  We introduce the idea that the knowable quantum reality depends not only on
the state but also on measurements. Mathematically, we map the states from the
ordinary Hilbert space into new states in what we call the measurement space.
The state vectors in the measurement space contain only the information
accessible with a particular set of measurements. The space can be used to find
the operational equivalent of any information theoretic quantity. We use the
formalism to define the operationally meaningful entanglement, explore its
properties and apply it to mode entanglement. There we find that a
mode-entangled state of a single particle has no operationally useful
entanglement unless additional degrees of freedom are introduced to the system.
