Entanglement Swapping

Entanglement Swapping

Notice that in the quantum teleportation scheme, we didn’t prohibit the target qubit from being entangled itself. If we teleport that qubit, then the entanglement will be “swapped” to the qubit at the destination. I’ve drawn a few illustrations to demonstrate this concept. First, here’s an illustration of how the conventional quantum teleportation circuit operates:

The left side depicts the simple case. Alice and Bob share an entangled pair a and b, corresponding to their names. After the CNOT and Hadamard gates, Alice entangles her target qubit ψ with a. Finally, after she measures her qubits, she knows which classical bit c to send to Bob so that he can transform his qubit into ψ.

The right side depicts the case when the target qubit is entangled with another qubit, say φ. Note that the procedure remains exactly the same. Alice doesn’t need to interact with φ, and she can still teleport ψ, which results in Bob owning a qubit that’s entangled with φ, even though neither person interacted with φ. Isn’t that bizarre?!

Finally, I’ll illustrate complete entanglement swapping. But beforehand, since we will need to keep track of a few more characters, here’s a brief introduction:

And here’s an illustration of complete entanglement swapping:

The setup consists of Alice and Bob sharing an entangled pair a and b. Likewise, Charles and Delilah share an entangled pair c and d. Additionally, Alice and Charles own arbitrary entangled qubits ψ and φ respectively. After a sequence of 2 teleportations, ψ and φ can be teleported to Bob and Delilah, who never interact, yet now share an arbitrary pair of entangled qubits.

Fortunately, all of these results have been experimentally reproduced!