# Questions tagged [measurement]

For questions related to measurement and its effects as relevant to quantum computation and quantum information.

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### Measuring one qubit in an entangled pair in another basis?

Qubits are usually measured in the computational basis, but we can change the basis by a unitary $U$ to measure in the basis formed by the columns of $U$. For example, if $| \psi \rangle = | 0 \rangle$...
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### Finding the "dual" basis of an overcomplete basis for Quantum State Tomography

This question is related to this stack exchange post: What does the POVM corresponding to single-qubit state tomography look like? From what I understand, when we are interested in reconstructing a ...
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### What is the relation between number of measurements and additive error, when estimating the expectation value of an observable?

Given an unknown state $\rho$ I would like to measure it using an observable $O$. I would perform measurements using the orthogonal basis of $O$ on multiple copies of $\rho$ and my answer would ...
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### How does this measurement in the Hadamard basis look like?

I am reading this paper by Mahadev. In going from (19) to (20) the author does a Hadamard measurement on two registers. I don't understand what exactly the Hadamard measurement does. The (simplified) ...
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### Can Noise in measurements lead to non-zero probability of a state that was not originally in the state vector?

Consider the following state vector $$\frac12 (|000\rangle + |011\rangle + |101\rangle + |110\rangle)$$ Can noise in measurements lead to a non-zero possibility of measuring the state $|111\rangle$ ...
1 vote
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### Adding measurement circuits to the singlet state circuit and want to create single circuit

I am trying to add the measurement of Alice and bob's circuit to the singlet circuit. Actually, I am trying to build the e91 protocol. But it is giving me an error like this. ...
1 vote
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### Can transformations of qubits in quantum computing be carried out instead by transformations of the measurement instrument?

As far as I understand, transforming and maintaining states of qubit devices in quantum computers is associated with all sort of problems such as transformation errors and decoherence. At the same ...
1 vote
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### Double Slit Experiment

Does anyone know if is possible to run one instance of this experiment with two groups of people. One group who is able to observe while the other group is somehow not and still get the expected ...
1 vote
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### Possible post - measurement states for Bell state $\frac{1}{\sqrt{2}}[|00\rangle + |11\rangle]$

This is in reference to page 241 of Introduction to classical and quantum computing by Thomas.G Wong. The author starts off with a Bell state $\frac{1}{\sqrt{2}}[|00\rangle + |11\rangle]$. In trying ...
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### How to know what eigenvalue corresponds to measurements of individual qubits in a multiqubit system?

I'm working through the book "Introduction to the Theory of Quantum Information Processing" by Bergou and Hillary, and I've encountered a scenario that I'm not sure how to approach. In ...
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### Single-shot error correction for the surface code with measurement errors

I am trying to implement a single-shot error correction for the surface code with data + measurement errors (both with prob. p), using the (build-in) BP+OSD decoder. I am mostly following these papers:...
1 vote
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1 vote
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### Motivation behind POVM and projective measurement

This is in reference to Quantum Computation and Quantum Information by Michael A. Nielsen and Isaac L. Chung [page 90, 92]. Any POVM elements $E_{m}$ are defined as $E_{m} = M_{m}^{\dagger}M_{m}$. A ...
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 ...
1 vote
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### Is it possible to produce an entangled state by measuring an unentangled state?

Wondering if it's possible to produce an entangled state by measuring an unentangled state. I tried a few examples, but it seems it's not possible.
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1 vote