Questions tagged [probability]

For questions associated with the calculation of probability, expected value, variance, standard deviation, or similar statistical quantities.

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quantum generalisation of random variables

What is the quantum information equivalent of a classical probability random variable ? Is it a density matrix or an observable ? If so can someone show me how to write a random variable that follows ...
yosh's user avatar
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1 answer
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Is there a general method for calculating expectation values for time-dependent wavefunctions?

Is there a general method for calculating expectation value? I have a workshop question, and I'm sure what a good process to follow is. It is given that $$|\psi(t = 0)\rangle = |0\rangle\,,\tag{1}$$ ...
qiclueless's user avatar
4 votes
0 answers
30 views

Kolmogorov dilations of positive definite kernels and Donsker's Delta

I'm having problems understanding a part of the proof of Kolmogorov’s dilation theorem (Theorem 3.2 given in this). We define a map $V_K:S\to H$ given by $V_K(x):=\delta_x+N$. Then, we compute the ...
Manuel E's user avatar
3 votes
2 answers
102 views

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 ...
YaGoi Root's user avatar
4 votes
1 answer
67 views

Minimum probability of measuring marked state in Grover's algorithm is 1/8

I recently came across the proposition that for a database containing $N$ elements with $m<\sqrt{N}$ marked elements, applying Grover's algorithm with any $T$ $(0<T<\sqrt{N}-1)$ iterations (...
requiemman's user avatar
2 votes
2 answers
101 views

How to compute marginal probabilities of Alice's qubit (in density operator language)?

Let $| \psi \rangle = \frac{1}{\sqrt{2}}|00\rangle + \frac{1}{2}|01\rangle + \frac{\sqrt{3}}{4} |10\rangle + \frac{1}{4}|11\rangle$ be a state vector describing a closed quantum mechanical system. ...
Physkid's user avatar
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Density matrix and State vector give different result in mesolve in QuTiP

qutip mesolve gives me different population evolve depending on that initial state is state vector or density matrix. And, in some situation, it gives me negative population. It doesn't make sense... ...
eechiki's user avatar
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1 vote
1 answer
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How to calculate probability of measuring $|1\rangle$ after application of $R_x$ gate

I was trying to understand how to calculate the probability of measuring $|1\rangle$ when executing the following circuit in Qiskit: ...
Khilesh Chauhan's user avatar
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0 answers
33 views

Saturating an inequality relating the operator norm and the total variation distance

Let $U$ be an $n$-qubit unitary, and let $p_U(x) = |\langle x | U | 0\rangle |^2$ be the probability of obtaining $x \in \{0,1\}^n$ on the all zero input. Given two $n$-qubit unitaries $U$ and $V$, it ...
trillianhaze's user avatar
2 votes
0 answers
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Given a unitary $U_p:|0\rangle\to\sum_\omega\sqrt{P(\omega)}|\omega\rangle$, what does $|0\rangle$ represent exactly?

Consider a random variable $X$ on a probability space $(\Omega, 2^\Omega, P)$. Let $H_\Omega$ be a Hilbert space with basis states ${| \omega \rangle}_{\omega \in \Omega}$, and fix a unitary $U_P$ ...
Simon's user avatar
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Improving operator norm bound on total variation distance

Let $U$ be an $n$-qubit unitary and $P_U(x) = |\langle x |U|0^n\rangle|$ the probability of measuring $x$ after acting $U$ on $|0^n\rangle$. For two $n$-qubit unitaries $U$ and $V$, one can prove that ...
trillianhaze's user avatar
0 votes
1 answer
70 views

Stabilizer States - Calculating measurement probabilities with the rank of the stabilizer table's X-block

Consider a $n$-Qubit stabilizer state $\rho = \ket{\psi}\bra{\psi}$ and its $n \times 2n$ boolean stabilizer tableau. Any Stabilizer State can be expressed as an equally weighted superposition $$ \ket{...
Coryn7's user avatar
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1 vote
2 answers
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Confusion on the probability of measuring first qubit of a separable mixed state

Let $\rho = \sum_{x \in \{0,1\}^n} P_x |x \rangle\langle x|$ be a separable mixed state over bit strings $x$ of size $n$. Suppose also that $U = U_1 \otimes \cdots \otimes U_n$ is a product of local ...
trillianhaze's user avatar
1 vote
1 answer
286 views

How do you find the possible measurement values of an observable?

$\newcommand{\ket}[1]{\left|#1\right>}$ Note: I considered posting this as an update to a prior question, but it seemed like it should be it's own post. So this is a very basic question, but one I'...
quantumstudent's user avatar
-2 votes
2 answers
94 views

Why are probabilities represented with alpha^2 and beta^2?

To preserve the Complementary Rule of probability, the sum of the probabilities of the outcomes (measured |0> or measured |1>) must equal 1 or 100%. That's why alpha^2+beta^2=1. However, why the ...
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1 vote
1 answer
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How much of quantum computing is based on probability?

I've recently discovered an interest in quantum computing and technology. Essentially, this means that I am trying to learn as much as I possibly can, one question at a time. I have heard that quantum ...
Logan's user avatar
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2 votes
1 answer
87 views

Bounding operator norm by total variation distance

Let $P_U(y \mid x) = |\langle y | U | x \rangle|^2$ denote the probability distribution of obtaining the bitstring $y \in \{0,1\}^n$ on a fixed input $x \in \{0,1\}^n$ w.r.t. the unitary $U$. For $n$-...
trillianhaze's user avatar
4 votes
0 answers
126 views

Probability inequality for Quantum Approximate Optimization Algorithm (QAOA)

In arXiv:2207.14734 the authors claim that it is "straightforward to show that" their equation 8 holds: $$\mathrm{Pr}_{x\sim q}[x:f(x)\geq \mu] \geq \frac{1}{M}$$ where we have an objective ...
Juri V's user avatar
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1 answer
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An inequality involving quantum channels

Consider two quantum circuits $\mathsf{C}$ and $\mathsf{D}$ applied to $|0^n\rangle$. Then, measure in the standard basis and, for $x \in \{0, 1\}^n$, consider two probabilities: \begin{equation} p_{x,...
BlackHat18's user avatar
  • 1,251
7 votes
1 answer
159 views

Generating random, but non-uniform state

I would like an algorithm that generates a random state, sampled according to some probability distribution which is not uniform in Hilbert space. Assume though that I have at my disposal a uniform (...
nervxxx's user avatar
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3 votes
1 answer
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Independence in state prepared by independently drawn Haar random gates

Consider independently drawn $2 \times 2$ Haar random unitaries $U_1, U_2, \ldots, U_n$ and $$V = U_1 \otimes U_2 \otimes \cdots U_n.$$ Consider the state $\sigma$ given by $$\sigma = V \rho V^{*}, $$ ...
BlackHat18's user avatar
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2 votes
3 answers
175 views

How to get exact measurement probabilities when having intermediate measurements with Qiskit?

Suppose we have a circuit with two qubits, A and B. Both are initialized to $|0\rangle$. Over qubit A we apply a single rotation gate (e.g. $R_y$) with an angle given by $x_0$, and then we entangle ...
dviqu's user avatar
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2 votes
2 answers
113 views

Computing a ratio involving Haar random unitaries

Consider an $n$-qubit Haar random unitary $U$. I am trying to compute the expression \begin{equation} \mathbb{E}\left[ \frac{\text{Tr}\left(|0^n\rangle \langle 0^n | ~U\rho U^*\right)}{\text{Tr}\left(\...
BlackHat18's user avatar
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5 votes
2 answers
163 views

Simultaneous measurements and Bell basis measurements to estimate $\lvert\text{Tr}(\sigma \rho)\rvert^2$ in Huang et al. paper

Theorem 2 of this paper says if one is able to prepare $\rho^{\otimes k}$ then it is possible to predict expectation values of all $n$-qubit Pauli observables using $O(n)$ number of copies of $\rho$. ...
user8183310's user avatar
1 vote
0 answers
54 views

How to implement the Mach Zehnder Interferometer in Qutip?

I was trying to implement the Mach-Zehnder Interferometer with a phase shifter in Qutip but I couldn't nail it. I just want to give two number states as input and at the end see the probability ...
can kanaroğlu's user avatar
1 vote
0 answers
48 views

Close in operator norm imply close in weak multiplicative sense?

Fix $\epsilon > 0$, and suppose $U$ and $S$ are $n$ qubit unitaries such that $\| U - S \| \leq \epsilon$ (operator norm). Furthermore, let $P_U(y \mid x) = |\langle y | U | x \rangle|^2$ be the ...
trillianhaze's user avatar
2 votes
1 answer
70 views

Conditional expectation for Haar random states

Let $U$ be an $n$ qubit Haar random circuit applied to $|0^n \rangle$. Thereafter, the state is measured in the standard basis. Let $p_0$ be the probability of getting $0$ in the first qubit. We know ...
BlackHat18's user avatar
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1 vote
0 answers
75 views

I am optimising a variational quantum circuit to learn a distribution $p(x)$, but it doesn't converge over a training set $\mathcal{X}$?

I am training a variational quantum circuit to learn distributions: given data $s(\vec{\lambda})$, what is the probability distribution for the parameterisation $\vec{\lambda}$, i.e. the posterior ...
JoJo's user avatar
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2 votes
0 answers
96 views

Distribution of partial trace of Haar unitary

I am sure this must have been covered in the mathematical literature, but hoping someone can direct me to the right place. Let us be given random unitaries $U$ on $n$ qubits (so dimension of the space ...
nervxxx's user avatar
  • 540
0 votes
1 answer
64 views

Survival probability quantum circuit

Suppose say that I have a quantum state $\vert\psi\rangle$ at time $t = 0$, which is now evolved by a hamiltonian $H$ $$e^{-iHt}\vert\psi\rangle$$. I can ask the question, how much of initial state is ...
FearlessVirgo's user avatar
1 vote
1 answer
62 views

Sum of probability in non-orthogonal basis

On standard basis, the sum of the probability of a vector $\newcommand\ket[1]{\left|#1\right\rangle}\ket{v} = a \ket{0} + b \ket{1}$ is $a^2 + b^2 = 1$, right? What about the two states of the basis ...
Gabe Ebag's user avatar
-1 votes
1 answer
91 views

Why is $|P_0- P_1|=1$?

I have a question we have $ |0 \rangle, |1 \rangle, |+ \rangle$ and $|- \rangle, $ defined as usual. Let $P_0$= probability that a state be in 0, $P_+$= probability that a state be in +, and same ...
user206904's user avatar
1 vote
1 answer
108 views

How to write post-measurement states when the measurement apparatus measures one of two observables?

If I want to measure an observable $A$ but the measurement apparatus has $(1-p)$ probability of measuring the observable $B$ and probability $p$ that a measurement of $A$ would be done. So how can I ...
username9's user avatar
0 votes
1 answer
46 views

Cannot derive probability graph for Hadamard gate given in Qiskit textbook

I am reading the Qiskit textbook(beta) and they have explained Hadamard gate using an amplitude tree. To show how two H-gates on a qubit give the output as 0 everytime they said to consider that it ...
Rai's user avatar
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3 votes
1 answer
99 views

How do I get correct measurement probabilities in ZX calculus?

I'm learning ZX-calculus, but I'm getting confused when trying to obtain some simple results to compute probabilities for different outcomes. Here's a simple example where I'm getting lost. Here, <...
jjgoings's user avatar
  • 221
1 vote
1 answer
50 views

Formal measurement of the probability of an outcome of a Qubit in the hadamard base

I am fairly new to Quantum Computing and have a question which might be trivial for all of you, but I am really struggling with it. From Quantum Computation and Quantum Information I have learned, ...
dribble290's user avatar
0 votes
1 answer
139 views

Exact Probabilities of Outcomes for Clifford Circuits with Mid-Circuit Measurements Using Stim

I am trying to find the exact probabilities of specific measurement outcomes for Clifford circuits with mid-circuit measurements. Essentially, I am looking for a function that takes an arbitrary ...
user206444's user avatar
4 votes
0 answers
36 views

Is there a general framework that allows us to compare probabilistic and deterministic algorithms fairly?

Many popular QC algorithms are probabilistic in nature, like Grover's, Shor's, QAOA ..etc For some of these we have formulas that give probabilities of success (like for Grover's and Shor's), and for ...
bubakazouba's user avatar
1 vote
0 answers
128 views

Finding the Exact Probability Distribution for the Outcomes of a Quantum Circuit with Mid-Circuit Measurements

I would like to find the exact probabilities of the possible outcomes of a circuit that includes mid-circuit measurements. So, as a specific example, consider the following circuit: I would like to ...
user206444's user avatar
2 votes
0 answers
101 views

Distinguishing $n$ pure states in an $n$ dimensional Hilbert space

Suppose we have $n$ pure states in an $n$ dimensional Hilbert space, and we would like to distinguish them using POVM or PVM. We get any one of the pure states with equal probability, and we may set ...
Stan's user avatar
  • 21
0 votes
1 answer
63 views

What is the actual probability of not losing information (in a depolarizing channel)

The probability that a depolarizing channel doesn't affect the information is usually assumed to be $1-3p$, while, for convenience, it is affected with same probability $p$ by any Pauli operator $X,Y,...
Daniele Cuomo's user avatar
-2 votes
1 answer
126 views

Probability outcome $0$? Post measurement state?

Does anyone know how to solve this exercise? Here is the question: Let $|\psi\rangle$ be an arbitrary pure $n$-qubit state, i.e. $$|\psi\rangle=\sum_{x_1,\ldots,x_n=0,1}\alpha_{x_1\cdots x_n}|x_1\...
Rodrigo's user avatar
4 votes
3 answers
388 views

How do we achieve mathematically that the probability that Eve learns $x$ is $\cos^{2}\left ( \frac{\pi }{8} \right )$?

I'm trying to understand this problem. Alice I attempting to send a 2 classical bit message to Bob using 1 qubit such that there are 4 states $\varphi_{00}$ $\varphi_{01}$ $\varphi_{10}$ $\varphi_{11}$...
Vishakha Lall's user avatar
5 votes
1 answer
81 views

What is the quantum analogue of $P_{XY} = P_{Y|X}P_X$

A standard trick in probability manipulation is to take some joint distribution $P_{XY}$ and express it as $P_{Y|X}P_X$. This trick is useful because when one looks at things like the ratio of $\frac{...
user1936752's user avatar
  • 2,597
3 votes
1 answer
50 views

Relating upper bound on the total variation distance with a bound on a Pauli observable

Consider an $n$ qubit state $|\psi\rangle$. Let us say we have the following upper bound on the total variation distance between the distribution generated by measuring $|\psi\rangle$ in the standard ...
BlackHat18's user avatar
  • 1,251
1 vote
1 answer
232 views

Implementing Quantum Walks at IBM

a question about quantum walks, would this circuit be correct to start a quantum walk in a hypercube? I saw something about increment and decrement, but I didn't quite understand how they would work ...
Ikky R's user avatar
  • 79
0 votes
1 answer
144 views

probability and post-measurement state with observable 𝐼⊗𝑋

Today I learned about these but I can't know how to solve this probem. $|𝜓⟩={\frac1 {√3}}(|01⟩+|10⟩+|11⟩) $ I want to measure observable 𝐼⊗𝑋 . What is the probability of measuring |+⟩ on the ...
user19382's user avatar
4 votes
1 answer
115 views

Question regarding the output probability of a quantum circuit

Consider a quantum circuit $\text{Q}$, run on $|0^{n}\rangle$. For a specific $x \in \{0, 1\}^{n}$, let's say we are interested in the probability $$p_x = |\langle x|~\text{Q}~|0^{n}\rangle|^{2}.$$ ...
Tom Clancy's user avatar
4 votes
1 answer
229 views

Why is sampling from probability distributions generated by specific quantum circuits classically intractable?

I was reading a paper by Benedetti et al. titled Parameterized quantum circuits as machine learning models. Its authors state the following: We also know that sampling from the probability ...
karolyzz's user avatar
  • 269
1 vote
1 answer
45 views

Statistical 'process' behind quantum gates

Thinking about gate creating entangled state, it looks to me that the inputs to the gate look like marginal/independent distributions, and the output looks like their joint distribution. Does this ...
sitems's user avatar
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