NVIDIA / NVIDIA/cuda-quantum

[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

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First steps

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  4. 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

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