Measurements

import cudaq

Kernel measurement can be specified in the Z, X, or Y basis using mz, mx, and my. If a measurement is specified with no argument, the entire kernel is measured in that basis. Measurement occurs in the Z basis by default.

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

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

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

Mid-circuit Measurement and Conditional Logic

In certain cases, it 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 ot 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 expexted.

@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))

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