dmitry | Jan. 5, 2021, 9:47 a.m.
Continuing on with the CERN video lecture series, this post covers the third lecture in their series.
Qubits in an entangled state allow for qubits to be sent (from Alice to Bob) even if we don't know what the qubit's state is and there is no quantum channel available (quantum teleportation of information). Alice can do this if she shares an entangled state qubit pair with Bob by first applying a CNOT gate and then an H gate to the qubit (this is a third entangled qubit, separate from a pair of classical bits that are also needed) she wants teleported. The teleportation is still done over a classical communication system. Quantum teleportation can also be extended to entangle two qubits between two people who do not communicate (Bob and Charlie). This is done by having one of the two people share an entangled qubit pair with a third party (Alice). If the third party then teleports her half of the shared qubit to the other of the two original people, the two original parties now have entangled qubits.
Superdense coding is the principle of sending classical bits between two parties that preshare an entangled Bell pair. One of the parties can then apply specific quantum gates to their half of the entangled pair before sending the qubit to the other party which is then measured (making the state collapse to the correct bit).
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