a three-level chiral molecule is subject to an optically induced gauge potential, and the internal dynamics of the molecule can be described as an adiabatic evolution in the reduced pseudospin subspace of the two lowest energy levels. We demonstrate numerically that such an induced gauge potential can lead to observable pseudospin-dependent and chirality-dependent generalized Stern-Gerlach effects for mixed left- and right-handed chiral molecules under realistic conditions.

Temperature dependence of coulomb drag between finite-length quantum wires.

We evaluate the Coulomb drag current in two finite-length Tomonaga-Luttinger-liquid wires coupled by an electrostatic backscattering interaction. The drag current in one wire shows oscillations as a function of the bias voltage applied to the other wire, reflecting interferences of the plasmon standing waves in the interacting wires. In agreement with this picture, the amplitude of the current oscillations is reduced with increasing temperature. This is a clear signature of non-Fermi-liquid physics because for coupled Fermi liquids the drag resistance is always expected to increase as the temperature is raised.

Identification of novel deletion breakpoints bordered by segmental duplications in the NF1 locus using high resolution array-CGH.

BACKGROUND: Segmental duplications flanking the neurofibromatosis type 1 (NF1) gene locus on 17q11 mediate most gene deletions in NF1 patients. However, the large size of the gene and the complexity of the locus architecture pose difficulties in deletion analysis. We report the construction and application of the first NF1 locus specific microarray, covering 2.24 Mb of 17q11, using a non-redundant approach for array design. The average resolution of analysis for the array is approximately 12 kb per measurement point with an increased average resolution of 6.4 kb for the NF1 gene. METHODS: We performed a comprehensive array-CGH analysis of 161 NF1 derived samples and identified heterozygous deletions of various sizes in 39 cases. The typical deletion was identified in 26 cases, whereas 13 samples showed atypical deletion profiles. RESULTS: The size of the atypical deletions, contained within the segment covered by the array, ranged from 6 kb to 1.6 Mb and their breakpoints could be accurately determined. Moreover, 10 atypical deletions were observed to share a common breakpoint either on the proximal or distal end of the deletion. The deletions identified by array-CGH were independently confirmed using multiplex ligation-dependent probe amplification. Bioinformatic analysis of the entire locus identified 33 segmental duplications. CONCLUSIONS: We show that at least one of these segmental duplications, which borders the proximal breakpoint located within the NF1 intron 1 in five atypical deletions, might represent a novel hot spot for deletions. Our array constitutes a novel and reliable tool offering significantly improved diagnostics for this common disorder.

Positive cross correlations in a three-terminal quantum dot with ferromagnetic contacts.

We study current fluctuations in an interacting three-terminal quantum dot with ferromagnetic leads. For appropriately polarized contacts, the transport through the dot is governed by dynamical spin blockade, i.e., a spin-dependent bunching of tunneling events not present in the paramagnetic case. This leads, for instance, to positive zero-frequency cross correlations of the currents in the output leads even in the absence of spin accumulation on the dot. We include the influence of spin-flip scattering and identify favorable conditions for the experimental observation of this effect with respect to polarization of the contacts and tunneling rates.

Influence of dephasing on shot noise in an electronic Mach-Zehnder interferometer.

We analyze shot noise under the influence of dephasing in an electronic Mach-Zehnder interferometer, of the type that was realized recently [Nature (London) 422, 415 (2003)]]. Using a model of dephasing by a fluctuating classical field, we show how the usual partition noise expression T(1-T) is modified. We study the dependence on the power spectrum of the field, which is impossible in simpler approaches such as the dephasing terminal, against which we compare. We remark on shot noise as a tool to distinguish thermal smearing from genuine dephasing.

Variable electrostatic transformer: controllable coupling of two charge qubits.

We propose and investigate a novel method for the controlled coupling of two Josephson charge qubits by means of a variable electrostatic transformer. The value of the coupling capacitance is given by the discretized curvature of the lowest energy band of a Josephson junction, which can be positive, negative, or zero. We calculate the charging diagram of the two-qubit system that reflects the transition from positive to negative through vanishing coupling. We also discuss how to implement a phase gate making use of the controllable coupling.

Multiple Andreev reflections in a carbon nanotube quantum dot.

We report resonant multiple Andreev reflections in a multiwall carbon nanotube quantum dot coupled to superconducting leads. The position and magnitude of the subharmonic gap structure is found to depend strongly on the level positions of the single-electron states which are adjusted with a gate electrode. We discuss a theoretical model of the device and compare the calculated differential conductance with the experimental data.

Non-Markoffian effects of a simple nonlinear bath.

We analyze a model of a nonlinear bath consisting of a single two-level system coupled to a linear bath (a classical noise force in the limit considered here). This allows us to study the effects of a nonlinear, non-Markoffian bath in a particularly simple situation. We analyze the effects of this bath onto the dynamics of a spin by calculating the decay of the equilibrium correlator of the z-component of the spin. The exact results are compared with those obtained using three commonly used approximations: a Markoffian master equation for the spin dynamics, a weak-coupling approximation, and the substitution of a linear bath for the original nonlinear bath.

Full counting statistics of a superconducting beam splitter.

We study the statistics of charge transport in a mesoscopic three-terminal device with one superconducting terminal and two normal-metal terminals. We calculate the full distribution of transmitted charges into the two symmetrically biased normal terminals. In a wide parameter range, we find large positive cross correlations between the currents in the two normal arms. We also determine the third cumulant that provides additional information on the statistics not contained in the current noise.

Separation quality of a geometric ratchet.

We consider an experimentally relevant model of a geometric ratchet in which particles undergo drift and diffusive motion in a two-dimensional periodic array of obstacles, and which is used for the continuous separation of particles subject to different forces. The macroscopic drift velocity and diffusion tensor are calculated by a Monte Carlo simulation and by a master-equation approach, using the corresponding microscopic quantities and the shape of the obstacles as input. We define a measure of separation quality and investigate its dependence on the applied force and the shape of the obstacles.

High resolution deletion analysis of constitutional DNA from neurofibromatosis type 2 (NF2) patients using microarray-CGH.

Neurofibromatosis type 2 (NF2) is an autosomal dominant disorder whose hallmark is bilateral vestibular schwannoma. It displays a pronounced clinical heterogeneity with mild to severe forms. The NF2 tumor suppressor (merlin/schwannomin) has been cloned and extensively analyzed for mutations in patients with different clinical variants of the disease. Correlation between the type of the NF2 gene mutation and the patient phenotype has been suggested to exist. However, several independent studies have shown that a fraction of NF2 patients with various phenotypes have constitutional deletions that partly or entirely remove one copy of the NF2 gene. The purpose of this study was to examine a 7 Mb interval in the vicinity of the NF2 gene in a large series of NF2 patients in order to determine the frequency and extent of deletions. A total of 116 NF2 patients were analyzed using high-resolution array-comparative genomic hybridization (CGH) on an array covering at least 90% of this region of 22q around the NF2 locus. Deletions, which remove one copy of the entire gene or are predicted to truncate the schwannomin protein, were detected in 8 severe, 10 moderate and 6 mild patients. This result does not support the correlation between the type of mutation affecting the NF2 gene and the disease phenotype. This work also demonstrates the general usefulness of the array-CGH methodology for rapid and comprehensive detection of small (down to 40 kb) heterozygous and/or homozygous deletions occurring in constitutional or tumor-derived DNA.

Fine mapping of the constitutional translocation t(11;22)(q23;q11).

Translocation t(11;22)(q23;q11) is the most common constitutional reciprocal translocation in man. Balanced carriers are phenotypically normal, except for decreased fertility, an increased spontaneous abortion rate and a possible predisposition to breast cancer in some families. Here, we report the high resolution mapping of the t(11;22)(q23;q11) breakpoint. We have localised the breakpoint, by using fluorescence in situ hybidisation (FISH) walking, to a region between D11S1340 and WI-8564 on chromosome 11, and D22S134 and D22S264 on chromosome 22. We report the isolation of a bacterial artificial chromosome (BAC) clone spanning the breakpoint in 11q23. We have narrowed down the breakpoint to an 80-kb sequenced region on chromosome 11 and FISH analysis has revealed a variation of the breakpoint position between patients. In 22q11, we have sequenced two BACs (BAC2280L11 and BAC41C4) apparently mapping to the region; these contain low copy repeats (LCRs). Southern blot analysis with probes from BAC2280L11 has revealed different patterns between normal controls and translocation carriers, indicating that sequences similar/identical to these probes flank the translocation breakpoint. The occurrence of LCRs has previously been associated with genomic instability and "unclonable" regions. Hence, the presence of such repeats renders standard translocation breakpoint cloning techniques ineffective. Thus, we have used high resolution fiber-FISH to study this region in normal and translocation cases by using probes from 22q11, LCRs and 11q23. We demonstrate that the LCR containing the gap in 22q11 is probably substantially larger than the previous estimates of 100 kb. Using fiber-FISH, we have localised the breakpoint in 22q11 to approximately 20-40 kb from the centromeric border of the LCR (i.e. the telomeric end of AC006547) and have confirmed the breakpoint position on 11q23.