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+"""Quantum state vectors

+

+The quantum state of :math:`n` quantum bits is represented as a 1D array of complex

+numbers of length :math:`2^n`; the components of the state vector in the

+computational basis.

+

+The computational basis for :math:`n` qubits is ordered by the number represented

+by the associated classical bitstring.

+"""

+

+import numpy as np

+

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

+

+

+def from_classical(bitstring):

+    """Return a quantum state corresponding to a classical bitstring.

+

+    Parameters

+    ----------

+    bitstring : sequence of bits

+        Can be a string like "01011", or a sequence of

+        integers.

+

+    Returns

+    -------

+    state : ndarray[(2**n,), complex]

+        The state vector in the computational basis.

+        Has :math:`2^n` components.

+    """

+

+    bitstring = "".join(map(str, bitstring))

+    n_qubits = len(bitstring)

+    try:

+        index = int(bitstring, base=2)

+    except ValueError:

+        raise ValueError("Input is not a classical bitstring") from None

+

+    state = np.zeros(1 << n_qubits, dtype=complex)

+    state[index] = 1

+    return state