  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 magnon-magnon 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.
