# Questions tagged [quantum-state]

Quantum systems can mathematically be described by their 'quantum state'. When the system is closed/isolated, the state is 'pure' and can be written as a sum (i.e. 'superposition') of basis vectors. When the system is a subsystem of an open system, the state is instead usually 'mixed' and cannot be written as a pure state, so has to be written as a density matrix. Consider using the density-matrix tag when relevant

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### How close or far apart are the distributions generated by two Haar random states?

Consider two $n$ qubit Haar-random quantum states $|\psi\rangle$ and $|\phi\rangle$. Let $D_{|\psi\rangle}$ and $D_{|\phi\rangle}$ be the two probability distributions (over $n$-bit strings) obtained ...
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### How are the eigenvalues of $\rho=\frac12(|a\rangle\!\langle a| +|b\rangle\!\langle b|)$ derived?

Let's say I have a density matrix of the following form: $$\rho := \frac{1}{2} (|a \rangle \langle a| + |b \rangle \langle b|),$$ where $|a\rangle$ and $|b\rangle$ are quantum states. I saw that ...
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### What is the meaning of measuring a Bell state with Pauli operators?

There will be a certain value of getting the probability when measuring any Bell's state with Pauli operators such as observable X, Y, or Z. What is the meaning behind all this measurement? the result ...
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### How to relate the result of measuring Pauli operators on Bell states with the quantum entanglement? [closed]

how to relate the result of measuring Pauli operators such as observable X and observable Y on the Bell's state with the quantum entanglement? In which way to say that there are correlated to each ...
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### What are the possible results of measuring $X$ and $Z$ on the state $|01\rangle+|10\rangle$?

When calculating the probability of getting +1 on X-basis on the first qubit of Bell's state $|01\rangle+|10\rangle$, the result is 1/2 with the state after measurement |++⟩ while the probability of ...
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### How to apply a operator to qubit system on the basis of current state of system?

Suppose I have three different operators $U_1, U_2,U_3$. Now, these three operators will be applied if my current state of the system is $|\psi_0\rangle,|\psi_1\rangle$ and $|\psi_2\rangle$ ...
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### What is the probability of finding the second qubit as $0$ in the state $|\psi\rangle=\frac1{\sqrt2}|00\rangle+\frac12|10\rangle-\frac12|11\rangle$?

Assuming two qubits start in the state: $|\psi\rangle = \frac{1}{\sqrt 2}|00\rangle + \frac{1}{2}|10\rangle- \frac{1}{2}|11\rangle$ What is the probability of measuring the second qubit as 0? And ...
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### Qiskit get qubits from statevector

How do I get the amplitude of each qubit, like plot_bloch_multivector(), but not output the tensor product of all qubits. In this case I need output ...
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Let us take the initial state with the particle located at the origin $|n=0\rangle$ and the coin state with spin up $|0\rangle$. So, $$|\psi(0)\rangle = |0\rangle|n=0\rangle,$$ where $|\psi(0)\... 2answers 109 views ### How can I find a quantum channel connecting two arbitrary quantum states? Given two arbitrary density matrices$\rho, \sigma\in \mathcal{H}$(they have unit trace and are positive), how do I go about finding a possible quantum channel$\mathcal{E}$such that$\mathcal{E}(\...
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In the paper by Stamatopoulos et al., the authors say that it is possible to load a distribution on a three qubit state to obtain: In Qiskit finance this is performed using the uncertainty model ...
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### Cirq - Measure Density Matrix Function Getting First Element

Hello I am using measure density matrix function like that: ...
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### Is it wrong to say that $a$ and $b$ are the square roots of the detection probabilities in a qubit state $|\psi \rangle = a|0 \rangle +b|1 \rangle$?

Is it wrong to say in $a$ and $b$ are the square roots of the probability of the qubit being in the state 0 and 1 when measured for a qubit in the state $|\psi \rangle = a|0 \rangle +b|1 \rangle$? ...
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### What is the difference between MPS and DMRG? [closed]

How can matrix product states simulate quantum circuits? I heard matrix product states come from DMRG (density matrix renormalization group). Here are my questions. What is the difference between MPS ...
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### How to represent the state vector form of a qubit in density matrix representation? [duplicate]

While I'm studying state vector and density matrix. I wonder how to write qubit state as density matrix. qubit state can be represented with state vector form. But how about density matrix?
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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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### Quantum State for Non-Regular Graphs

While studying quantum walk on graphs, I am able to find how d-regular graphs state can be constructed in quantum domain. But not able to find anything on non-regular graph. For ex-: how this graph ...
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### Quantum analogues of information theoretic measures: are log probabilities replaced with the density matrix?

Below is a question and an answer. How does quantum information relate to, diverge from or reduce to Shannon information, which used log probabilities? What people are more often interested in are ...
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### Cirq-Measuring a State with Rotation Matrix

I have this state: $$p |\text{GHZ}\rangle \langle \text{GHZ}| + (1-p)\rho$$ And after creating this state I have this code lines: ...
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