  We develop a non-conventional description of the vacuum energy in quantum
field theory in terms of quantum entropy. Precisely, we show that the vacuum
energy of any non-interacting quantum field at zero temperature is proportional
to the quantum entropy of the qubit degrees of freedom associated with virtual
fluctuations. We prove this for fermions first, and then extend the derivation
to quanta of any spin. Finally, we use these results to obtain the first law of
thermodynamics for a non-interacting quantum vacuum at zero temperature.
