Decoherence in Josephson-Junction Qubits due to Critical Current Fluctuations

We compute the decoherence caused by 1/f fluctuations at low frequency f in the critical current I_0 of Josephson junctions incorporated into flux, phase, charge and hybrid flux-charge superconducting quantum bits (qubits). The dephasing time \tau_{\phi} scales as I_0/ \Omega \Lambda S_{I_0}^{1/2}(1 Hz),where \Omega / 2\pi is the energy level splitting frequency, S_{I_0}(1 Hz) is the spectral density of the critical current noise at 1 Hz, and \Lambda \equiv |I_0 d \Omega / \Omega d I_0| is a parameter computed for given parameters for each type of qubit that specifies the sensitivity of the level splitting to critical current fluctuations. Computer simulations show that the envelope of the coherent oscillations of any qubit after time t scales as \exp (-t^2/ 2 \tau_{\phi}^2) when the dephasing due to critical current noise dominates the dephasing from all sources of dissipation. We compile published results for fluctuations in the critical current of Josephson tunnel junctions fabricated with different technologies and a wide range in I_0 and A, and show that their values of S_{I_0}(1 Hz) scale to within a factor of three of [ 144 (I_0/\mu{\rm A})^2/ (A/ \mu{\rm m}^2)](pA)^2/Hz at 4.2 K. We empirically extrapolate S_{I_0}^{1/2}(1 Hz) to lower temperatures using a scaling T(K)/4.2. Using this result, we find that the predicted values of \tau_{\phi} at 100 mK range from 0.8 to 12 \mus, and are usually substantially longer than values measured experimentally at lower temperatures

Phase Coherence and Andreev Reflection in Topological Insulator Devices

Topological insulators (TIs) have attracted immense interest because they host helical surface states. Protected by time-reversal symmetry, they are robust to non-magnetic disorder. When superconductivity is induced in these helical states, they are predicted to emulate p-wave pairing symmetry, with Majorana states bound to vortices. Majorana bound states possess non-Abelian exchange statistics which can be probed through interferometry. Here, we take a significant step towards Majorana interferometry by observing pronounced Fabry-Perot oscillations in a TI sandwiched between a superconducting and normal lead. For energies below the superconducting gap, we observe a doubling in the frequency of the oscillations, arising from the additional phase accumulated from Andreev reflection. When a magnetic field is applied perpendicular to the TI surface, a number of very sharp and gate-tunable conductance peaks appear at or near zero energy, which has consequences for interpreting spectroscopic probes of Majorana fermions. Our results demonstrate that TIs are a promising platform for exploring phase-coherent transport in a solid-state system.Comment: 9 pages, 7 figure

On a Possibility to Measure Thermoelectric Power in SNS Structures

Two dissimilar Josephson junctions, which are connected to a heater can act as precise batteries. Because of the difference in thermoelectric power of these batteries, circuit with two dissimilar batteries, under heat flow $\Delta T\sim 10^{-5}K$ would have a net EMF $10^{-11} V$ around the zero-resistance loop leading to a loop's magnetic flux oscillating in time. It is shown its theoretical value is proportional to both the temperature difference as well as the disparity in the thermoelectric powers of the two junctions.Comment: 5 page

Evidence for an anomalous current phase relation in topological insulator Josephson junctions

Josephson junctions with topological insulator weak links can host low energy Andreev bound states giving rise to a current phase relation that deviates from sinusoidal behaviour. Of particular interest are zero energy Majorana bound states that form at a phase difference of $\pi$. Here we report on interferometry studies of Josephson junctions and superconducting quantum interference devices (SQUIDs) incorporating topological insulator weak links. We find that the nodes in single junction diffraction patterns and SQUID oscillations are lifted and independent of chemical potential. At high temperatures, the SQUID oscillations revert to conventional behaviour, ruling out asymmetry. The node lifting of the SQUID oscillations is consistent with low energy Andreev bound states exhibiting a nonsinusoidal current phase relation, coexisting with states possessing a conventional sinusoidal current phase relation. However, the finite nodal currents in the single junction diffraction pattern suggest an anomalous contribution to the supercurrent possibly carried by Majorana bound states, although we also consider the possibility of inhomogeneity.Comment: 6 pages, 4 figure

Robust Fabry-Perot interference in dual-gated Bi2_2Se3_3 devices

We study Fabry-Perot interference in hybrid devices, each consisting of a mesoscopic superconducting disk deposited on the surface of a three-dimensional topological insulator. Such structures are hypothesized to contain protected zero modes known as Majorana fermions bound to vortices. The interference manifests as periodic conductance oscillations of magnitude $\sim 0.1$ $e^2/h$. These oscillations show no strong dependence on bulk carrier density or sample thickness, suggesting that they result from phase coherent transport in surface states. However, the Fabry-Perot interference can be tuned by both top and back gates, implying strong electrostatic coupling between the top and bottom surfaces of topological insulator.Comment: 5 pages, 3 figures. Accepted by Appl. Phys. Let