| CARVIEW |
Select Language
HTTP/2 200
date: Tue, 29 Jul 2025 01:29:23 GMT
content-type: text/html; charset=utf-8
content-encoding: gzip
cf-ray: 9668d84efc05c16f-BLR
cf-cache-status: DYNAMIC
cache-control: private
set-cookie: prov=2a1f0a34-b578-4c32-86f2-a0b217c5781a; expires=Wed, 29 Jul 2026 01:29:23 GMT; domain=.stackexchange.com; path=/; secure; httponly
set-cookie: prov=2a1f0a34-b578-4c32-86f2-a0b217c5781a; Path=/; HttpOnly; Domain=stackexchange.com
set-cookie: __cf_bm=w4BtXSq4XPCM3VXQ03ow88N5UxW_yurjAESgB9c8.kY-1753752563-1.0.1.1-P4.iNUeRsosJLzNvD9c14h6judrNzy_heKz9s5RJsRLFZbACjJOglwWHHspTYk0cenrnX43QcFLT5dH_gtPRKV.n.6hHanEplskUHKGmQac; path=/; expires=Tue, 29-Jul-25 01:59:23 GMT; domain=.stackexchange.com; HttpOnly; Secure; SameSite=None
set-cookie: _cfuvid=yl_JrdyMzkM6m.MMFwrmY7oNvNuXABHMmVJ_1I5BOFE-1753752563460-0.0.1.1-604800000; path=/; domain=.stackexchange.com; HttpOnly; Secure; SameSite=None
strict-transport-security: max-age=31536000; includeSubDomains
vary: Accept-Encoding
content-security-policy: upgrade-insecure-requests; frame-ancestors 'self' https://stackexchange.com
x-clacks-overhead: GNU Terry Pratchett
x-frame-options: SAMEORIGIN
x-request-guid: ac40a9e3-c56d-4362-9d24-a1753f97981d
x-worker-origin-response-time: 392000000
x-dns-prefetch-control: off
server: cloudflare
Newest 'measurement' Questions - Quantum Computing Stack Exchange
Skip to main content
Stack Exchange Network
Stack Exchange network consists of 183 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.
Visit Stack Exchange
Teams
Q&A for work
Connect and share knowledge within a single location that is structured and easy to search.
Learn more about TeamsQuestions tagged [measurement]
For questions related to measurement and its effects as relevant to quantum computation and quantum information.
765 questions
- Bountied 0
- Unanswered
- Frequent
- Score
- Trending
- Week
- Month
- Unanswered (my tags)
2
votes
1
answer
64
views
How to transform the density matrix from one basis to another basis
Suppose I have a density matrix for 2 qubits which is 4 by 4, in $Z$ basis $\left(|0\rangle,|1\rangle\right)$. Now I want to transform the density matrix to $X$ basis $\left(|+\rangle,|-\rangle\right)$...
10
votes
1
answer
215
views
Is it hard to invert unitary effects implemented using feedback?
It's well know that if you have a circuit made up of only unitary operations, it's trivial to invert the circuit. Just reverse the order of operations, and invert each individual operation.
However, ...
1
vote
1
answer
56
views
Modeling readout errors for non-destructive measurements in Stim
I am currently trying to implement a QEC code in stim that is based on non-destructive measurements as core operations and want to add readout errors on these measurements as noise. For destructive ...
1
vote
0
answers
52
views
How to measure in practice the average of an observable?
Given a $n$ qubit quantum register in state $|\psi\rangle$, I have a basic question about assessing the average of an observable $\hat{O}$ with respect to state $|\psi\rangle$ :
\begin{equation}
\...
- 537
0
votes
1
answer
83
views
Shor code: arbitrary quantum error-correction
For describing Shor code for arbitrary error-correction, Nielsen and Chuang in page 434 say: we describe the noise by a trace-preserving quantum operation $\mathcal{E}$
$$\mathcal{E}(|\psi\rangle\...
0
votes
0
answers
45
views
Equivalence of projective measurement with general measurement
Nielsen and Chuang in page 91 say a very important thing: "projective measurements augmented by unitary operations turn out to be completely equivalent to general measurements, as shown in ...
0
votes
1
answer
106
views
Qiskit gives different statevectors when performing partial trace
I am building a circuit using Qiskit that contains many computational qubits and several ancilla qubits. Here is a toy example:
...
1
vote
0
answers
49
views
applying Haar random unitary to projection and pure states and compute its trace
I have this question when I read the proof of Theorem 4 in This paper. Here are the settings:
\begin{align}
&|\phi\rangle^{BC}: \text{The reduction of a pure state $|\phi\rangle^{ABCD}$ where A,C ...
0
votes
2
answers
68
views
Rotations that don’t change the outcomes probabilities in X and Z
Is there a rotation operator that doesn't change the outcome probability when measuring either on basis X or Z in the scenario where I only change the sign of the angle?
I was thinking of $R_y(\pm\...
2
votes
1
answer
187
views
Complexity of checking if a circuit is deterministic or not?
Suppose I am given a list of gates for some circuit. I start with some fixed state, say $\vert 000..\rangle$, and measure the output in the computational basis. I want to know if the circuit is ...
3
votes
1
answer
98
views
Hong Ou Mandel interference using Bell sates
In the standard Hong Ou Mandel interferometer, if identical photons are sent as input to a beam splitter, they will bunch at the output and so the rate of coincidences between the two outputs will ...
0
votes
2
answers
111
views
After applying CNOT, is it equivalent to measure the control or the target?
This is a follow up to What happens after applying CNOTs and measuring either register?
I have been playing with some problem that involves intermediate measurements after applying a CNOT. Here are ...
3
votes
2
answers
139
views
What happens after applying CNOTs and measuring either register?
Suppose I have a multi-qubit quantum state of the form:
$$
|\psi\rangle = \sum_i a_i |i\rangle |0^n\rangle |\phi_i\rangle\;,
$$
where $|\phi_i\rangle$ is some pure state (on a separate subsystem) that ...
3
votes
1
answer
402
views
Legal to take inner product between a two-qubit and single-qubit state?
Given a single-qubit state $|\psi\rangle=\alpha|0\rangle+\beta|1\rangle$, is it fine to calculate something like:
$$\langle\Phi^+|\psi\rangle = \frac{1}{\sqrt{2}}(\alpha\langle0| + \beta\langle1|),$$
...
2
votes
1
answer
114
views
Prove the relation between trace norm and fidelity for pure states
I encountered an exercise as follows:
Consider two generic pure states ( $|\Psi\rangle$, $|\Phi\rangle $).
Prove the following exact relation between the fidelity and the trace
distance:
$$\frac{1}{2}...
- The Overflow Blog
-
- Featured on Meta
-
-
Related Tags
quantum-state × 193
qiskit × 102
entanglement × 71
quantum-gate × 56
povm × 54
error-correction × 52
programming × 48
quantum-operation × 43
linear-algebra × 38
probability × 35
nielsen-and-chuang × 35
bell-basis × 33
quantum-circuit × 30
density-matrix × 30
quantum-algorithms × 28
circuit-construction × 28
ibm-q-experience × 28
projection-operator × 27
pauli-gates × 24
information-theory × 23
stabilizer-code × 21
simulation × 20
state-discrimination × 19
noise × 18
more related tags
Hot Network Questions
- Strange behavior when using tikz-network and babel (french)
- Was there a cultural significance to the mockery of baldness?
- Where does Nietzsche indicate the character of the Superman (Übermensch)?
- What is the weight of holy water?
- What are the canonical commutation relations mathematically?
- Writing a visually-selected couple of files out to a file with an open buffer
- How to formally define aggregated index sets based on shared attributes?
- A national poll of 1000 French returns 25% of "Yes". Is it enough to say that it's quite impossible locally to be 0%, if we don't know the variance?
- wifi problem with regulatory domain settings
- Why do we name and separate invisible bands even though the EM spectrum is continuous?
- Time Machine backup disk not writeable any longer
- Calculus Books or Websites Recommendation
- Would saying I want to go into industry on a statement of purpose negatively affect PhD admissions?
- What is the grammar of が after an い-adjective?
- PoE center tap vs dedicated V+/V-
- Can you identify this aircraft landing at Llanbedr Airfield?
- Partitioning heaps - Why?
- Sci-fi novel human-centered, with alien empire using stargates, humans try to avoid overruling
- How does one justify the rationality of buying insurance?
- Projective representations in the algebraic formalism
- What did Chava say to Adam?
- Why did many arcade games have separate sound CPUs?
- If I buy spell components for a different price than the official one, does that change anything about how they work as a component?
- Can I combine 2 car reservations on adjacent dates with Hertz?