Polarization fluctuation as an entanglement measure for N-level systems
It is known that subsystem fluctuations are related to entanglement for any N-level system entangled with an environment. We define an observable wherein its fluctuations can be directly related to known measures such as the purity, Rényi-2 entropy, and concurrence. This quantity, which we call the Schmidt polarization fluctuation shows an exact relationship between fluctuations and entanglement that is simple to construct and is model-independent. It can also be considered as a proper entanglement measure and is experimentally accessible.