Enhancement of magnon-magnon entanglement inside a cavity

Abstract

Magnon-photon coupling has been experimentally realized inside a cavity and the emerging field known as cavity spintronics has attracted significant attention for its potential docking with quantum information science. However, one seldom knows whether this coupling implies an entanglement state among magnons and photons or not, which is crucial for its usage in quantum information.Here we study the entanglement properties among magnons and photons in an antiferromagnet-light system and find that the entanglement between magnon and photon is nearly zero while the magnonmagnon entanglement is very strong and it can be even further enhanced through the coupling with the cavity photons. The maximum enhancement occurs when the antiferromagnet reaches resonant with the light. The essential physics can be well understood within the picture of cavity induced cooling of magnon-magnon state near its joint vacuum with stronger entanglement. Our results are significant to extend the cavity spintronics to quantum manipulation and further provide an alternate to manipulate the deep strong correlations of continuous variable entanglement with a generic stable condition and easy tunability.Introduction.-Antiferromagnetic (AFM) spintronics emerges for its better stability and fast dynamics over its ferromagnetic counterpart [1,2]. Of particular interest are antiferromagnetic spin waves (magnons) that show much richer physics than ferromagnets, such as the spin pumping at the interface of an AFM/normal metal bilayer [3], magnon spin current enhancement through an AFM layer [4][5][6][7], long-distance magnon transport [8][9][10], and magnon-driven magnetic structure motion [11][12][13]. It has recently been theoretically proposed [14] and experimentally verified [15] that antiferromagnetic magnons can be strongly coupled to the light by placing an AFM element into a cavity, which serves a promising candidate to realize coherent information transfer between magnons and photons for its superb stability and tunability of magnons properties through external knobs. One promising application of the hybridized magnonphoton polariton is to connect it with the quantum information science [16,17], similar to the cavity electrodynamics that rose 20 years ago and has found its role in the implementation of quantum qubits and quantum computing circuits [18]. To push the development of cavity spintronics along this line, it is crucial to have a clear insight into the quantum correlations among magnons and photons, especially their entanglement properties, which are central resources for quantum computing and quantum technologies.

Enhanced entanglement and asymmetric EPR steering between magnons

“…The enhancement of magnon-magnon entanglement can be well understood through cavity cooling of the Bogoliubov modes toward their joint ground state [39]. We can reformulate the magnon eigenmodes as α = S (r)aS † (r), β = S (r)bS † (r), where S (r) = exp[r(ab − a † b † )] is a unitary twomode squeezing operator.…”

“…Hybrid magnonical systems have become a promising platform in the study of the macroscopic quantum phenomenon due to the large size of the magnon systems. Recently, several schemes have been proposed to generate different types of quantum correlations between magnons and photons or phonons [36][37][38][39]. However, whether asymmetric EPR steering can be achieved in such hybrid systems is still unknown.…”

Reconfigurable magnonic mode-hybridisation and spectral control in a bicomponent artificial spin ice

“…3 c, g, l and p), where reversed and unreversed wide-bar modes approach a single frequency they do not overlap. The modes instead bend away from a Kittel-like form as they approach, leaving an anticrossing gap 9 , 31 , 51 – 53 which has been predicted to occur in ASI due to a microstate-dependent band structure 31 . Figure 4 shows the anticrossing region in type 3 microstates.…”

Magnon blockade in a hybrid ferromagnet-superconductor quantum system

“…Magnon, regarded as the quantized spin wave, is the dynamic intrinsic excitation of the magnetic ordered body [10], which has been the subject of extensive investigations in many fields of research, for instance, cavity optomagnonics [11][12][13], hybrid ferromagnetic-superconducting system [14][15][16][17], and Dirac or Weyl magnons in topological insulators [18][19][20]. Many novel phenomena and important applications, ranging from optical cooling of magnon [21] and magnon-induced high-order sideband generation [22][23][24] to magnon gradient memory [25,26] and observation of topological magnon insulator states [27] have been theoretically or experimentally verified.Recently, the investigation of the quantum characteristics of the magnon-polariton system has fascinated widespread concern and made substantial progress [28][29][30][31][32]. For example, the generations of magnon-photon-phonon entanglement [28] and squeezed magnon-phonon states [31] from the cavity magnomechanics, as well as the steady Bell state generation via magnon-photon coupling [32] have been proposed.…”

“…Recently, the investigation of the quantum characteristics of the magnon-polariton system has fascinated widespread concern and made substantial progress [28][29][30][31][32]. For example, the generations of magnon-photon-phonon entanglement [28] and squeezed magnon-phonon states [31] from the cavity magnomechanics, as well as the steady Bell state generation via magnon-photon coupling [32] have been proposed.…”