5,191 research outputs found
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
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
Description of Quantum Entanglement with Nilpotent Polynomials
We propose a general method for introducing extensive characteristics of
quantum entanglement. The method relies on polynomials of nilpotent raising
operators that create entangled states acting on a reference vacuum state. By
introducing the notion of tanglemeter, the logarithm of the state vector
represented in a special canonical form and expressed via polynomials of
nilpotent variables, we show how this description provides a simple criterion
for entanglement as well as a universal method for constructing the invariants
characterizing entanglement. We compare the existing measures and classes of
entanglement with those emerging from our approach. We derive the equation of
motion for the tanglemeter and, in representative examples of up to four-qubit
systems, show how the known classes appear in a natural way within our
framework. We extend our approach to qutrits and higher-dimensional systems,
and make contact with the recently introduced idea of generalized entanglement.
Possible future developments and applications of the method are discussed.Comment: 40 pages, 7 figures, 1 table, submitted for publication. v2: section
II.E has been changed and the Appendix on "Four qubit sl-entanglement
measure" has been removed. There are changes in the notation of section IV.
Typos and language mistakes has been corrected. A figure has been added and a
figure has been replaced. The references have been update
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
Thermopower in the Coulomb blockade regime for Laughlin quantum dots
Using the conformal field theory partition function of a Coulomb-blockaded
quantum dot, constructed by two quantum point contacts in a Laughlin quantum
Hall bar, we derive the finite-temperature thermodynamic expression for the
thermopower in the linear-response regime. The low-temperature results for the
thermopower are compared to those for the conductance and their capability to
reveal the structure of the single-electron spectrum in the quantum dot is
analyzed.Comment: 11 pages, 3 figures, Proceedings of the 10-th International Workshop
"Lie Theory and Its Applications in Physics", 17-23 June 2013, Varna,
Bulgari
Dynamical critical scaling and effective thermalization in quantum quenches: the role of the initial state
We explore the robustness of universal dynamical scaling behavior in a
quantum system near criticality with respect to initialization in a large class
of states with finite energy. By focusing on a homogeneous XY quantum spin
chain in a transverse field, we characterize the non-equilibrium response under
adiabatic and sudden quench processes originating from a pure as well as a
mixed excited initial state, and involving either a regular quantum critical or
a multicritical point. We find that the critical exponents of the ground-state
quantum phase transition can be encoded in the dynamical scaling exponents
despite the finite energy of the initial state. In particular, we identify
conditions on the initial distribution of quasi-particle excitation which
ensure Kibble-Zurek scaling to persist. The emergence of effective thermal
equilibrium behavior following a sudden quench towards criticality is also
investigated, with focus on the long-time dynamics of the quasi-particle
excitation. For a quench to a regular quantum critical point, this observable
is found to behave thermally provided that the system is prepared at
sufficiently high temperature, whereas thermalization fails to occur in
quenches taking the system towards a multi-critical point. We argue that the
observed lack of thermalization originates in this case in the asymmetry of the
impulse region that is also responsible for anomalous multicritical dynamical
scaling.Comment: 18 pages, 13 eps color figures, published versio
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