# Tag Info

## Hot answers tagged faster-than-light

10

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 ...

8

In the situation described in the book, Alice and Bob share the state $$|\psi\rangle = \frac{|00\rangle+|11\rangle}{\sqrt{2}}.$$ Using the definition $|\pm\rangle=(|0\rangle\pm|1\rangle)/\sqrt2$ and simple algebra we can see that $|\psi\rangle$ can also be written $$|\psi\rangle = \frac{|{++}\rangle+|{--}\rangle}{\sqrt{2}}.$$ Now, if Alice measures $|\... 5 In conclusion, NO and NO, respectively. Quantum teleportation does not mean instantaneous disappearance and reappearance of information at another spatial point. Quantum teleportation in plain English means moving a quantum state from one place to another using quantum operations and quantum measurement, which are connected by classical as well as quantum ... 4 Entanglement does not transmit information, as follows from the No-communication theorem. Lieb-Robinson bound is a limit on speed at which perturbation propagates using short-range interactions, for example in spin lattice. I doubt it means something for protocols such as BB84; you can transmit quantum information by sending polarized photons, and photons ... 3 It sounds like the source of your misunderstanding is the following Bob is able to harness the information of a qubit which contains more information than regular classical bits This statement is not true. There is a theorem known as Holevo's theorem which states that the maximum information that can be obtained from a single qubit is one bit. 3 The mistake occurs when you compute the reduced state on Alice2's system. For simplicity we'll assume all systems are qubits and in addition$|x\rangle = |0\rangle$. In this setting, Bob2 prepares the state$|\phi\rangle = \frac{1}{\sqrt{2}}(|00\rangle + |11 \rangle)$and sends the first qubit to Alice. Tracing out the system of Bob2 we find$\$ \rho_{A_2} = \...

1

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 ...

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