Quantum Optics Group
Norman Bridge Laboratory of Physics
California Institute of Technology
A great article explaining this experiment can be found at ABC news. See the October 23, 1998 issue of Science magazine for the article in full, or read the Caltech press release for a summary.
In quantum teleportation, an unknown quantum state is faithfully transferred from a sender (Alice) to a receiver (Bob). To perform the teleportation, Alice and Bob must have a classical communication channel and must also share quantum entanglement -- in the protocol we employ*, each possesses one half of a two-particle entangled state. Alice makes an appropriate projective measurement (Bell measurement) of the unknown state together with her component of the shared entangled state. The result of this measurement is a random piece of classical information which Alice sends to Bob over their classical communication channel. Bob uses this information to choose a unitary transformation which he performs on his component of the shared entangled state, thus transforming it into an output state identical to the original (unknown) input. Notice that the input state is destroyed by Alice's projective measurement, so that teleportation does not result in "cloning" of a quantum state.
(*Teleportation protocol of C. H. Bennett et al., PRL 70, 1895 (1993).)
Teleportation with Squeezed Light
We have implemented quantum teleportation with light beams serving as both the entangled pair and the input (and output) state. Squeezed light is used to generate the entangled (EPR) beams which are sent to Alice and Bob. A third beam, the input, is a coherent state of unknown complex amplitude. This state is teleported to Bob with a high fidelity only achievable via the use of quantum entanglement.
Entangled EPR beams are generated by combining two beams of squeezed light at a 50/50 beamsplitter. EPR beam 1 propagates to Alice's sending station, where it is combined at a 50/50 beamsplitter with the unknown input state, in this case a coherent state of unknown complex amplitude. Alice uses two sets of balanced homodyne detectors to make a Bell-state measurement on the amplitudes of the combined state. Because of the entanglement between the EPR beams, Alice's detection collapses Bob's field (EPR beam 2) into a state 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.
Fidelity (Quantum vs. Classical?)
Quantum teleportation 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. Note that the fidelity is an average over all input states and so measures the ability to transfer an arbitrary, unknown superposition from Alice to Bob.