# Is prior information lost during/after entanglement?

1. Jan 3, 2015

### sanpkl

During entanglement photons get into an indeterminate state. The outcome of the photon state, on measurement, would now be random.

Does this mean that, some of the, information the striking photon was carrying prior to transferring its energy to two entangled photons irretrievably lost? Even in principle

Last edited: Jan 3, 2015
2. Jan 3, 2015

### vanhees71

No! If you have an entangled state, you have maximal possible knowledge about the entangled photons. According to quantum theory there's no "sharper" determination of a quantum system's (in this case two photons) state than a pure state (represented by a projector as statistical operator or equivalently the corresponding ray in Hilbert space).

In addition, usually the photons are created in an entangled state, i.e., they where never there before, and thus there cannot get lost "prior information".