Quantum Computing Stack Exchange
Quantum Computing Stack Exchange

Questions tagged [measurement]

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

660 questions
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Are true Projective Measurements possible experimentally?

I have heard various talks at my institution from experimentalists (who all happened to be working on superconducting qubits) that the textbook idea of true "Projective" measurement is not what happens in real-life experiments. Each time I asked…
Dr. T. Q. Bit
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Can one ever find the elements of a superposition state?

Given some set of basis states $\{\vert 0\rangle, \vert 1\rangle, \vert 2\rangle...\vert N\rangle\}$ and an unknown superposition of the form $\frac{1}{\sqrt{2}}(\vert i \rangle + \vert j \rangle)$, what exactly forbids us from computing $i$ and…
Al Jones
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Projective vs general measurements - a missing piece

This may be a very basic and common question (also discussed a lot), but strikingly enough I couldn't find the answer in the books or elsewhere. The projective measurement is given by the PVM on the space $H$: $$\sum P_i = I,$$ where $P_i$ are…
Danylo Y
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Do weak measurements (with/without weak values) have any application in quantum computation?

I have seen their applications in quantum state tomography but not in computation as such.
RAMAN CHOUDHARY
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How is the number of measurement outcomes linked to the rank of the observable?

I am thinking about the following question: Assuming that we have some given state $\rho$ and we perform a measurement with $k$ outcomes on this state. Then we can describe the measurement in outcomes as eigenvalues of the measurable, i.e.,…
LeoW.
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Measuring in an arbitrary basis

I'm new to quantum computing, so apologize if my question seems obvious. Say we have a state $|\Psi\rangle$ and we want to measure it in the computational basis. From what I've been able to understand so far is that, when we make a measure in the…
Skyris
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What does simultaneous measurement of a mutually commuting set of observables mean?

If I have a mutually commuting set of observables $\{A, B, C, D,...\}$ and when we talk of simultaneous measurements of these observables, do we mean a single apparatus is required for simultaneous measurement or a series of apparatuses are…
RAMAN CHOUDHARY
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If $[M,N]=0$, is the probability distribution obtained measuring $M$ after $N$ the same as the one obtained measuring $N$?

Let $|\psi\rangle$ be a fixed state and $M$ and $N$ two commuting operators corresponding to projective measurements. Consider the probability distribution $p$ obtained by measuring $N$ on $|\psi\rangle$. Is this probability distribution exactly the…
eternalstudent
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How to check states?

I don't know much about physics so maybe the question is stupid, but I wonder how engineers detect that the state function did not collapse due to the environment while a calculation is performed? Theoretically, a measurement will break the system.…
user2827
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What does it mean to make a simultaneous measurement?

In Quantum Computing quantum measurements are described by a collection of measurement operators $\{M_n\}$ such that the probability of the outcome $m$ is given by $p(m)=\langle\psi|M_m^\dagger M_m|\psi\rangle$ and the state of the system after…
Musk Nation
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Is there a list of known quantum measurements?

Quantum Measurement can be divided into General Measurements, Projective measurements and general POVMs. And there are also some special kinds of quantum measurements that have their own name, such as : adiabatic measurement (protective…
narip
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How can we only use 8192 shots for an experiment with 14 or more qubits?

Let's say you want to do an experiment with 14+ qubits. You apply some arbitrary unitary operator $U \in (\mathbb{C}^2)^{\otimes n} \times (\mathbb{C}^2)^{\otimes n}$ to the state $|\psi\rangle \in (\mathbb{C}^2)^{\otimes n} $. That is $$ U|\psi…
KAJ226
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Does the energy of a qubit change when it undergoes a collapse?

Can the state of a qubit affect its energy? Please elaborate on your answer
SAI SAGOLE
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Are projective measurement bases always orthonormal?

Are projective measurement bases always orthonormal?
ahelwer
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Elitzur-Vaidman bomb

In the original paper (Quanum Mechanical Interaction-Free Measurements - Elitzur, Vaidman, p.991), they make an algebraic substitution for the 'appearance' or 'scattering' of the bomb (green arrow): Can anyone shed any light on what this…
algo_rithm
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