[cpp, run] Reversed qubits in qvector when passed to cudaq::integer
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Description
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Describe the bug
cudaq::to_integer sets the most significant bit for the first qubit when run from kernel.
Steps to reproduce the bug
// RUN: nvq++ -Wl,--export-dynamic -fenable-cudaq-run --target quantinuum --emulate %s -o %t && %t
#include <cudaq.h>
#include <iostream>
__qpu__ int64_t test_to_integer() {
cudaq::qvector qubits(4);
x(qubits[0]);
return cudaq::to_integer(mz(qubits));
}
int main() {
auto resultsK = cudaq::run(2, test_to_integer);
auto resultsNK = cudaq::to_integer(std::vector<bool>{true, false, false, false});
std::cout << "Kernel: " << resultsK[0] << std::endl;
std::cout << "Non-Kernel: " << resultsNK << std::endl;
return 0;
}
// Output:
// Kernel: 8
// Non-Kernel: 1
Expected behavior
Same output for kernel and non-kernel cases
Is this a regression? If it is, put the last known working version (or commit) here.
Not a regression
Environment
- CUDA-Q version:
- Python version:
- C++ compiler:
- Operating system:
Suggestions
No response
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start with the provided C++ reproducer and compare cudaq::to_integer(mz(qubits)) in the kernel with cudaq::to_integer(std::vector) outside the kernel. Confirm that both cases return the same integer, with the first measured qubit contributing the same bit position.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- cpp
- Domain
- backend-api-design
- Issue type
- Bug
- Difficulty
- 3/5
- Estimated time
- 1-2 days
- Activity status
- Stale
- Clarity
- Clearly specified
- Newbie friendliness
- 48/100