45 research outputs found
Spin current cross-correlations as a probe of magnon coherence
Motivated by the important role of the normalized second order coherence
function, often called , in the field of quantum optics, we propose a
method to determine magnon coherence in solid-state devices. Namely, we show
that the cross-correlations of pure spin-currents injected by a ferromagnet
into two metal leads, normalized by their dc value, replicate the behavior of
when magnons are driven far from equilibrium. We consider two
scenarios: driving by ferromagnetic resonance, which leads to the coherent
occupation of a single mode, and driving by heating of the magnons, which leads
to an excess of incoherent magnons. We find an enhanced normalized
cross-correlation in the latter case, thereby demonstrating bunching of
nonequilibrium thermal magnons due to their bosonic statistics. Our results
contribute to the burgeoning field of quantum magnonics, which seeks to explore
and exploit the quantum nature of magnons
Magnon-Mediated Indirect Exciton Condensation through Antiferromagnetic Insulators
Electrons and holes residing on the opposing sides of an insulating barrier
and experiencing an attractive Coulomb interaction can spontaneously form a
coherent state known as an indirect exciton condensate. We study a trilayer
system where the barrier is an antiferromagnetic insulator. The electrons and
holes here additionally interact via interfacial coupling to the
antiferromagnetic magnons. We show that by employing magnetically uncompensated
interfaces, we can design the magnon-mediated interaction to be attractive or
repulsive by varying the thickness of the antiferromagnetic insulator by a
single atomic layer. We derive an analytical expression for the critical
temperature of the indirect exciton condensation. Within our model,
anisotropy is found to be crucial for achieving a finite , which increases
with the strength of the exchange interaction in the antiferromagnetic bulk.
For realistic material parameters, we estimate to be around 7 K, the same
order of magnitude as the current experimentally achievable exciton
condensation where the attraction is solely due to the Coulomb interaction. The
magnon-mediated interaction is expected to cooperate with the Coulomb
interaction for condensation of indirect excitons, thereby providing a means to
significantly increase the exciton condensation temperature range.Comment: 7+13 Pages, 2+1 figures. Added discussion of retardation effects.
Accepted for publication in Phys. Rev. Let
Real-time privacy preserving framework for Covid-19 contact tracing
The recent unprecedented threat from COVID-19 and past epidemics, such as SARS, AIDS, and Ebola, has affected millions of people in multiple countries. Countries have shut their borders, and their nationals have been advised to self-quarantine. The variety of responses to the pandemic has given rise to data privacy concerns. Infection prevention and control strategies as well as disease control measures, especially real-time contact tracing for COVID-19, require the identification of people exposed to COVID-19. Such tracing frameworks use mobile apps and geolocations to trace individuals. However, while the motive may be well intended, the limitations and security issues associated with using such a technology are a serious cause of concern. There are growing concerns regarding the privacy of an individual\u27s location and personal identifiable information (PII) being shared with governments and/or health agencies. This study presents a real-time, trust-based contact-tracing framework that operates without the use of an individual\u27s PII, location sensing, or gathering GPS logs. The focus of the proposed contact tracing framework is to ensure real-time privacy using the Bluetooth range of individuals to determine others within the range. The research validates the trust-based framework using Bluetooth as practical and privacy-aware. Using our proposed methodology, personal information, health logs, and location data will be secure and not abused. This research analyzes 100,000 tracing dataset records from 150 mobile devices to identify infected users and active users
Master equation approach to magnon relaxation and dephasing
There has been a recent upsurge of interest in the quantum properties of
magnons for quantum information processing. An important issue is to examine
the stability of quantum states of magnons against various relaxation and
dephasing channels. Since the interaction of magnons in magnetic systems may
fall in the ultra-strong and even deep-strong coupling regimes, the relaxation
process of magnon states is quite different from the more common quantum
optical systems. Here we study the relaxation and dephasing of magnons based on
the Lindblad formalism and derive a generalized master equation that describes
the quantum dynamics of magnons. Employing this master equation, we identify
two distinct dissipation channels for squeezed magnons, i.e., the local
dissipation and collective dissipation, which play a role for both ferromagnets
and antiferromagnets. The local dissipation is caused by the independent
exchange of angular momentum between the magnonic system and the environment,
while the collective dissipation is dressed by the parametric interactions of
magnons and it enhances the quantumness and thermal stability of squeezed
magnons. Further, we show how this formalism can be applied to study the pure
dephasing of magnons caused by four-magnon scattering and magnon-phonon
interactions. Our results provide the theoretical tools to study the
decoherence of magnons within a full quantum-mechanical framework and further
benefit the use of quantum states of magnons for information processing.Comment: 13 pages, 3 figure
Electrically switchable entanglement channel in van der Waals magnets
Two-dimensional layered van der Waals (vdW) magnets demonstrate their potential to allow the study
of both fundamental and applied physics due to their remarkable electronic properties. However, the connection of vdW magnets to spintronics and quantum information science is not clear. In particular, it
remains elusive whether there are interesting magnetic phenomena belonging only to vdW magnets but
absent in widely studied crystalline magnets. Here, we consider the quantum correlations of magnons
in a layered vdW magnet and identify an entanglement channel of magnons across the magnetic layers,
which can be effectively tuned and even deterministically switched on and off by both magnetic and electric means. This is a unique feature of vdW magnets, in which the underlying physics is well understood
in terms of the competing roles of exchange and anisotropy fields that contribute to magnon excitation.
Furthermore, we show that such a tunable entanglement channel can mediate the electrically controllable
entanglement of two distant qubits, which also provides a protocol to indirectly measure the entanglement of magnons. Our findings provide an avenue to electrically manipulate qubits and further open up
opportunities to utilize vdW magnets for quantum information scienc
Quantum magnonics: when magnon spintronics meets quantum information science
Spintronics and quantum information science are two promising candidates for
innovating information processing technologies. The combination of these two
fields enables us to build solid-state platforms for studying quantum phenomena
and for realizing multi-functional quantum tasks. For a long time, however, the
intersection of these two fields was limited. This situation has changed
significantly over the last few years because of the remarkable progress in
coding and processing information using magnons. On the other hand, significant
advances in understanding the entanglement of quasi-particles and in designing
high-quality qubits and photonic cavities for quantum information processing
provide physical platforms to integrate magnons with quantum systems. From
these endeavours, the highly interdisciplinary field of quantum magnonics
emerges, which combines spintronics, quantum optics and quantum information
science.Here, we give an overview of the recent developments concerning the
quantum states of magnons and their hybridization with mature quantum
platforms. First, we review the basic concepts of magnons and quantum
entanglement and discuss the generation and manipulation of quantum states of
magnons, such as single-magnon states, squeezed states and quantum many-body
states including Bose-Einstein condensation and the resulting spin
superfluidity. We discuss how magnonic systems can be integrated and entangled
with quantum platforms including cavity photons, superconducting qubits,
nitrogen-vacancy centers, and phonons for coherent information transfer and
collaborative information processing. The implications of these hybrid quantum
systems for non-Hermitian physics and parity-time symmetry are highlighted,
together with applications in quantum memories and high-precision measurements.
Finally, we present an outlook on the opportunities in quantum magnonics.Comment: 93 pages, 35 figures, Physics Reports (in press
Antiferromagnetic magnons as highly squeezed Fock states underlying quantum correlations
Employing the concept of two-mode squeezed states from quantum optics, we
demonstrate a revealing physical picture for the antiferromagnetic ground state
and excitations. Superimposed on a N{\'e}el ordered configuration, a spin-flip
restricted to one of the sublattices is called a sublattice-magnon. We show
that an antiferromagnetic spin-up magnon is comprised by a quantum
superposition of states with spin-up and spin-down
sublattice-magnons, and is thus an enormous excitation despite its unit net
spin. Consequently, its large sublattice-spin can amplify its coupling to other
excitations. Employing von Neumann entropy as a measure, we show that the
antiferromagnetic eigenmodes manifest a high degree of entanglement between the
two sublattices, thereby establishing antiferromagnets as reservoirs for strong
quantum correlations. Based on these novel insights, we outline strategies for
exploiting the strong quantum character of antiferromagetic (squeezed-)magnons
and give an intuitive explanation for recent experimental and theoretical
findings in antiferromagnetic magnon spintronics
Complete suppression and N\'eel triplets-mediated exchange in antiferromagnet-superconductor-antiferromagnet trilayers
An antiferromagnetic insulator (AFI) bearing a compensated interface to an
adjacent conventional superconductor (S) has recently been predicted to
generate N\'eel triplet Cooper pairs, whose amplitude alternates sign in space.
Here, we theoretically demonstrate that such N\'eel triplets enable control of
the superconducting critical temperature in an S layer via the angle between
the N\'eel vectors of two enclosing AFI layers. This angle dependence changes
sign with the number of S monolayers providing a distinct signature of the
N\'eel triplets. Furthermore, we show that the latter mediate a similarly
distinct exchange interaction between the two AFIs' N\'eel vectors.Comment: 7 pages, 4 figure
Pure dephasing of magnonic quantum states
For a wide range of nonclassical magnonic states that have been proposed and demonstrated recently, a new time scale besides the magnon lifetime - the magnon dephasing time - becomes important, but this time scale is rarely studied. Considering exchange interaction and spin-phonon coupling, we evaluate the pure magnon dephasing time and find it to be smaller than the magnon lifetime at temperatures of a few kelvins. By examining a magnonic cat state as an example, we show how pure dephasing of magnons destroys and limits the survival of quantum superpositions. Thus it will be critical to perform quantum operations within the pure dephasing time. We further derive the master equation for the density matrix describing such magnonic quantum states taking into account the role of pure dephasing. This methodology can be generalized to include additional dephasing channels that experiments are likely to encounter in the future. Our findings enable one to design and manipulate robust quantum states of magnons for information processing
Ubiquitous Superconducting Diode Effect in Superconductor Thin Films
The macroscopic coherence in superconductors supports dissipationless
supercurrents which could play a central role in emerging quantum technologies.
Accomplishing unequal supercurrents in the forward and backward directions
would enable unprecedented functionalities. This nonreciprocity of critical
supercurrents is called superconducting (SC) diode effect. We demonstrate
strong SC diode effect in conventional SC thin films, such as niobium and
vanadium, employing external magnetic fields as small as 1 Oe. Interfacing the
SC layer with a ferromagnetic semiconductor EuS, we further accomplish
non-volatile SC diode effect reaching a giant efficiency of 65%. By careful
control experiments and theoretical modeling, we demonstrate that the critical
supercurrent nonreciprocity in SC thin films could be easily accomplished with
asymmetrical vortex edge/surface barriers and the universal Meissner screening
current governing the critical currents. Our engineering of the SC diode effect
in simple systems opens door for novel technologies. Meanwhile, we reveal the
ubiquity of Meissner screening effect induced SC diode effect in
superconducting films, which should be eliminated with great care in the search
of exotic superconducting states harboring finite-momentum Cooper pairing.Comment: 27 pages, 16 figure