It is known that classical information can be completely hidden from asubsystem in two distinct ways. The information may be moved to another location, orit may be encoded as correlations between a pair of subsystems. Most generally, theinformation will be hidden as a combination of these two. Can we hide quantuminformation in the same way? Consider a physical process which maps an arbitraryquantum state to a fixed state. If the final state is independent of the input, thenwe prove that this missing, or hidden, information is wholly encoded in theremainder of Hilbert space with no information stored in the correlations betweenthe two subsystems. We call this the “no-hiding theorem”. Thus, unlike classicalinformation, quantum mechanics allows only one way to completely hide an arbitraryquantum state from one of its subsystems, i.e., by moving it to the remainingsubsystems. We will discuss various applications of this theorem in quantumteleportation, thermalisation and finally in black hole information loss paradox.
