arXiv subjects

Ahmed Kenawy

Publications and source records attributed to Ahmed Kenawy.

5 recordsLinked to original sources

arXiv2025

On-demand analog space-time in superconducting networks: grey holes, dynamical instability and exceptional points

There has been considerable effort to mimic analog black holes and wormholes in solid state systems. Lattice realizations in particular present specific challenges. One of those is that event horizons in general have both white and black hole (grey hole) character, a feature guaranteed by the Nielsen-Ninomiya theorem. We here explore and extend the capability of superconducting circuit hardware to implement on-demand spacetime geometries on lattices, combining nonreciprocity of gyrators with the nonlinearity of Josephson junctions. We demonstrate the possibility of the metric sharply changing within a single lattice point, thus entering a regime where the modulation of system parameters is "trans-Planckian", and the Hawking temperature ill-defined. Instead of regular Hawking radiation, we find an instability in the form of an exponential burst of charge and phase quantum fluctuations over short time scales - a robust signature even in the presence of an environment. Moreover, we present a loop-hole for the typical black/white hole ambiguity in lattice simulations: exceptional points in the dispersion relation allow for the creation of pure black (or white) hole horizons, at the expense of a radical change in the dynamics of the wormhole interior.

cond-mat.mes-hall↗

arXiv2023

Time-dependent driving and topological protection in the fractional Josephson effect

The control of any type of quantum hardware invariably necessitates time-dependent driving. If the basis depends on the control parameter, the presence of a time-dependent control field yields an extra term in the Schrödinger equation that is often neglected. Here, we examine the effect of this term in a flux-controlled Majorana junction. We show that a time-varying flux gives rise to an electromotive force which is amplified when truncating to the junction's low-energy degrees of freedom. As a result, it compromises the robustness of the ground-state degeneracy present in the absence of the drive. The resulting flattening of the energy spectrum can be measured by a strong suppression of the dc supercurrent.

cond-mat.supr-con↗

arXiv2022

Electromotive force in driven topological quantum circuits

Time-dependent control of superconducting quantum circuits is a prerequisite for building scalable quantum hardware. The quantum description of these circuits is complicated due to the electromotive force (emf) induced by time-varying magnetic fields. Here, we examine how the emf modifies the fractional Josephson effect. We show that a time-varying flux introduces a new term that depends on the geometry of both the circuit and the applied magnetic field. This term can be probed via current and charge measurements in closed-loop and open-circuit geometries. Our results refine the current understanding of how to properly describe time-dependent control of topological quantum circuits.

cond-mat.mes-hall↗

arXiv2022

Circuit quantization with time-dependent flux:the parallel-plate SQUID

Quantum circuit theory has emerged as an essential tool for the study of the dynamics of superconducting circuits. Recently, the problem of accounting for time-dependent driving via external magnetic fields was addressed by Riwar-DiVincenzo in their paper - 'Circuit quantization with time-dependent magnetic fields for realistic geometries' in which they proposed a technique to construct a low-energy Hamiltonian for a given circuit geometry, taking as input the external magnetic field interacting with the geometry. This result generalises previous efforts that dealt only with discrete circuits. Moreover, it shows through the example of a parallel-plate SQUID circuit that assigning individual, discrete capacitances to each individual Josephson junction, as proposed by treatments of discrete circuits, is only possible if we allow for negative, time-dependent and even singular capacitances. In this report, we provide numerical evidence to substantiate this result by performing finite-difference simulations on a parallel-plate SQUID. We furnish continuous geometries with a uniform magnetic field whose distribution we vary such that the capacitances that are to be assigned to each Josephson junction must be negative and even singular. Thus, the necessity for time-dependent capacitances for appropriate quantization emerges naturally when we allow the distribution of the magnetic field to change with time.

quant-ph↗

arXiv2018

Vibration-induced modulation of magnetic anisotropy in a magnetic molecule

We theoretically analyze the spectrum of a magnetic molecule when its charge and spin can couple to the molecular vibrations. More specifically, we show that the interplay between charge-vibron and spin-vibron coupling leads to a renormalization of the magnetic anisotropy parameters of the molecule. This effect is discussed for a model device consisting of an individual magnetic molecule embedded in a junction. We study the transport properties of the device and illustrate how the differential conductance is affected by the vibrationally induced renormalization of the magnetic anisotropy. Depending on the total molecular spin and the bare (intrinsic) magnetic anisotropy, the induced modulation can lead to visible shifts and crossings in the spectrum, and it can even be the cause of a transport blockade. It is therefore of particular interest to use mechanically controllable break junctions, since in such a case, the relevant coupling between the molecular spin and vibrations can be controlled via deformations of the molecule when stretching or compressing the junction.

cond-mat.mes-hall↗