# Questions tagged [quantum-state]

Questions about or related to quantum states. Consider using the density-matrix tag when relevant.

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### About swap gate

As I was reading Qiskit textbook and there is this problem ... How can I swap a $|+\rangle$ to a $|-\rangle$
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### Expectation values of non-local operators in Qiskit

Is there a convenient way in Qiskit to calculate the expectation value for a non-local operator, i.e. I would like to calculate: $$\langle \Psi|O|\Psi \rangle$$ More precisely, I would like to ...
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### State tomography with Pauli basis measurements for a high number of qubits

My end goal is to recover the quantum state in its computational basis or reduced density matrix of a high number qubit circuit in a real QPU. Taking into account that the number of qubits will be ...
1 vote
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### Is there an alternative algorithm to extract a probability of states of qubit after measuring?

Let a simple quantum circuit consist of one qubit and one classical bit. Initially, a qubit has the $|0\rangle$-state and then goes through some sequence of quantum gates, see the circuit below. The ...
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### Entangling attack on BB84 protocol

I am trying to solve the exercise 5.3 from the book "A Gentle Introduction to Quantum Computing". The exercise reads as follows: Suppose Eve attacks the BB84 quantum key distribution of ...
1 vote
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### Quantum measurement on Bloch Sphere

I'm reading the paper "Data re-uploading for a universal quantum classifier". This paper proposes the classification process as follows $x_i$ with different rotation angle $\theta_i$ ...
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### How to read data from multiple qubits and feed it to a MLP

Being new in the field of quantum computing, I tried with the implementation of "https://qiskit.org/textbook/ch-machine-learning/machine-learning-qiskit-pytorch.html" using a single qubit. ...
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### Why are orthogonal quantum states represented as collinear in the Bloch sphere?

We know that the angle between two orthogonal qubit states is 90 degrees. Why then, when we use the Bloch sphere, the angle becomes 180 degrees?
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### Construct the unitaries U by decomposition in TSP Qiskit?

I'm stuck in here and really don't understand this stuff. In order to construct the unitary matrices for Quantum Phase Estimation, Qiskit decomposes the diagonal unitary matrices U(j) by tensor ...
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### Why is Dirac notation used for quantum states?

I am thinking that I am using Dirac notation to represent $|0\rangle$ and $|1\rangle$ and we are using column vector but why actually using column vector where first one is $(0 \,1)$ for $|1\rangle$ ...
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### How simulation of noisy quantum circuits is done in Qiskit using the statevector method

While performing VQE calculations of medium-sized molecules like H2O, using Qiskit AerSimulator with noise, I noticed that even for a large number of shots, the speed of simulation using statevector ...
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### CNOT entangling a third qubit and how measurement impacts entanglement

I'm trying to understand entanglement, superposition and the effects of measurement on entangled qubits a bit better. About entanglement: I know that the circuit below will entangle the two qubits: ...
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### A path towards building quantum Computing graduation project for undergraduates

I need help. I'm a computer science student with a Data science major. I have a final graduation project this year. With that, I want to create a project in the Quantum computing field. I'm already ...
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### How we can switch the digital multi-qubit gates to analog multi-qubit blocks?

What is the difference between the digital and Analog Quantum gate? and the multi-qubit gates involved in a VQE ansatz can be switched to analog multi-qubit blocks. so How we can switch the digital ...
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### What is the rank of the pure state?

Consider pure or nearly pure quantum state, is it usually low-rank? Can you give a example of a concrete state and its rank?
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### What actually StatePreparation perform in qiskit?

I used this method to initialize classical data in a circuit. I would like to know what's under the hood and the complexity of this procedure, but I can't find anything on the internet which suits my ...
1 vote
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### What is the value of a control qubit as a Controlled-X Gate or Tofolli Gate parameter?

For example in a Controlled-X or Tofolli gate, qubits can be assigned as control bits. But before the measurement ; what are the values of these qubits in the gate tensor since they can not be 1 or 0 ?...
1 vote
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### Quantum gate count minimization

In classical digital logic, we have K-map and Quine–McCluskey algorithm to minimize boolean equation, thereby reducing the number of logic gates needed. Is there such a thing in quantum computing?. ...
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### It is possible to obtain amplitudes from superposition with certain probability?

In many papers and books on quantum theory it is written that it is not possible to obtain amplitudes exactly when we have a qubit in superposition. In a course we said that actually it is possible, ...
1 vote
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### Is there any way to always output |1> if the measurement has a probability to output 1?

Let's say I have an oracle that outputs 1 on a specific input (there can be multiple inputs x so that f(x)=1), else it outputs 0. If there is no such input so that f(x)=1, the quantum state of the ...
57 views

### Set relative phase of qubit to zero without measurement?

Is there a quantum way to set the unknown relative phase of a qubit (assumed in a pure state) to zero, without measurement? The relative phase is not known, otherwise I would subtract it using a phase ...
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### A doubt regarding Mach-Zehnder Interferometer functioning

I am tying to understand the working of Mach-Zehnder Interferometer in the physical sense (how it behaves and why it does so etc.). The source I am referring to is this: (https://www.cs.princeton.edu/...
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