109 research outputs found
Quantum Magnetism, Spin Waves, and Light
Both magnetic materials and light have always played a predominant role in
information technologies, and continue to do so as we move into the realm of
quantum technologies. In this course we review the basics of magnetism and
quantum mechanics, before going into more advanced subjects. Magnetism is
intrinsically quantum mechanical in nature, and magnetic ordering can only be
explained by use of quantum theory. We will go over the interactions and the
resulting Hamiltonian that governs magnetic phenomena, and discuss its
elementary excitations, denominated magnons. After that we will study
magneto-optical effects and derive the classical Faraday effect. We will then
move on to the quantization of the electric field and the basics of optical
cavities. This will allow us to understand a topic of current research
denominated Cavity Optomagnonics. These notes were written as the accompanying
material to the course I taught in the Summer Semester 2018 at the
Friedrich-Alexander University in Erlangen. The course is intended for Master
or advanced Bachelor students. Basic knowledge of quantum mechanics,
electromagnetism, and solid state at the Bachelor level is assumed. Each
section is followed by a couple of simple exercises which should serve as to
"fill in the blanks" of what has been derived, plus specific references to
bibliography, and a couple of check-points for the main concepts developed. The
figures are pictures of the blackboard taken during the lecture.Comment: Class notes, revised version, typos corrected, figures adde
Cavity Optomagnonics
In the recent years a series of experimental and theoretical efforts have centered around a new topic: the coherent, cavity-enhanced interaction between optical photons and solid state magnons. The resulting emerging field of Cavity Optomagnonics is of interest both at a fundamental level, providing a new platform to study light-matter interaction in confined structures, as well as for its possible relevance for hybrid quantum technologies. In this chapter I introduce the basic concepts of Cavity Optomagnonics and review some theoretical developments
Antiferromagnetic cavity optomagnonics
Currently there is a growing interest in studying the coherent interaction between magnetic systems and electromagnetic radiation in a cavity, prompted partly by possible applications in hybrid quantum systems. We propose a multimode cavity optomagnonic system based on antiferromagnetic insulators, where optical photons couple coherently to the two homogeneous magnon modes of the antiferromagnet. These have frequencies typically in the THz range, a regime so far mostly unexplored in the realm of coherent interactions, and which makes antiferromagnets attractive for quantum transduction from THz to optical frequencies. We derive the theoretical model for the coupled system, and show that it presents unique characteristics. In particular, if the antiferromagnet presents hard-axis magnetic anisotropy, the optomagnonic coupling can be tuned by a magnetic field applied along the easy axis. This allows us to bring a selected magnon mode into and out of a dark mode, providing an alternative for a quantum memory protocol. The dynamical features of the driven system present unusual behavior due to optically induced magnon-magnon interactions, including regions of magnon heating for a red-detuned driving laser. The multimode character of the system is evident in a substructure of the optomagnonically induced transparency window
Magnon-Phonon Quantum Correlation Thermometry
A large fraction of quantum science and technology requires low-temperature environments such as those afforded by dilution refrigerators. In these cryogenic environments, accurate thermometry can be difficult to implement, expensive, and often requires calibration to an external reference. Here, we theoretically propose a primary thermometer based on measurement of a hybrid system consisting of phonons coupled via a magnetostrictive interaction to magnons. Thermometry is based on a cross-correlation measurement in which the spectrum of back-action driven motion is used to scale the thermomechanical motion, providing a direct measurement of the phonon temperature independent of experimental parameters. Combined with a simple low-temperature compatible microwave cavity readout, this primary thermometer is expected to become a promising alternative for thermometry below 1 K
Cavity optomagnonics with magnetic textures: coupling a magnetic vortex to light
Optomagnonic systems, where light couples coherently to collective
excitations in magnetically ordered solids, are currently of high interest due
to their potential for quantum information processing platforms at the
nanoscale. Efforts so far, both at the experimental and theoretical level, have
focused on systems with a homogeneous magnetic background. A unique feature in
optomagnonics is however the possibility of coupling light to spin excitations
on top of magnetic textures. We propose a cavity-optomagnonic system with a non
homogeneous magnetic ground state, namely a vortex in a magnetic microdisk. In
particular we study the coupling between optical whispering gallery modes to
magnon modes localized at the vortex. We show that the optomagnonic coupling
has a rich spatial structure and that it can be tuned by an externally applied
magnetic field. Our results predict cooperativities at maximum photon density
of the order of by proper engineering of these
structures.Comment: 16 pages, 11 figures, published versio
Coupled Spin-Light dynamics in Cavity Optomagnonics
Experiments during the past two years have shown strong resonant
photon-magnon coupling in microwave cavities, while coupling in the optical
regime was demonstrated very recently for the first time. Unlike with
microwaves, the coupling in optical cavities is parametric, akin to
optomechanical systems. This line of research promises to evolve into a new
field of optomagnonics, aimed at the coherent manipulation of elementary
magnetic excitations by optical means. In this work we derive the microscopic
optomagnonic Hamiltonian. In the linear regime the system reduces to the
well-known optomechanical case, with remarkably large coupling. Going beyond
that, we study the optically induced nonlinear classical dynamics of a
macrospin. In the fast cavity regime we obtain an effective equation of motion
for the spin and show that the light field induces a dissipative term
reminiscent of Gilbert damping. The induced dissipation coefficient however can
change sign on the Bloch sphere, giving rise to self-sustained oscillations.
When the full dynamics of the system is considered, the system can enter a
chaotic regime by successive period doubling of the oscillations.Comment: Extended version, as publishe
Magnon heralding in cavity optomagnonics
In the emerging field of cavity optomagnonics, photons are coupled coherently
to magnons in solid-state systems. These new systems are promising for
implementing hybrid quantum technologies. Being able to prepare Fock states in
such platforms is an essential step towards the implementation of quantum
information schemes. We propose a magnon-heralding protocol to generate a
magnon Fock state by detecting an optical cavity photon. Due to the
peculiarities of the optomagnonic coupling, the protocol involves two distinct
cavity photon modes. Solving the quantum Langevin equations of the coupled
system, we show that the temporal scale of the heralding is governed by the
magnon-photon cooperativity and derive the requirements for generating high
fidelity magnon Fock states. We show that the nonclassical character of the
heralded state, which is imprinted in the autocorrelation of an optical "read"
mode, is only limited by the magnon lifetime for small enough temperatures. We
address the detrimental effects of nonvacuum initial states, showing that high
fidelity Fock states can be achieved by actively cooling the system prior to
the protocol.Comment: 17 pages, 14 figures. Correction of typos, version as publishe
Quantum thermodynamics of the driven resonant level model
We present a consistent thermodynamic theory for the resonant level model in
the wide band limit, whose level energy is driven slowly by an external force.
The problem of defining 'system' and 'bath' in the strong coupling regime is
circumvented by considering as the 'system' everything that is influenced by
the externally driven level. The thermodynamic functions that are obtained to
first order beyond the quasistatic limit fulfill the first and second law with
a positive entropy production, successfully connect to the forces experienced
by the external driving, and reproduce the correct weak coupling limit of
stochastic thermodynamics.Comment: Final version as publishe
Scattering theory of adiabatic reaction forces due to out-of-equilibrium quantum environments
The Landauer-Buettiker theory of mesoscopic conductors was recently extended
to nanoelectromechanical systems. In this extension, the adiabatic reaction
forces exerted by the electronic degrees of freedom on the mechanical modes
were expressed in terms of the electronic S-matrix and its first non-adiabatic
correction, the A-matrix. Here, we provide a more natural and efficient
derivation of these results within the setting and solely with the methods of
scattering theory. Our derivation is based on a generic model of a slow
classical degree of freedom coupled to a quantum-mechanical scattering system,
extending previous work on adiabatic reaction forces for closed quantum
systems.Comment: Minor typos fixed, published versio
Scattering theory of current-induced forces in mesoscopic systems
We develop a scattering theory of current-induced forces exerted by the
conduction electrons of a general mesoscopic conductor on slow "mechanical"
degrees of freedom. Our theory describes the current-induced forces both in and
out of equilibrium in terms of the scattering matrix of the phase-coherent
conductor. Under general nonequilibrium conditions, the resulting mechanical
Langevin dynamics is subject to both non-conservative and velocity-dependent
Lorentz-like forces, in addition to (possibly negative) friction. We illustrate
our results with a two-mode model inspired by hydrogen molecules in a break
junction which exhibits limit-cycle dynamics of the mechanical modes.Comment: 4+ pages, 1 figure; v2: minor modification
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