p2

conditioned on Alice’s measurement outcome. After receiving the classical result from Alice,
Bob is able to construct the teleported state via a simple phase-space displacement of the EPR
field 2. Quantum teleportation in this scheme is theoretically perfect, yielding an output state
which equals the input with a fidelity F = 1. In practice, fidelities less than one are realized due
to imperfections in the EPR pair, Alice’s Bell measurement, and Bob’s unitary transformation.
By contrast, a sender and receiver who share only a classical communication channel cannot hope
to transfer an arbitrary quantum state with a fidelity of one. For coherent states, the classical
teleportation limit is F = 0.5, while for light polarization states it is F = 0.67. The quantum nature
of the teleportation achieved in this case is demonstrated by the experimentally determined
fidelity of F = 0.58, greater than the classical limit of 0.5 for coherent states. The fidelity is an
average over all input states and so measures the ability to transfer an arbitrary, unknown
superposition from Alice to Bob. This technique achieves the teleportation of continuous
quantum state variables, as opposed to the discrete quantum state variables used in the Bennett et
al. (1993) teleportation protocol and its variants. The teleportation of a squeezed state of light
from one beam of light to another demonstrates the teleportation of a continuous feature (of light)
that comes from the superpositions of an infinite number of basic states of the electromagnetic
field, such as those found in squeezed states. This line of research also involves the experimental
demonstration of the mapping of quantum states from photonic to atomic media via entanglement
and teleportation. Hald et al. (1999) reported on the experimental observation of a spin-squeezed
macroscopic ensemble of 10 7 cold atoms, whereby the ensemble is generated via quantum state
entanglement/teleportation from non-classical light to atoms.
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