Measuring Kernels

Python

import cudaq

C++

#include <cudaq.h>

Kernel measurement can be specified in the Z, X, or Y basis using mz, mx, and my. Measurement occurs in the Z basis by default.

Python

@cudaq.kernel
def kernel():
    qubits = cudaq.qvector(2)
    mz(qubits)

C++

__qpu__ void kernel0() {
  cudaq::qvector qubits(2);
  mz(qubits[0]);
}

Specific qubits or registers can be measured rather than the entire kernel.

Python

@cudaq.kernel
def kernel():
    qubits_a = cudaq.qvector(2)
    qubit_b = cudaq.qubit()
    mz(qubits_a)
    mx(qubit_b)

C++

__qpu__ void kernel1() {
  cudaq::qvector qubits_a(2);
  cudaq::qubit qubits_b;
  mz(qubits_a);
  mx(qubits_b);
}

Mid-circuit Measurement and Conditional Logic

In certain cases, it is helpful for some operations in a quantum kernel to depend on measurement results following previous operations. This is accomplished in the following example by performing a Hadamard on qubit 0, then measuring qubit 0 and saving the result as b0. Then, qubit 0 can be reset and used later in the computation. In this case it is flipped to a 1. Finally, an if statement performs a Hadamard on qubit 1 if b0 is 1.

The results show qubit 0 is one, indicating the reset worked, and qubit 1 has a 75/25 distribution, demonstrating the mid-circuit measurement worked as expected.

Python

@cudaq.kernel
def kernel() -> list[bool]:
    q = cudaq.qvector(2)

    h(q[0])
    b0 = mz(q[0])
    reset(q[0])
    x(q[0])

    if b0:
        h(q[1])

    return mz(q)


from collections import Counter

results = cudaq.run(kernel, shots_count=1000)
# Convert results to bitstrings and count
bitstring_counts = Counter(
    ''.join('1' if bit else '0' for bit in result) for result in results)

print(f"Bitstring counts: {dict(bitstring_counts)}")

C++

Bitstring counts:
{
  10: 771
  11: 229
}

Output

Python

Bitstring counts: {'11': 247, '10': 753}

C++

Bitstring counts:
{
  10: 771
  11: 229
}