# 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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### Quantum communications and "knowledge" of receiving a qubit

Question from a computer scientist (not a physicist). Imagine two nodes on a quantum network. Alice sends Bob a qubit in some state of superposition over a quantum channel. Is Bob able to sense that ...
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### What is the difference between having a single-qubit state and knowing a result of a measurement you want to perform on it?

In the quantum teleportation protocol Alice can send Bob an unknown quantum state $|\psi\rangle$. If the only thing Bob does with $|\psi\rangle$ is to measure it in some basis, I guess it would be ...
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### Complexity of quantum circuits in a specific protocol

Consider the following simultaneous communication problem. Alice and Bob do not share any entanglement or any common randomness, and cannot communicate directly with each other. As inputs, $x$ is ...
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### Optimality of teleportation for entanglement-assisted quantum channel coding

Setting: Consider a quantum channel $N_{A\rightarrow B}$ between Alice and Bob. Let Alice and Bob also share arbitrary amounts of entanglement assistance through the state $\phi_{A_EB_E}$ (these can ...
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### Tradeoff between error and rates of quantum communication

Suppose Alice and Bob share $n$ copies of a noiseless quantum channel $I_{A\rightarrow B}$ which can be used to send quantum states and $H_A\cong H_B$ i.e. the input and output Hilbert spaces are the ...
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### How to prove that $\frac{| x_0 \rangle + | x_1 \rangle}{\sqrt{2}}$ hides one of $x_0$ or $x_1$?

I create a quantum state $| \psi \rangle = \frac{| x_0 \rangle + | x_1 \rangle}{\sqrt{2}}$ for a randomly chosen $x_0,x_1$ of 50 bits. I give this quantum state $|\psi \rangle$ to you and you return ...
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### Quantum teleportation and the reality of quantum states

This question is perhaps philosophical but it's been confusing me. Suppose Alice is teleporting some qubit state $|\phi\rangle$ to Bob via the quantum teleportation protocol. After Alice applies the ...
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### Distributed quantum computing with classical communication

I'm interested in the field of distributed quantum computing, i.e. using multiple smaller quantum devices/circuits to cooperate to be able to perform some task that would require a larger device (...
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### Simulating a Quantum Network over a real Classical Network Using Simulaqron

I'm trying to simulate a quantum network over a real classical network using the Simulaqron framework. I know the documentations says this is possible but I am having trouble finding examples of how ...
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### Entanglement and teleportation transmission costs

Let say we have a product state $$|\phi^+\rangle_{12}|\phi^+\rangle_{34}$$ with particles (1,4) belong to $A$ and (2,3) belong to $B$. Where $A$ and $B$ are say at distance of $L$ from each other. ...
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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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### Is communication possible with entanglement?

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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### In the BB84 protocol, for what error thresholds can Alice and Bob not establish secure bits?

Suppose Alice and Bob can communicate over a quantum channel. The quantum channel is noiseless in the absence of eavesdropping, a condition that cannot be guaranteed. They can also communicate over a ...
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### Schumacher compression - comparing with Shannon compression

Background Shannon's source coding theorem tells us the following. We shall consider a binary alphabet for simplicity. Suppose Alice has $n$ independent and identically distributed instances of a ...
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### How to deal with noise in BB84 protocol?

BB84 protocol works followingly: Alice generate $N$ random qubits $a_i$ in state either $|0\rangle$ or $|1\rangle$ . These will be a key. Then she prepares $N$ random bits $b_{i}$. If $b_i=0$, an ...
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### Generalising the standard quantum teleportation protocol: what are the possible unitaries?

Consider the standard quantum teleportation scheme. Let the first register hold the state $\newcommand{\ket}{|#1\rangle}\ket\psi$ to be teleported, and the second register be the one shared between ...
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### What is the quantum state transmitted to Bob in BB84 protocol?

Sender (Alice) wants to exchange private encryption key with Bob using BB84 protocol. She generate a binary string (1010100010010111),and encode using Hadamard gate and identity gate (HHHIIHIHHIIIHIIH)...
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### What is the practical interest of superdense coding?

When you enter the field of quantum communications, you run at some point into the concept of superdense coding. Basically it is a way to encode classical bits on the qubits of a quantum channel, ...
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### Does the Lieb-Robinson bound constrain the speed of entanglement information transmission?

I just learned from the existence of the theoretical Lieb-Robinson bound, which indicates that the speed at which information can be propagated in non-relativistic quantum systems cannot exceed this ...
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### What is a flying qubit?

DiVincenzo criteria for quantum communication mentions flying qubits: a. The ability to interconvert stationary and flying qubits b. The ability to faithfully transmit flying qubits between specified ...
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### Can superdense coding be made more efficient?

Superdense encoding allows us to transmit the information of two classical bits using a single qubit with a pre-shared Bell state qubit pair. We can duplicate the construct to transmit $2n$ classical ...
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### Understanding the idea of "local hidden state model"

In Cavalcanti and Skrzypczyk (2017), section IIA, pag. 3, while introducing the idea of quantum steering, the authors introduce the idea of Local Hidden State model as following (emphasis mine): A ...
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### Entanglement-assisted hashing bound for asymmetric depolarizing channels

I reading the paper EXIT-Chart Aided Quantum Code Design Improves the Normalised Throughput of Realistic Quantum Devices, which proposes the use of QTCs in order to do quantum error correction for ...
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### Eavesdropping in superdense coding: why can't the third party infer anything about the message?

There's an exercise in Nielsen & Chuang asking us to show that if some third party intercepts Alice's qubit she's sending to Bob while attempting to transmit (2 bits of) classical information, ...
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### Why does entanglement not increase the classical capacity of a channel?

In a recent paper, the authors quote an older work of Bennett, Shor and others and make the following statement While entanglement assistance can increase achievable rates for classical point-to-...
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### Does entanglement allow communication of user specified information or not?

At first when I heard about entanglement, I thought "Neat, so we can make a manipulation on our quantum computer here in such a way that can be interpreted as a binary output somewhere else, and have ...
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### Why is measurement needed in teleportation?

One of the postulates of QC is, that measurements in every circuit can be postponed or never performed in a circuit while achieving the same functionality of the circuit (at least this the way I ...
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### Transmission of information over long distances

I am reading that entangled particles can share information across long distances and the speed is usually faster than the speed of light...so am I right in assuming that future communications in the ...
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### Real-life examples of classical-quantum channels

In quantum information theory, classical-quantum channels are considered to be channels whose input is the realizations $x\in\mathcal{X}$ of a classical random variable to a quantum state $\rho_x^B$, ...
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### BB84 protocol over LiFi

The question is already explained in the title, but I'm curious regarding the possibility to adopt the BB84 protocol over LiFi (in theory). Could it be possible or are there some limitations, such as ...
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### What is the quantum bandwidth of a planar array of noisy qubits, assuming free classical communication?

A common task to perform during quantum computation on the surface code is moving qubits from one place to another. There are standard ways to do this within the surface code, but I was wondering what ...
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### How to test with the "Quantum Internet"?

I want to test some ideas using the quantum internet. I know that is not widely available now. Can it be simulated? Are there any simulation systems which allow - Entanglement Testing states ...
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### Protocol for entaglement swapping

Suppose there are 3 parties, of which 2 pairs share an EPR pair and can communicate classically. What is a protocol that results in the third pair sharing an EPR pair? That is the problem I'm given, ...
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### Collective measurements: importance and realization

I am reading the paper Polar codes for classical-quantum channels by Wilde and Guha, and it is stated the fact that collective measurements are necessary in order to aciheve the Holevo symmetric ...
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### How many bits do Alice and Bob needs to compare to make sure the channel is secure in BB84?

I was trying to self-study qmc by reading the Quantum Computing A Gentle Introduction book, in section 2.4 it talks about the quantum key distribution protocol BB84. After (I thought) I understood it ...