NVIDIA / NVIDIA/cuda-quantum

Support cudaq.qvector for the multi-qubit custom op

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#2,484 2 comments 0 reactions 1 assignee View on GitHub

@sacpis is already working on this.

Since Mar 16, 2026.

enhancement stale-notified
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C++
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Description

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Describe the feature

Extended from ticket https://github.com/NVIDIA/cuda-quantum/issues/1475

Problem
Accepting cudaq.qvector to provide flexibility and convenience for multi-qubit custom op within a kernel.

Description
Now in v0.9.1, error: invalid number of arguments occurs when the code below is executed with cudaq.draw, cudaq.sample, etc:

num_qubits = 2
cudaq.register_operation("custom_op", np.identity(2**num_qubits))

@cudaq.kernel
def kernel(num_qubits: int):
    qubits = cudaq.qvector(num_qubits)
    custom_op(qubits)  # Doesn't work

Then we need to manually pass the qubits (or cudaq.qubit) one by one:

@cudaq.kernel
def kernel(num_qubits: int):
    qubits = cudaq.qvector(num_qubits)
    custom_op(qubits[0], qubits[1])

Same for the ctrl method if cudaq.qvector is involved:

num_qubits = 2
cudaq.register_operation("custom_op", np.identity(2**num_qubits))

@cudaq.kernel
def kernel(num_qubits: int):
    qubits = cudaq.qvector(num_qubits+1)
    custom_op.ctrl(qubits[0], qubits[1:3])  # Doesn't work

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