Orsolya Kálmán

Networks of quantum nanorings: programmable spintronic devices.

An array of quantum rings with local (ring by ring) modulation of the spin orbit interaction (SOI) can lead to novel effects in spin state transformation of electrons. It is shown that already small (3 x 3, 5 x 5) networks are remarkably versatile from this point of view: Working in a given network geometry, the input current can be directed to any of the output ports, simply by changing the SOI strengths by external gate voltages. Additionally, the same network with different SOI strengths can be completely analogous to the Stern-Gerlach device, exhibiting spatial-spin entanglement.

Stability of spintronic devices based on quantum ring networks

Transport properties in mesoscopic networks are investigated, where the strength of the (Rashba-type) spin-orbit coupling is assumed to be tuned with external gate voltages. We analyze in detail to what extent the ideal behavior and functionality of some promising network-based devices are modified by random (spin-dependent) scattering events and by thermal fluctuations. It is found that although the functionality of these devices is obviously based on the quantum coherence of the transmitted electrons, there is a certain stability: moderate level of errors can be tolerated. For mesoscopic networks made of typical semiconductor materials, even cryogenic temperatures can smear out the desired transport properties. When the energy distribution of the input carriers is narrow enough, it turns out that the devices can operate close to their ideal limits even at relative high temperature. As an example, we present results for two different networks: one that realizes a Stern-Gerlach device and another that simulates a spin quantum walker. Finally we propose a simple network that can act as a narrow band energy filter even in the presence of random scatterers.

Two-dimensional quantum rings with oscillating spin-orbit interaction strength: A wave function picture

We determine the relevant spinor valued wave functions for a two-dimensional quantum ring in the presence of Rashba-type spin-orbit interaction (SOI). The case of constant SOI strength is considered first, then we investigate the physical consequences of time-dependent (oscillating) SOI strength. Floquets method is applied to find time-dependent eigenspinors, and it is shown that the Floquet quasienergies and thus their differences (the generalized Rabi frequencies) are determined by the radial boundary conditions. Time evolution of various initial states is calculated.

Quantum Galvanometer by Interfacing a Vibrating Nanowire and Cold Atoms

We evaluate the coupling of a Bose-Einstein condensate (BEC) of ultracold, paramagnetic atoms to the magnetic field of the current in a mechanically vibrating carbon nanotube within the frame of a full quantum theory. We find that the interaction is strong enough to sense quantum features of the nanowire current noise spectrum by means of hyperfine-state-selective atom counting. Such a nondestructive measurement of the electric current via its magnetic field corresponds to the classical galvanometer scheme, extended to the quantum regime of charge transport. The calculated high sensitivity of the interaction in the nanowire-BEC hybrid systems opens up the possibility of quantum control, which may be further extended to include other relevant degrees of freedom.

Parametric amplification of the mechanical vibrations of a suspended nanowire by magnetic coupling to a bose-einstein condensate

We show how the vibrational modes of a nanowire may be coherently manipulated with a Bose-Einstein condensate of ultracold atoms. We consider the magnetomechanical coupling between paramagnetic atoms and a suspended nanowire carrying a dc current. Atomic spin flips produce a backaction onto the wire vibrations, which can lead to mechanical mode amplification. In contrast to systems considered before, the condensate has a finite energy bandwidth in the range of the chemical potential and we explore the consequences of this on the parametric drive. Applying the resolvent method, we determine the threshold coupling and we also find a significant frequency shift of the vibration due to magnetomechanical dressing.

Magnetic-noise-spectrum measurement by an atom laser in gravity

Bose-Einstein condensates of ultracold atoms can be used to sense fluctuations of the magnetic field by means of transitions into untrapped hyperfine states. It has been shown recently that counting the outcoupled atoms can yield the power spectrum of the magnetic noise. We calculate the spectral resolution function, which characterizes the condensate as a noise measurement device in this scheme. We use the description of the radio-frequency outcoupling scheme of an atom laser, which takes into account the gravitational acceleration. Employing both an intuitive and the exact three-dimensional and fully quantum mechanical approach, we derive the position-dependent spectral resolution function for condensates of different size and shape.