  Entanglement is the key resource for quantum technologies and is at the root
of exciting many-body phenomena. However, quantifying the entanglement between
two parts of a real-world quantum system is challenging when it interacts with
its environment, as the latter mixes cross-boundary classical with quantum
correlations. Here, we efficiently quantify quantum correlations in such
realistic open systems using the operator space entanglement spectrum of a
mixed state. If the system possesses a fixed charge, we show that a subset of
the spectral values encode coherence between different cross-boundary charge
configurations. The sum over these values, which we call "configuration
coherence", can be used as a quantifier for cross-boundary coherence.
Crucially, we prove that for purity non-increasing maps, e.g., Lindblad-type
evolutions with Hermitian jump operators, the configuration coherence is an
entanglement measure. Moreover, it can be efficiently computed using a tensor
network representation of the state's density matrix. We showcase the
configuration coherence for spinless particles moving on a chain in presence of
dephasing. Our approach can quantify coherence and entanglement in a broad
range of systems and motivates efficient entanglement detection.
