# Does entanglement allow enhanced communication efficiency?

From what I gather, communication is not possible with quantum mechanics. With the experiment on teleportation, entanglement is referred to as coordination and not communication. However, my belief is that communication is taking place. For example, given if a qubit has more information than a bit, this will mean that after Alice entangles/sends the qubit and prior to Bob measuring it, information is communicated. Am I missing something?

Yes. I have studied superdense coding. I also understand about the 2 classical bits. Let me add clarity to my question. Alice has a qubit which is sent to Bob. The qubit is not a regular bit and it contains more information than a bit. So, at the end of the day, Bob ends up receiving a qubit. And what I was saying in my original question, let's assume Bob does not read it but rather uses this qubit in a circuit. Hence, Bob is able to harness the information of a qubit which contains more information than regular classical bits. Hence, I see that communication of information is taking place. Is there something I am missing and/or can I clarify further my question. Thank you!

• Have you studied superdense coding? Alice and Bob preshare entanglement; Alice sends a qubit to Bob (one particle), and Bob performs a joint measurement of the two qubits (his and hers) to get two classical bits of info. Commented Jul 16, 2020 at 22:18
• Thank you for my response. Can you please read the added clarification I added an answer to my question.I don;t know how to clean this up.
– Jose
Commented Jul 17, 2020 at 21:22

First, about teleportation, you say that you think quantum communication takes place in the protocol, but it doesn't. They only share an EPR pair they created together, hence the coordination and after, what Alice sends to Bob when communication takes place are classical bits, she sends the measured bits of the 2 qubits she has, so the only communication we have here is classical.

Next, even if Bob has a qubit sent by Alice, even though he technically has more information than with a classical bit, he cannot access it, because the only way to do so is to measure the qubit and by doing that you collapse the qubit over one state and you lose all the other information that were previously in the qubit. Doing operations over the qubit will not change this fact, because you won't know what qubit you were working with.

Is this clear enough? Does this answer your question or do you have more?

It sounds like the source of your misunderstanding is the following

Bob is able to harness the information of a qubit which contains more information than regular classical bits

This statement is not true. There is a theorem known as Holevo's theorem which states that the maximum information that can be obtained from a single qubit is one bit.

In your argument, you are missing the relativistic causality. Bob and Alice simply cannot decide who did the measurement first because their space-time measurement events are connected by the space-like curve. (because they do the measurements targeting the faster than light communication.) Depending on the inertial reference frame, Bob can be first or Alice can be first.