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():
    q = cudaq.qvector(2)

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

    if b0:
        h(q[1])


print(cudaq.sample(kernel))

C++

__qpu__ void kernel2() {
  cudaq::qvector q(2);
  h(q[0]);
  auto b0 = mz(q[0]);
  cudaq::reset(q[0]);
  x(q[0]);

  if (b0) {
    h(q[1]);
  }
}

int main() {
  auto result = cudaq::sample(kernel2);
  result.dump();
  return 0;
}

Output

Python

{ 
  __global__ : { 10:728 11:272 }
   b0 : { 0:505 1:495 }
}

C++

{
  __global__ : { 10:728 11:272 }
   b0 : { 0:505 1:495 }
}