ABSTRACT:
Quantum teleportation of an unknown broadband electromagnetic field is
investigated. The continuous-variable 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**, 10302 (2000)].