  In this Letter we present a new quantity that shows whether two general qubit
systems are entangled, which we call harmony. It captures the notion of
separability and maximal entanglement. It is also shown that harmony is
monogamous for 3-qubit states. Thus, harmony serves as a new entanglement
measure. In addition, since it is written as a simple function of the density
operator, it is in practice easier to compute than other previously known
measures.
