Returns a circuit that prepares a multi-qubit Bell state from the zero
    state.

def ghz_circuit(qubits: Qubits) -> Circuit:
    """Returns a circuit that prepares a multi-qubit Bell state from the zero
    state.
    """
    circ = Circuit()

    circ += H(qubits[0])
    for q0 in range(0, len(qubits)-1):
        circ += CNOT(qubits[q0], qubits[q0+1])

    return circ