the paper is this one, I suppose
http://arxiv.org/abs/quant-ph/0302179

##
Advertising

it was expected that the vacuum behaves
as a noisy channel
http://arxiv.org/abs/quant-ph/0301065
and in general entanglements are sensible
to Lorentzian boosts (but not much)
http://arxiv.org/abs/quant-ph/0302095
http://arxiv.org/abs/quant-ph/0205179
and unforfutanely quantum entropy/information
is not a Lorentz covariant concept
http://arxiv.org/abs/quant-ph/0212023
bbbut, and this are the good news,
there are detailed papers (J. Hardy)
about the possibility of entanglement
between 'usual' quantum states and
'vacuum' states, like i.e.
psi = m |1,a>|0,b> + n |0,a>|1,b> + p |0,a>|0,b>
where |1,a> and |0,a>
are one-particle and vacuum state directed to a-lice
and |1,b> and |0,b> ... directed to b-ob