  In certain special circumstances, such as in the vicinity of a black hole or
in a uniformly accelerating frame, vacuum fluctuations appear to give rise to a
finite-temperature environment. This effect, currently without experimental
confirmation, can be interpreted as a manifestation of quantum entanglement
after tracing out vacuum modes in an unobserved region. In this work, we
identify a class of experimentally accessible quantum systems where thermal
density matrices emerge from vacuum entanglement. We show that reduced density
matrices of lower-dimensional subsystems embedded in $D$-dimensional gapped
Dirac fermion vacuum, either on a lattice or continuum, have a thermal form
with respect to a lower-dimensional Dirac Hamiltonian. Strikingly, we show that
vacuum entanglement can even conspire to make a subsystem of a gapped system at
zero temperature appear as a hot gapless system. We propose concrete
experiments in cold atom quantum simulators to observe the vacuum entanglement
induced thermal states.
