https://davidmerwin.pieces.cloud/?p=007f4db189

Python Snippet.md

Python Snippet

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# Revised code snippet: Calculate PauliString expectation value from state vector

```python
def expectation_from_state_vector(self, state_vector: np.ndarray, qubit_map: Mapping[TKey, int]) -> float:
    """
    Evaluate the expectation value of this PauliString given a state vector.

    Args:
        state_vector: An array representing a valid state vector.
        qubit_map: A map from all qubits used in this PauliString to the
        indices of the qubits that `state` is defined over.

    Returns:
        The expectation value of the input state.
    """

    # If state_vector is 1D, reshape it to be a multidimensional tensor
    if len(state_vector.shape) == 1:
        num_qubits = state_vector.shape[0].bit_length() - 1
        state_vector = np.reshape(state_vector, (2,) * num_qubits)

    # Create a copy of the state vector
    ket = np.copy(state_vector)

    # Apply Pauli operators to the state vector
    for qubit, pauli in self.items():
        # Create a buffer to store the intermediate calculation
        buffer = np.empty(ket.shape, dtype=state_vector.dtype)
        
        # Specify the target tensor and the qubit axis for applying the unitary
        args = protocols.ApplyUnitaryArgs(
            target_tensor=ket, available_buffer=buffer, axes=(qubit_map[qubit],)
        )
        
        # Apply the Pauli operator to the state vector
        ket = protocols.apply_unitary(pauli, args)

    # Calculate the expectation value of the input state using tensor dot product
    expectation_value = np.tensordot(state_vector.conj(), ket, axes=len(ket.shape)).item()

    # Multiply the expectation value by the coefficient of the PauliString
    return self.coefficient * expectation_value

Note: The code has been revised to include clear variable names, added comments, and improved code structure to enhance human readability.

| Associated Context                         |                                          |
| ------------------------------------------ | ---------------------------------------- |
| Type | Code Snippet  ( ***.py***  )  |
| Associated Tags | `cirq` `https://github.com/davidmerwin` `http://quantumai.google/cirq` |
| Description | 'No Description Provided' |
| Related Links | [https://github.com/davidmerwin](https://github.com/davidmerwin)<br>[http://quantumai.google/cirq](http://quantumai.google/cirq) |
| Related People | [David Merwin](mailto:davidmerwin1992@gmail.com), [David Jeffrey Merwin](mailto:merwin1992@protonmail.com), [CIRQ](mailto:cirq-maintainers@googlegroups.com), [Jack C](mailto:chengzp.1@gmail.com) |
| Sensitive Information | No Sensitive Information Detected  |
| Shareable Link | https://davidmerwin.pieces.cloud/?p=007f4db189  |