Prof Vlatko Vedral FInstP

Abstract:

We present various schemes for measuring the quantum state of a single mode of the electromagnetic field. These involve measuring the photon statistics for the mode before and after an interaction with either one or two two-level atoms. The photon statistics conditioned on the final state of the atoms, for two choices of the initial set of atomic states, along with the initial photon statistics, may be used to calculate the complete quantum state in a simple manner. Alternatively, when one atom is used, two unconditioned sets of photon statistics, each after interaction with a single atom in different initial states, along with the initial photon statistics may be used to calculate the initial state in a simple manner. When the cavity is allowed to interact with just one atom, only pure cavity states which do not contain zeros in the photon distribution may be reconstructed. When two atoms are used we may reconstruct pure states which do not contain adjacent zeros in the photon distribution. Coherent states and number states are among those that may be measured with one-atom interaction, and squeezed states and 'Schrödinger cats' are among those that may be measured with a two-atom interaction.

Abstract:

The form of Bell's inequality does not sufficiently measure quantum correlations because there are states that do not violate the inequality, but can be purified by a combination of local interactions and classical communications to yield a state that does not violate the inequality. It is possible to say that a quantum state is entangled or not, but the amount of entanglement cannot easily be determined for general mixed states. Any measure of entanglement has to satisfy specified conditions on the basis of quantum optics before constructing a whole class of good entanglement measures that are geometrically intuitive.