Magnetic Sensors Based on Long Josephson Tunnel Junctions - An Alternative to SQUIDs

The properties of Josephson devices are strongly affected by geometrical effects. A loop-shaped superconducting electrode tightly couples a long Josephson tunnel junction with the surrounding electromagnetic field. Due to the fluxoid conservation, any change of the magnetic flux linked to the loop results in a variation of the shielding current circulating around the loop, which, in turn, affects the critical current of the Josephson junction. This method allows the realization of a novel family of robust superconducting devices (not based on the quantum interference) which can function as a general-purpose magnetic sensors. The best performance is accomplished without compromising the noise performance by employing an in-line-type junction few times longer than its Josephson penetration length. The linear (rather than periodic) response to magnetic flux changes over a wide range is just one of its several advantages compared to the most sensitive magnetic detectors currently available, namely the Superconducting Quantum Interference Devices (SQUID). We will also comment on the drawbacks of the proposed system and speculate on its noise properties.Comment: 13 pages, 4 figure

Multiple junction biasing of superconducting tunnel junction detectors

We describe a new biasing scheme for single photon detectors based on superconducting tunnel junctions. It replaces a single detector junction with a circuit of three junctions and achieves biasing of a detector junction at subgap currents without the use of an external magnetic field. The biasing occurs through the nonlinear interaction of the three junctions, which we demonstrate through numerical simulation. This nonlinear state is numerically stable against external fluctuations and is compatible with high fidelity electrical readout of the photon-induced current. The elimination of the external magnetic field potentially increases the capability of these types of photon detectors and eases constraints involved in the fabrication of large detector arrays.Comment: 15 pages, including 3 figure

Surgical Correction of Severe Bilateral Thumb Pincer-Nail Deformity

A 53 year old woman presented to the office with complaint of severe bilateral thumb nail pain and the deformity shown (Figure 1). She denied previous trauma to her thumb nails and had no history of previous bacterial or fungal infections of her nails. Based on the curvature of her nails, which increased from proximal to distal, a diagnosis of bilateral thumb pincer nail deformity was made

Investigation of low temperature quantum crossover in Josephson junctions

The evidence for macroscopic quantum tunneling (MQT) in Josephson junctions at low temperatures has been reassessed. Swept bias escape distributions have been modeled with an algorithm-based simulation and the results compared with data from representative published experiments. Signatures expected of a crossover to MQT are not found in the analyzed data.Comment: 7 pages, 6 figure

Classical analysis of phase-locking transients and Rabi-type oscillations in microwave-driven Josephson junctions

We present a classical analysis of the transient response of Josephson junctions perturbed by microwaves and thermal fluctuations. The results include a specific low frequency modulation in phase and amplitude behavior of a junction in its zero-voltage state. This transient modulation frequency is linked directly to an observed variation in the probability for the system to switch to its non-zero voltage state. Complementing previous work on linking classical analysis to the experimental observations of Rabi-oscillations, this expanded perturbation method also provides closed form analytical results for attenuation of the modulations and the Rabi-type oscillation frequency. Results of perturbation analysis are compared directly (and quantitatively) to numerical simulations of the classical model as well as published experimental data, suggesting that transients to phase-locking are closely related to the observed oscillations.Comment: 18 pages total, 8 figures (typos corrected; minor revisions to figures and equations

A classical statistical model for distributions of escape events in swept-bias Josephson junctions

We have developed a model for experiments in which the bias current applied to a Josephson junction is slowly increased from zero until the junction switches from its superconducting zero-voltage state, and the bias value at which this occurs is recorded. Repetition of such measurements yields experimentally determined probability distributions for the bias current at the moment of escape. Our model provides an explanation for available data on the temperature dependence of these escape peaks. When applied microwaves are included we observe an additional peak in the escape distributions and demonstrate that this peak matches experimental observations. The results suggest that experimentally observed switching distributions, with and without applied microwaves, can be understood within classical mechanics and may not exhibit phenomena that demand an exclusively quantum mechanical interpretation.Comment: Eight pages, eight figure

Screening magnetic fields by a superconducting disk: a simple model

We introduce a simple approach to evaluate the magnetic field distribution around superconducting samples, based on the London equations; the elementary variable is the vector potential. This procedure has no adjustable parameters, only the sample geometry and the London length, $\lambda$, determine the solution. The calculated field reproduces quantitatively the measured induction field above MgB$_2$ disks of different diameters, at 20K and for applied fields lower than 0.4T. The model can be applied if the flux line penetration inside the sample can be neglected when calculating the induction field distribution outside the superconductor. Finally we show on a cup-shape geometry how one can design a magnetic shield satisfying a specific constraint

Long Josephson Tunnel Junctions with Doubly Connected Electrodes

In order to mimic the phase changes in the primordial Big Bang, several "cosmological" solid-state experiments have been conceived, during the last decade, to investigate the spontaneous symmetry breaking in superconductors and superfluids cooled through their transition temperature. In one of such experiments the number of magnetic flux quanta spontaneously trapped in a superconducting loop was measured by means of a long Josephson tunnel junction built on top of the loop itself. We have analyzed this system and found a number of interesting features not occurring in the conventional case with simply connected electrodes. In particular, the fluxoid quantization results in a frustration of the Josephson phase, which, in turn, reduces the junction critical current. Further, the possible stable states of the system are obtained by a self-consistent application of the principle of minimum energy.Comment: 34 pages, 9 figures, Phys. Rev. B April 201

Critical current in Nb-Cu-Nb junctions with non-ideal interfaces

We report on experimental studies of superconductor (Nb) - normal metal (Cu) - superconductor (Nb) junctions with dirty interfaces between the different materials. By using a set of simultaneously prepared samples, we investigated the thickness dependence as well as the temperature dependence of the critical currents in the junctions. Good agreement between the decay of the measured critical currents and theoretical calculations was obtained without any fitting parameters

Analysis of measurement errors for a superconducting phase qubit

We analyze several mechanisms leading to errors in a course of measurement of a superconducting flux-biased phase qubit. Insufficiently long measurement pulse may lead to nonadiabatic transitions between qubit states $|1>$ and $|0>$, before tunneling through a reduced barrier is supposed to distinguish the qubit states. Finite (though large) ratio of tunneling rates for these states leads to incomplete discrimination between $|1>$ and $|0>$. Insufficiently fast energy relaxation after the tunneling of state $|1>$ may cause the repopulation of the quantum well in which only the state $|0>$ is supposed to remain. We analyze these types of measurement errors using analytical approaches as well as numerical solution of the time-dependent Schr\"{o}dinger equation.Comment: 14 pages, 14 figure