Questions tagged [measurement]

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

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Parameterized swap test and perfect swap test

Suppose one has parameterized a swap test by using an ansatz $U(\theta) = \exp(-i\theta \text{ CSWAP})$, and one tries to find an angle $\theta$ such that one can distinguish given two quantum states ...
41 views

The meaning of non-destructive measurements in QEC

I am starting to study quantum error correction and it's the first time that I see the operators (usually Pauli) as projective measurements which seem to just identify a syndrome but do not destroy ...
170 views

Can joint measurement be achieved in two labs far apart?

Consider the following scenario: Alice and Bob are in two labs far apart, and they each have one qubit. Can joint measurement (for bipartite projective measurement, they are measurements that cannot ...
39 views

Retrieve image from Histogram data of Qiskit

I am trying to understand the quantum image processing. Please refer the link- https://qiskit.org/textbook/ch-applications/image-processing-frqi-neqr.html. I am not able to understand how to process ...
37 views

How do we define qubit measurements in a plane?

When does $\vec{a} \cdot \vec{\sigma}$ define a measurement in x-y, y-z, and x-z planes?
44 views

Measurement of the second quantum register in the Shor's algorithm [duplicate]

I've read that the measurement of the ancilla qubits is not fundamental for Shor's algorithm, but I don't understand how the algorithm works if I remove it. Without those measurements, do I have $r$ ...
49 views

Error with IBM's Quantum Composer Qiskit Histogram when Resets are involved?

I am having trouble understanding what IBM's histogram is doing. I have independently verified, in agreement with the statevector given from IBM, that after all these steps the statevector should be ...
25 views

What is a histogram of a job a plot of, if not the squared amplitudes of the final state-vector?

The histogram of a circuit is the result of running the circuit (with measurement) many times, right? Does this correspond to the squared amplitudes of the final state-vector? If not, why?
31 views

What happens to the elements of the simulated state vector when we set a qubit to $|0\rangle$?

In IBM's Qiskit online simulator, we have the (non-reversible) ability to set a specific qubit to $| 0\rangle$. This is convenient but I'm left confused as to what happens to the elements of the ...
62 views

Density matrix after measuring Bell state in CHSH game

In these notes, the author says the following about the CHSH game Does Alice and Bob’s ability to succeed more than 75% of the time mean that they are communicating? Well, we know it’s not possible ...
54 views

Partial measurement can be replaced with constant overhead

While reading the chapter on Quantum Computation (starting on page 401) of the draft version of the Arora & Barak book I came across exercise §4 on page 431 that reads as: Suppose that $f$ is ...
167 views

Does one measurement affect the following measurement errors in mid-circuit measurement?

Suppose I have a quantum circuit with a few measurements (say $N$) on a single qubit. Before all the measurements, I generated calibration circuits and the 2 by 2 calibration matrix for that qubit of ...
34 views

Mathematics of Measurement then Partial Trace

Say we have the following quantum state: $$|\psi\rangle = \frac{1}{\sqrt{2}}(|00\rangle +|10\rangle)$$ To measure the first qubit and then further trace out the first qubit, my notes have the ...
29 views