37 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 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
Light propagation and magnon-photon coupling in optically dispersive magnetic media
Achieving strong coupling between light and matter excitations in hybrid systems is a benchmark for the implementation of quantum technologies. We recently proposed (Bittencourt, Liberal, and Viola-Kusminskiy, arXiv:2110.02984) that strong single-particle coupling between magnons and light can be realized in a magnetized epsilon-near-zero (ENZ) medium, in which magneto-optical effects are enhanced. Here we present a detailed derivation of the magnon-photon coupling Hamiltonian in dispersive media both for degenerate and nondegenerate optical modes, and show the enhancement of the coupling near the ENZ frequency. Moreover, we show that the coupling of magnons to plane-wave nondegenerate Voigt modes vanishes at specific frequencies due to polarization selection rules tuned by dispersion. Finally, we present specific results using a Lorentz dispersion model. Our results pave the way for the design of dispersive optomagnonic systems, providing a general theoretical framework for describing and engineering ENZ-based optomagnonic systems.Open access publication funded by the Max Planck Society. V.A.S.V.B. and S.V.K. acknowledge financial support from the Max Planck Society. I.L. acknowledges support from ERC Starting Grant No. 948504, Ramón y Cajal Fellowship No. RYC2018-024123-I, and Project No. RTI2018-093714-301J-I00 sponsored by MCIU/AEI/FEDER/UE
Langevin dynamics of a heavy particle and orthogonality effects
The dynamics of a classical heavy particle moving in a quantum environment is determined by a Langevin equation which encapsulates the effect of the environment-induced reaction forces on the particle. For an open quantum system, these include a Born-Oppenheimer force, a dissipative force, and a stochastic force due to shot and thermal noise. Recently, it was shown that these forces can be expressed in terms of the scattering matrix of the system by considering the classical heavy particle as a time-dependent scattering center, allowing to demonstrate interesting features of these forces when the system is driven out of equilibrium. At the same time, it is well known that small changes in a scattering potential can have a profound impact on a fermionic system due to the Anderson orthogonality catastrophe. In this work, by calculating the Loschmidt echo, we relate Anderson orthogonality effects with the mesoscopic reaction forces for an environment that can be taken out of equilibrium. In particular, we show how the decay of the Loschmidt echo is characterized by fluctuations and dissipation in the system and discuss different quench protocols
Engineering Entangled Coherent States of Magnons and Phonons via a Transmon Qubit
We propose a scheme for generating and controlling entangled coherent states
(ECS) of magnons, i.e. the quanta of the collective spin excitations in
magnetic systems, or phonons in mechanical resonators. The proposed hybrid
circuit architecture comprises a superconducting transmon qubit coupled to a
pair of magnonic Yttrium Iron Garnet (YIG) spherical resonators or mechanical
beam resonators via flux-mediated interactions. Specifically, the coupling
results from the magnetic/mechanical quantum fluctuations modulating the qubit
inductor, formed by a superconducting quantum interference device (SQUID). We
show that the resulting radiation-pressure interaction of the qubit with each
mode, can be employed to generate maximally-entangled states of magnons or
phonons. In addition, we numerically demonstrate a protocol for the preparation
of magnonic and mechanical Bell states with high fidelity including realistic
dissipation mechanisms. Furthermore, we have devised a scheme for reading out
the prepared states using standard qubit control and resonator field
displacements. Our work demonstrates an alternative platform for quantum
information using ECS in hybrid magnonic and mechanical quantum networks
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