[PAST EVENT] Condensed Matter Seminar

November 18, 2014
Small Hall, Room 122
300 Ukrop Way
Williamsburg, VA 23185Map this location

This seminar consists of two parts: i) on signatures of Majorana fermions in topological Josephson junctions, and ii) on the possibility to generate entangled quasiparticles in three-terminal Josephson junctions.

Majorana fermions have been introduced in 1937 by E. Majorana as real solutions of the relativistic Dirac equation. Even if their existence remains hypothetic in high energy physics, recent developments in condensed matter physics suggest their presence as emergent excitations in solid state devices. During the last decade, they have attracted a lot of interest due to their non-Abelian statistics, allowing promising applications in fault-tolerant quantum computing.

For instance, Majorana fermions appears at the boundaries of topological superconductors. When two topological superconductors are connected, zero-energy Majorana bound states localized on either side of the junction form an Andreev bound state. As this current carrying state is 4?-periodic in the superconducting phase difference, it was speculated that, at finite dc bias voltage, the junction exhibits a fractional Josephson effect. We will show that any finite phase velocity induce a dynamic coupling between the bound state and the continuum of states above the superconducting gap amplitude. This intrinsic coupling provides unavoidable mechanism that alter the fractional Josephson effect. We will discuss, in terms of the circuit parameters, signatures of the fractional Josephson effect that could be relevant for current experimental investigations: the even-odd effect in Shapiro steps and the emergence of a peak at fractional Josephson frequency in the current noise spectrum. Furthermore, other manifestations of the Majorana bound states forming at the edges will be discussed.

In addition, substantial part of the talk will be devoted to quantum entanglement in three terminal Josephson junctions. Quantum entanglement is a crucial ingredient in modern quantum communication proposals. A quantitative test of entanglement is provided by the Bell inequalities which were successfully measured in quantum optics. However, in condensed matter physics, the question is still open. In solid state devices, the current is the natural observable and Bell inequalities are expressed via current-current correlation functions. Moreover, superconductivity offers a natural playground for the search of entangled particles: Cooper pairs are singlet states which are naturally entangled in spin space. We will show that, current-current cross correlations can be positive and amplified in coherent three terminal Josephson junctions. This finding opens the possibility for further investigations on quantum entanglement in those systems.