Newest Questions
13,091 questions
3
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views
How to construct a block-diagonal embedded Fourier matrix?
We know the quantum circuit representation of Fourier operator. Now, I want the quantum circuit for the following unitary matrix using single and two qubit gates:
$$U =
\begin{pmatrix}
F_3 & 0 \\
...
- 31
1
vote
2
answers
52
views
QFT of two superposed states
Given a N qubits quantum register, in state $1/\sqrt{2}(|a\rangle+|b\rangle)$, my goal is to find sum of a and b (a+b) or subtraction (b-a).
Taking classical example, let [7, 13], the discret FT will ...
- 491
2
votes
1
answer
108
views
stim : confusion about the use of observables
The role of adding observables is not clear to me. Here are some points of confusion :
(1)From the circuit I generate a detector error model using dem from which I generate two
matrices (pcm and lcm) ...
- 2,984
1
vote
1
answer
75
views
Drawbacks of writing a compiler in Smalltalk
Sorry for the vague title, what I’m really asking is: when writing a compiler for a quantum DSL, many languages have chosen an imperative or functional host language. Few, if any, have chosen a purely ...
- 13
0
votes
0
answers
25
views
Do identical reduced system trajectories ρₛ(t) guarantee identical recoverability in small open quantum systems?
In a small open quantum system with a system qubit $S$ and an environment split into an accessible fragment $F$ and an inaccessible remainder $R$, consider the following situation.
Two global ...
0
votes
0
answers
47
views
Using restarts to prove QMA in PP
I am going through Ryan O'Donnell's quantum computing class homeworks, and on problem 6 of this assignment you are asked to prove $QMA$ in $PP$ using the fact that $RestartingPP = PP$, where $...
0
votes
0
answers
55
views
What is the biggest pain point for quantum developers? [closed]
I'm working on building an AI platform to streamline/automate the workflow for quantum developers. I was curious to learn from those who are actively involved in the field (building circuits, writing ...
1
vote
0
answers
30
views
Why have my Qiskit VQE results changed, even though I haven’t changed anything?
I am computing the 1-RDM of the H₂ molecule (STO-3G, 0.735 Å) using VQE in Qiskit + Qiskit Nature. A few months ago, when I ran the exact same code on the same laptop, the spin-summed 1-RDM always had ...
- 207
0
votes
1
answer
26
views
Definition of lg in IBM quantum learning resources
In its discussion of Shor's algorithm IBM's quantum learning platform uses the definition
$$
n = \lg(N-1) = \lfloor{\log(N-1)}\rfloor+1
$$
to describe the number of bits needed to encode the value N-1....
- 145
1
vote
2
answers
39
views
How to show used qubits in a transpiled circuit in Qiskit?
If a QuantumCircuit is created in Qiskit and then compiled using transpile the returned circuit has ...
- 23
0
votes
0
answers
49
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Understanding theorem 6 of postBQP=PP paper
In this paper, $\mathsf{BQP}_p$ is defined to be similar to $\mathsf{BQP}$ in all aspects except that the Born's measurement rule is changed. For a normalised state
$$|{\psi}\rangle =\sum_x \alpha_x |...
- 953
1
vote
3
answers
166
views
Minimal ancilla-free CCCNOT (C³X) decomposition using controlled-S conjugation
While experimenting in Quirk, I found a very compact and fully functional decomposition of the three-controlled NOT gate (also known as CCCNOT or C³X).
This construction requires no ancilla qubits and ...
1
vote
0
answers
48
views
In Google Stim, is there a separate algorithm for handling measurement errors that occur before the final round?
Based on the paper “Sparse Blossom: Correcting a Million Errors per Core Second with Minimum-Weight Matching,” my understanding is that the MWPM decoder performs minimum-weight matching between the ...
- 15
0
votes
0
answers
29
views
Is operator entanglement entropy related to a Choi state?
$\newcommand{\kket}[1]{|#1\rangle\!\rangle}
\newcommand{\bbra}[1]{\langle\!\langle #1|}
\newcommand{\hilbert}{\mathcal{H}}
\newcommand{\ket}[1]{|#1\rangle}
$
I'm interested in an intrinsic notation ...
- 1
1
vote
1
answer
65
views
Confirm $f \otimes g$ is a linear map
In the textbook, "Dancing with Qubits," it states:
"Let $V, W, X,$ and $Y$ be finite dimensional vector spaces over $\mathbb{F}$. If $f: V \rightarrow X$ and $g: W \rightarrow Y$ are ...
- 391
0
votes
0
answers
18
views
Methods for generating Random Tensor Network and trace diagrams
I’m looking for effective ways to draw Random Tensor Networks and trace diagrams that show tensor contractions and Haar averages clearly. The goal is to generate clean, editable visuals where tensors ...
- 324
1
vote
1
answer
70
views
Stabilizer flow details when using detslice figures
I only have a rough idea that this is a back propagations of the pauli flow. But some details really confuse me.
What's the difference between the filled and hollow circles. For example, tick 96 we a ...
- 31
0
votes
1
answer
49
views
Asymptotic scaling of surface codes, why p and not 2p/3?
In Fowler's paper, "Surface codes: Towards practical large-scale quantum computation" (link), a rudimentary probabilistic model for logical errors is described. My only issue with this is ...
- 41
1
vote
0
answers
38
views
Help debugging flat dissociation curve from IBM Quantum Runtime Batch
I've been working on a VQE implementation to calculate the dissociation curve of the H₂ molecule using Qiskit. The simulated curve (using Aer) looks great and behaves as expected. However, when I ...
- 11
0
votes
1
answer
49
views
Is there a way to know the propagated errors at a certain place of a circuit in Stim?
What I want to realize in Stim is:
Given a circuit, errors are sampled. Then, at a certain place of a circuit (e.g., qubit 6 after a certain CNOT gate), I want to know the errors there if we propagate ...
- 301
0
votes
0
answers
84
views
+50
Converting qubit eigenstate to pyscf eigenstate
I want to convert a PySCF wavefunction in terms of CIstrings to a representation of a qubit state after Jordan-Wigner transformation of the Hamiltonian.
Qiskit-nature keeps the molecular ordering of ...
- 53
2
votes
1
answer
46
views
What is AIRouting doing step by step?
I am trying to understand how AIRouting tool from qiskit is optimal for routing. Therefore I am trying to understand its choice of swap/ie the sequence of action applied. I feed a random cnot gates ...
- 21
0
votes
1
answer
108
views
stim: understanding DEM probability calculations
I'm trying to understand how the probabilities in a stim DEM are calculated.
I tried three simple cases; I understand the first, but can't seem to go beyond that :
case 1: I understand the ...
- 2,984
0
votes
0
answers
86
views
Can Schmidt decomposition geometry predict quantum computational advantage?
The Problem
Predicting which quantum systems will achieve computational advantage remains unsolved. Traditional metrics (gate fidelity, coherence times, quantum volume) achieve only 57-74% ...
0
votes
0
answers
30
views
Measurement-induced dephasing rate: factor of 2 discrepancy in two published papers
I am studying measurement-induced dephasing in the dispersive regime of circuit QED.
In the paper Yan et al., Nat. Commun. 7, 12964 (2016), equation (5), they use the expression
$$
\Gamma_\phi = \bar{...
- 11
0
votes
0
answers
24
views
Why there is no difference between PES of hydrogen molecule and hydrogen molecular cation?
I'm using GroundStateEigensolver to calculate Potential Energy Line for hydrogen molecular cation. But the results are the same with that of hydrogen molecule. I'm using following code:
...
- 63
0
votes
0
answers
35
views
Drop‑in replacement for Monte Carlo using Quantum Amplitude Estimation (Qiskit). Looking for feedback & critique
I’m an independent enthusiast exploring quantum algorithms. I want to share a personal idea/hypothesis and get honest feedback. I’m not claiming I’ve found “the truth” — I’m trying to understand ...
2
votes
3
answers
523
views
Can we know if two subsystems are entangled by only looking at one of them?
Today I bring a question similar to other posted before(i.e. How to know if any two arbitrary qubits are entangled?, How do I show that a two-qubit state is an entangled state?) but not exactly equal.
...
- 164
2
votes
1
answer
93
views
Semi-Auto Magic State
I'm currently reading this paper. The figure shown below performs bit-wise addition and then uncomputes the result. Specifically, it computes $s_{k} = ((a_{k-1} \oplus s_{k-1}) \cdot (b_{k-1} \oplus ...
- 23
0
votes
0
answers
36
views
Reproducing the PBR theorem on IBM hardware: is the wavefunction “real” even on noisy qubits?
I’ve been exploring whether the quantum state (ψ) represents information or reality by trying to replicate the Pusey–Barrett–Rudolph (PBR) test in Qiskit.
The recent preprint Yang, Yuan & Barnes (...
- 1,371
0
votes
1
answer
66
views
How to Implement Y Flows in Lattice Surgery CX?
I am currently trying to implement the Y-Flows for the Lattice Surgery CX and having trouble understanding what MPP measurements I need for what logical flows and how I might realize them.
...
- 324
2
votes
1
answer
116
views
Is Extropic's "thermodynamic computing" just a classical stochastic computer dressed in quantum buzzwords?
I was looking at this new company Extropic and their claims (Video, Paper).
They claim they are using thermodynamics to keep a bit in a "superposition" of 0 and 1, but then explicitly state ...
1
vote
1
answer
42
views
qiskit pVQD: problems with parameter optimization of the ansatz
I am facing a problem with the PVQD algorithm from qiskit algorithms. I am following the pVQD qiskit tutorial, which works fine on my device.
Specifically, the ansatz circuit in the tutorial is ...
1
vote
1
answer
61
views
How does the surface code protect against correlated errors acting on two logical qubits?
In the surface code, each logical qubit is typically encoded in a separate patch of physical qubits. The code can correct up to ⌊(d − 1)/2⌋ errors on a single logical qubit patch, where d is the code ...
- 11
0
votes
0
answers
43
views
Does "Peaked quantum advantage using error correction" support public verification of quantumness?
Peaked quantum advantage using error correction by Deshpande et al. (arXiv link) is a recent, October 2025 entry in the program of designing interactive proofs of quantumness. The proposal combines ...
- 16.3k
0
votes
1
answer
52
views
Quantum Phase Estimation
Implement the Quantum Phase Estimation (QPE) algorithm using the unitary operator S and the eigenstate
∣1⟩ as the target qubit. Use 3 counting qubits and 1 target qubit.
Assumptions / notation:
Assume ...
- 11
3
votes
0
answers
125
views
How to implement $U|i\rangle=|2^i\rangle$ without ancilla qubits?
I want to implement a unitary operation $U$ on $n$ qubits, which performs the following mapping on computational basis states:
$$U|i\rangle = |2^i\rangle,\quad \forall i \in [0, n-1]$$
When $i\geq n$, ...
- 313
1
vote
0
answers
78
views
Preparing a logical state of a stabilizer code in qiskit
I'm using qiskit to study some aspects of a simple error correcting (stabilizer) code. I have a code which works for particular simple examples, but I am struggling to generalize the preparation of ...
- 173
3
votes
3
answers
187
views
Why do we only need to correct for diagonal Pauli errors (X, Z)?
Why do we only need to account for Pauli X and Z errors for full QEC?
The basis of quantum error correction is that we only need to correct for X and Z errors. We can choose Paulis as a basis set for ...
- 53
0
votes
0
answers
35
views
Formulating Semidefinite Programming Constraints for Probabilistic Quantum Channel Simulation under Coherence Resource Constraints
I am developing a theoretical framework that conceptualizes quantum coherence as an entropy-coupled flow with distinct scalar and radiative coherence modes, aiming to unify coherence phenomena in ...
1
vote
1
answer
170
views
Propagation of Clifford in `stim`
From what I understand, errors are simulated in stim using Pauli frames representing Pauli errors that propagate through the circuit (without signs). That way, we ...
- 712
3
votes
1
answer
46
views
Clarification on "Bob must do the same thing to every pair of electrons" in Quantum Computing for Everyone (Superdense Coding)
I am reading $\textit{Quantum Computing for Everyone}$ by Chris Bernhardt and I came across the following passage in the section on $\textbf{Superdense coding}$:
Alice and Bob initially have one ...
- 241
1
vote
0
answers
25
views
Transforming from non-additive to stabilizer code
Is there a way to code-switch from non-additive codes to stabilizer codes, such as surface codes?
If so, is it always possible or is it only possible in certain cases?
- 33
1
vote
1
answer
87
views
3 Bit Quantum Gate
Build a three-qubit quantum circuit that determines whether two input qubits have the same value (i.e., both are ∣0⟩ or both are ∣1⟩). Use the first two qubits as the input bits and the third qubit as ...
- 11
0
votes
1
answer
63
views
Unable to generate parity check matrices with generator matrices for CSS-T Code
In this paper, the authors provide a method for constructing qLDPC CSS-T codes. They provide example generator matrices for the binary linear codes $C_1$ and $C_2$ such that $Q(C_1, C_2)$ is a CSS-T ...
- 321
2
votes
1
answer
76
views
Ignoring Detectors with PyMatching and Stim
I'm trying to implement a heralded-erasure-based surface code. In order to perform heralded 2-qubit gates, I used Craig Gidney's workaround, which essentially involves a string of possible correlated ...
- 41
0
votes
0
answers
25
views
How to choose the initial state of the reverse trajectory when applying fluctuation theorems and calculating entropy production?
In the literature on fluctuation theorems and stochastic entropy production, I often find different choices for the initial state of the reverse trajectory. This choice seems to depend on how the ...
3
votes
0
answers
52
views
What are standard syndrome extraction circuits for 4.8.8. color codes?
I want to simulate a quantum memory gadget for a 4.8.8. color code, as defined in Fig. 2(a) of Ref.[1], and I am looking for useful candidates for syndrome extraction circuits. Ideally, some that are ...
- 197
3
votes
1
answer
116
views
A question about CSS codes
Consider an $[[n, k, d]]$ quantum CSS code with $C^{\perp} \subset C$. If I consider the state,
\begin{equation}
\frac{1}{\sqrt{|C^{\perp}|}} \sum_{x \in C^{\perp}} |x\rangle;
\end{equation}
is this a ...
- 1,629
1
vote
1
answer
92
views
Logical membrane tracking for logical phase gate on surface code patch
In the article https://arxiv.org/pdf/2112.12160 there are many gadgets defined to implement the logical gates on a surface code patch under the CBQC settings. I am trying to implement the logical ...
- 37