@@ -1,3 +1,34 @@

 """Quantum measurements"""

-__all__ = []  # type: ignore

+from typing import Tuple

+

+import numpy as np

+

+from . import operator

+from .state import normalize, num_qubits

+

+__all__ = ["measure"]  # type: ignore

+

+

+def measure(

+    rng: np.random.RandomState, qubit: int, state: np.ndarray

+) -> Tuple[bool, np.ndarray]:

+    """Measure the given qubit in the computational basis."""

+    nqs = num_qubits(state)

+    if qubit >= nqs:

+        raise ValueError("Cannot measure qubit {qubit} of an {nqs}-qubit state.")

+

+    proj_1 = operator.apply(m_1, [qubit], state)

+

+    p_1 = state.conj() @ proj_1

+

+    if rng.random() < p_1:

+        return (True, normalize(proj_1))

+    else:

+        proj_0 = operator.apply(m_0, [qubit], state)

+        p_0 = 1 - p_1

+        return (False, normalize(proj_0))

+

+

+m_1 = np.array([[0, 0], [0, 1]], complex)

+m_0 = np.array([[1, 0], [0, 0]], complex)