Introduction to Quantum Information: Part 5/7

SUPERDENSE CODING

Superdense coding is a very cool protocol! It allows to securely transfer two classical bits of information by only sending one qubit (even though, you need a previously shared entangled pair)!

It is quite a hard protocol to describe in detail, but I will try it using its quantum circuit

Let us suppose that Alice wants to send securely two classical bits to Bob.
The protocol reads as follow:

• two entangled qubits are generated and sent to both parties ("Prepare and Share a Bell Pair");
• Alice applies a NOT or a phase-flip gate to her qubit depending on the bits she wants to transmit ("Sender Encodes Bits"), then she sends her qubit;
• Bob receives the qubit from Alice, thus he applies a CNOT (with the qubit from Alice as control) and a Hadamard gate (only to the qubit from Alice);
• Bob performs a measurement on both qubits retrieving the classical bits Alice wanted to sent.

A comment: to transmit two bits, there is an exchange of a qubit and nothing else, no communication of any sort.
Moreover, it is a secure protocol because if anybody else would intercept Alice's qubit, this malicious actor could not retrieve enough information to know the message. Yet Bob would not receive it, too, but the secrecy of the message is not compromised.

In a blockchain perspective, it could be used to transmit information (keys, transactions, etc.) without an actual message to be eavesdropped. Nonetheless, there are even more sophisticated algorithms and protocols to achieve this result. In this context, superdense coding can be considered a starting point.

That's all for today!
Stay tuned and see you in the next post! We will meet with the most groundbreaking quantum algorithm: SHOR'S ALGORITHM!

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