# Questions tagged [communication]

Quantum communication is a generalization of quantum cryptography. It refers to protocols and techniques for transmission of an unknown quantum state (of a quantum system) from one place to another.

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### How a third party can learn the coefficient of a shared $2n$-qubits state of Alice and Bob using a classical message from each one?

Suppose Alice and Bob share the $2n$-qubits state $$|\phi _{x,y,b} \rangle = \frac{1}{2}(|x\rangle + (-1)^b |y\rangle)$$ where $x,y$ are $2n$-length strings of $0$ and $1$, $b$ is a bit, and $x \ne y$....
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### Pseudo telepathy with 3 people that share the state $\frac{1}{2}(|000\rangle -|011\rangle-|101\rangle-|110\rangle)$

The problem is similar to the pseudo-telepathy where Alice, Bob and Charlie are given bits $a,b,c \in \{0,1\}$ such that $a \oplus b \oplus c=0$, and their goal is to output the bits $A,B,C$ without ...
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### Classical capacity of quantum channel - Holevo quantity vs accessible information of a channel

Just above Eq (20.7) in Mark Wilde's book, while discussing the classical capacity of a quantum channel, he says: These results then suggest that the ultimate classical capacity of the channel is the ...
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1 vote
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### Can you project on an orthogonal basis for a multipartite system using only local measurements and classical communication?

Say Alice possesses one qubit, and Bob two, and that the joint state is $|\psi_{A, B_1, B_2}\rangle = \alpha|n_1\rangle + \beta |n_2\rangle$, where $|n_1\rangle$ and $|n_2\rangle$ are orthonormal ...
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### What is a "quantum communication link"?

The IBM roadmap for scaling their quantum computers refer to a quantum communication link to combine together multiple smaller quantum chips. What is the technology that enables connecting together ...
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### Increasing quantum channel capacity

I have a few confusions regarding quantum communication. Please bear with me as I ask what might appear to be basic or naive questions: In the case of classical communication the capacity of wireless ...
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### Distributing entanglement using entanglement swapping

I've read papers that consider the problem of sharing entanglement between A and C with an intermediate B. A and B share an entangled state and so do B and C. Then, entanglement swapping is used to ...
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### Entangling two particles at a distance

Entangled quantum particles are very interesting as they only make sense if you look only at the math. Usually, when we talk of "entangling" two qubits, we mean that we have two qubits, $A$ ...
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1 vote
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### Average-case error and worst-case error in entangment-assisted classical communication

I am reading this paper where one considers a quantum channel $\mathcal{N}$. We are interested in the maximum rate at which we can send classical messages over this channel in the one-shot setting i.e....
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### Possible applications for small photonic cluster-states

Assume I have a "resource state generator", enabling me to produce photonic cluster states in arbitrary shape with O(1-10) photons. Are there any possible applications for such a device in ...
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### Is it advantageous for a state to have higher Bell CHSH violation?

For a density matrix $\hat \rho$, one can construct a "Bell operator" $\hat B$, such that the violation of the inequality $\langle \hat B \rangle \le 2$ is a clear indication of quantum ...
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### Can $2^n$ bits be sent with $n$ instances of quantum teleportation?

So, right now these are two pieces of information I've been told are correct: Quantum teleportation can send a single qubit from Alice to Bob, with two classical bits $n$ qubits can store $2^n$ ...
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### Is it well known classical communication over a quantum channel at a rate below its classical channel capacity can have an exponentially small error?

Classical channel capacity $C(\mathcal N)$ of a quantum channel $\mathcal N$ is defined to be a rate below which any classical communication can succeed with an arbitrarily small error. Specifically, ...
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### Do quantum teleportation require transmission media?

It said on wikipedia: Because classical information needs to be sent, quantum teleportation cannot occur faster than the speed of light. So does it mean quantum communication specifically ...
1 vote
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### Does a noiseless quantum channel violate no-cloning theorem?

A quantum channel is defined as CPTP map from Alice to Bob $\mathcal{N}: \mathcal{H}_A \to \mathcal{H}_B$. In particular, a noiseless quantum channel is such that $\mathcal{N}(\rho_A) = \rho_A$. My ...
1 vote
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### $2$ ebits $+$ $1$ bit $= 2$ bits?

The Set-Up Let's say Alice and Bob share $k$ ebits, i.e., they have one-qubit each of each of the $k$ Bell states $\frac{\vert 00\rangle+\vert 11\rangle}{\sqrt{2}}$. Now, Alice wants to send $2n$ bits ...
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### Is it possible to prevent quantum communication detection?

From what I understood, one of the advantages of quantum communication is that one can (mathematically/physically) prove that the quantum communication/message has not been intercepted/tampered with. (...
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### What are examples of zero capacity quantum channels with Choi rank less than $d$?

All the currently known examples of quantum channels with zero quantum capacity are either PPT or anti-degradable. These notions can be conveniently defined in terms of the Choi matrix of the given ...
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### Can someone elaborate how to use Qutrits to do teleportation? And the unitary operation Bob needs to do? [duplicate]

I can understand doing teleportation by using qubits, however, I am stuck there when it comes to qutrits and the operation Bob needs to perform. Suppose Alice and Bob are sharing the most entangled ...
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### How are inputs during the quantum communication between Alice and Bob placed in the Galois Field?

I have to write a report on "Classical Interaction Cannot Replace a Quantum Message" by Gavinsky (arxiv link). The author gave a lecture, and at 14:55 explains that a communication problem ...
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### Why do quantum networks matter?

I'm very new to quantum computing, sorry if this is a dumb question. I was reading in some lecture notes I found online that a quantum network could be created by using teleportation multiple times to ...
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### If Alice and Bob share only classical communication resources, is shared entanglement always equivalent to shared randomness?

If Alice and Bob share only classical communication resources such as noisy or perfect classical channels, is shared entanglement always equivalent to shared randomness? In other words, must there be ...
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### Teleportation followed by measurement: Lowering communication cost

Suppose Alice and Bob have access to shared entanglement and a classical channel and wish to simulate the following quantum protocol. Alice sends over to Bob an $n$-qubit state which is not known to ...
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### Non-ideal coin tossing

Can someone please check if the following makes sense? We have a non-ideal coin-tossing scheme as follows. Alice and Bob know what $|0\rangle,|1\rangle$ are. Bob wins when the coin is 1. Honest Alice ...
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### Non-ideal bit commitment and coin tossing

Can someone please explain rigorously the reasoning behing non-ideal ($\epsilon$-concealing and $\delta$-binding) bit commitment scheme and the impossibility of coin tossing. What is bias in coin ...
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### How do I compare the secret key rate (kbps) with the secret key rate per channel use?

I am doing some research for an undergrad project and have hit a conceptual stumbling block. I have recently been looking at some of the achievements in practical QKD using trusted node quantum ...
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### 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 ...
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