Broadband teleportation

Quantum teleportation of an unknown broadband electromagnetic field is investigated. The continuousvariable teleportation protocol by Braunstein and Kimble @Phys. Rev. Lett. 80, 869 ~1998!# for teleporting the quantum state of a single mode of the electromagnetic field is generalized for the case of a multimode field with finite bandwith. We discuss criteria for continuous-variable teleportation with various sets of input states and apply them to the teleportation of broadband fields. We first consider as a set of input fields ~from which an independent state preparer draws the inputs to be teleported! arbitrary pure Gaussian states with unknown coherent amplitude ~squeezed or coherent states!. This set of input states, further restricted to an alphabet of coherent states, was used in the experiment by Furusawa et al. @Science 282, 706 ~1998!#. It requires unit-gain teleportation for optimizing the teleportation fidelity. In our broadband scheme, the excess noise added through unit-gain teleportation due to the finite degree of the squeezed-state entanglement is just twice the ~entanglement! source’s squeezing spectrum for its ‘‘quiet quadrature.’’ The teleportation of one half of an entangled state ~two-mode squeezed vacuum state!, i.e., ‘‘entanglement swapping,’’ and its verification are optimized under a certain nonunit gain condition. We will also give a broadband description of this continuous-variable entanglement swapping based on the single-mode scheme by van Loock and Braunstein @Phys. Rev. A 61, 10 302 ~2000!#.