Devices for satellite-assisted quantum networks

Quantum networks, quantum nodes interconnected by quantum channels, offer powerful means of secure communications and quantum computations. They are crucial elements in a broad area of quantum technologies including quantum simulations and metrologies. In particular, quantum links with satellites take the network into a global or greater scale, extending the capability of transmitting information. It also provides experimental platforms of testing quantum physics in a relativistic regime. The realization of satellite-assisted quantum networks requires devices that are interfaced with quantum optical channels to satellites. This thesis discusses the development of four essential devices, three of which are in line with Canada's Quantum Encryption and Science Satellite (QEYSSat) mission. First, polarization-entangled photon sources are developed to transmit one of the paired photons over ground-based fiber-optic networks and the other over ground-to-satellite free-space links. A practical and versatile interferometric scheme is designed and demonstrated, which allows constructing highly non-degenerate sources with only conventional polarization optics. A method of directly producing entangled photon-pairs from optical fibers without interferometers is studied with thorough numerical analysis to show feasibility of experimental demonstration. An entangled photon source for the QEYSSat mission is conceptually designed, and several key parameters to fulfill a set of performance requirements are theoretically studied and experimentally verified. Secondly, this thesis presents two characterization platforms for optical components that are designed and implemented for the QEYSSat mission. One is to precisely measure transmitted wavefronts of large optics including telescopes. A proof-of-principle experiment is conducted with accurate modelling of measurement apparatus via three-dimensional raytracing, and quantitative agreement between the experiment and simulations validates our methodology. The other provides polarization characterizations for a variety of optical components including lenses, mirrors, and telescopes with consistent precision. A detailed description of subsystems including calibrations and test procedures is provided. Polarization-test results of several components for the QEYSSat are discussed. Third, quantum frequency transducers are developed for single-photon quantum key distributions with QEYSSat links. The devices are designed to translate the wavelength of single-photons emitted from quantum dot single-photon sources to QEYSSat channel wavelength via four-wave mixing Bragg-scattering process. Two optical media are concerned: a silicon nitride ring resonator and a photonic crystal fiber. Thorough numerical simulations are performed to estimate the device performance for both cases. A proof-of-principle demonstration of the frequency translation is conducted with a commercial photonic crystal fiber. Finally, a quantum simulator, serving as a quantum node in satellite-assisted quantum networks, is designed in a silicon nitride nanophotonic platform with cesium atoms. The designed photonic structure tailors electromagnetic vacuum such that photon-mediated forces between atoms causes collective motions mediating site-selective SU(2) spin-spin interactions. A coherent spin-exchange rate between atoms and collective dissipation rate of atoms are precisely estimated via finite-element time domain simulations. Furthermore, two schemes of trapping atoms in the vicinity of the designed structure are studied with calculations of potential energies and phonon tunneling rates. Experimental progress toward realization of the proposed system is summarized. The presented research activities of designing, analyzing, and implementing devices demonstrates the readiness of satellite-assisted quantum networks. This work contributes to creating quantum channels by entanglements with interfaces of various quantum systems in line with a broader scope of establishing a global quantum internet and quantum space exploration

Origin of the increased velocities of domain wall motions in soft magnetic thin-film nanostripes beyond the velocity-breakdown regime

It is known that oscillatory domain-wall (DW) motions in soft magnetic thin-film nanostripes above the Walker critical field lead to a remarkable reduction in the average DW velocities. In a much-higher-field region beyond the velocity-breakdown regime, however, the DW velocities have been found to increase in response to a further increase of the applied field. We report on the physical origin and detailed mechanism of this unexpected behavior. We associate the mechanism with the serial dynamic processes of the nucleation of vortex-antivortex (V-AV) pairs inside the stripe or at its edges, the non-linear gyrotropic motions of Vs and AVs, and their annihilation process. The present results imply that a two-dimensional soliton model is required for adequate interpretation of DW motions in the linear- and oscillatory-DW-motion regimes as well as in the beyond-velocity-breakdown regime.Comment: 16 pages, 3 figure

Quantitative understanding of magnetic vortex oscillations driven by spin-polarized out-of-plane dc current: Analytical and micromagnetic numerical study

We studied magnetic vortex oscillations associated with vortex gyrotropic motion driven by spin-polarized out-of-plane dc current by analytical and micromagnetic numerical calculations. Reliable controls of the tunable eigenfrequency and orbital amplitude of persistent vortex oscillations were demonstrated. This work provides an advanced step towards the practical application of vortex oscillations to persistent vortex oscillators in a wide frequency (f) range of 10 to 2000 MHz and with high values of f/(delta f).Comment: 27 pages, 4 figures, 2 table

Electronic interferometer capacitively coupled to a quantum dot

We theoretically study electron interference in a ballistic electronic interferometer capacitively coupled to a quantum dot. The visibility of the interference is reduced when the dot has degenerate ground states with different excess charges. The degree of the reduction depends on system parameters such as the strength of the capacitive coupling, and the dependence is analyzed in the regime where the dwell time of electrons in the dot is much longer than the electron flight time through the interferometry region coupled to the dot. The result is consistent with recent experimental data.Comment: 4 pages, 2 figure

Total Reflection and Negative Refraction of Dipole-Exchange Spin Waves at Magnetic Interfaces: Micromagnetic Modeling Study

We demonstrated that dipole-exchange spin waves traveling in geometrically restricted magnetic thin films satisfy the same laws of reflection and refraction as light waves. Moreover, we found for the first time novel wave behaviors of dipole-exchange spin waves such as total reflection and negative refraction. The total reflection in laterally inhomogeneous thin films composed of two different magnetic materials is associated with the forbidden modes of refracted dipole-exchange spin waves. The negative refraction occurs at a 90 degree domain-wall magnetic interface that is introduced by a cubic magnetic anisotropy in the media, through the anisotropic dispersion of dipole-exchange spin waves.Comment: 13 pages, 5 figure

Criterion for transformation of transverse domain wall to vortex or antivortex wall in soft magnetic thin-film nanostripes

We report on the criterion for the dynamic transformation of the internal structure of moving domain walls (DWs) in soft magnetic thin-film nanostripes above the Walker threshold field, Hw. In order for the process of transformation from transverse wall (TW) to vortex wall (VW) or antivortex wall (AVW) occurs, the edge-soliton core of the TW-type DW should grow sufficiently to the full width at half maximum of the out-of-plane magnetizations of the core area of the stabilized vortex (or antivortex) by moving inward along the transverse (width) direction. Upon completion of the nucleation of the vortex (antivortex) core, the VW (AVW) is stabilized, and then its core accompanies the gyrotropic motion in a potential well (hill) of a given nanostripe. Field strengths exceeding the Hw, which is the onset field of DW velocity breakdown, are not sufficient but necessary conditions for dynamic DW transformation

Reliable low-power control of ultrafast vortex-core switching with the selectivity in an array of vortex states by in-plane circular-rotational magnetic fields and spin-polarized currents

The authors investigated the technological utility of counterclockwise (CCW) and clockwise (CW) circular-rotating fields (HCCW and HCW) and spin-polarized currents with an angular frequency ??H close to the vortex eigenfrequency ??D, for the reliable, low-power, and selective switching of the bistate magnetization (M) orientations of a vortex core (VC) in an array of soft magnetic nanoelements. CCW and CW circular gyrotropic motions in response to HCCW and HCW, respectively, show remarkably contrasting resonant behaviors, (i.e., extremely large-amplitude resonance versus small-amplitude nonresonance), depending on the M orientation of a given VC. Owing to this asymmetric resonance characteristics, the HCCW (HCW) with ??H ??? ??D can be used to effectively switch only the up (down) core to its downward (upward) M orientation, selectively, by sufficiently low field (???10 Oe) and current density (??? 107 A cm2). This work provides a reliable, low power, effective means of information storage, information recording, and information readout in vortex-based random access memory, simply called VRAM.open906

Understanding eigenfrequency shifts observed in vortex gyrotropic motions in a magnetic nanodot driven by spin-polarized out-of-plane dccurrent

We observed sizable eigenfrequency shifts in spin-polarized dc-current-driven vortex gyrotropic motions in a soft magnetic nanodot, and clarified the underlying physics through micromagnetic numerical calculations. It was found that the vortex eigenfrequency is changed to higher (lower) values with increasing Oersted field (OH) strength associated with the out-of-plane dc current for the vortex chirality parallel (antiparallel) to the rotation sense of the OH circumferential in-plane orientation. The eigenfrequency shift was found to be linearly proportional to the current density j0 in the linear regime as in ?? D ≃?? j0 / G, where G is the gyrovector constant and is a positive constant, e.g., 1.9?? 10-8 erg/A for a model Permalloy dot of 300 nm diameter and 20 nm thickness. This behavior originates from the sizable contribution of the OH to the effective potential energy of a displaced vortex core in the gyrotropic motion. The present results reveal that D, an intrinsic dynamic characteristic of a given nanodot vortex state, is controllable by changes in both the density and direction of spin-polarized out-of-plane dc currents.open191