text: #   Licensed under the Apache License, Version 2.0 (the "License");
  #   you may not use this file except in compliance with the License.
  #   You may obtain a copy of the License at
  #
  #       http://www.apache.org/licenses/LICENSE-2.0
  #
  #   Unless required by applicable law or agreed to in writing, software
  #   distributed under the License is distributed on an "AS IS" BASIS,
  #   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  #   See the License for the specific language governing permissions and
  #   limitations under the License.
  """Gates that target four qubits."""
  
  from typing import Optional, Union
  
  import numpy as np
  import sympy
  
  import cirq
  from cirq._compat import proper_repr
  
  
  class DoubleExcitationGate(cirq.EigenGate):
      """Evolve under ``-|0011⟩⟨1100|`` + h.c. for some time."""
  
      def __init__(
              self,
              *,  # Forces keyword args.
              exponent: Optional[Union[sympy.Symbol, float]] = None,
              rads: Optional[float] = None,
              degs: Optional[float] = None,
              duration: Optional[float] = None) -> None:
          """Initialize the gate.
  
          At most one of exponent, rads, degs, or duration may be specified.
          If more are specified, the result is considered ambiguous and an
          error is thrown. If no argument is given, the default value of one
          half-turn is used.
  
          Args:
              exponent: The exponent angle, in half-turns.
              rads: The exponent angle, in radians.
              degs: The exponent angle, in degrees.
              duration: The exponent as a duration of time.
          """
  
          if len([1 for e in [exponent, rads, degs, duration] if e is not None
                 ]) > 1:
              raise ValueError('Redundant exponent specification. '
                               'Use ONE of exponent, rads, degs, or duration.')
  
          if duration is not None:
              exponent = 2 * duration / np.pi
          else:
              exponent = cirq.chosen_angle_to_half_turns(half_turns=exponent,
                                                         rads=rads,
                                                         degs=degs)
  
          super().__init__(exponent=exponent)
  
      def num_qubits(self):
          return 4
  
      def _eigen_components(self):
          minus_one_component = np.zeros((16, 16))
          minus_one_component[3, 3] = minus_one_component[12, 12] = 0.5
          minus_one_component[3, 12] = minus_one_component[12, 3] = -0.5
  
          plus_one_component = np.zeros((16, 16))
          plus_one_component[3, 3] = plus_one_component[12, 12] = 0.5
          plus_one_component[3, 12] = plus_one_component[12, 3] = 0.5
  
          return [(0, np.diag([1, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1, 1, 0, 1, 1, 1])),
                  (-1, minus_one_component), (1, plus_one_component)]
  
      def _apply_unitary_(self,
                          args: cirq.ApplyUnitaryArgs) -> Optional[np.ndarray]:
          if cirq.is_parameterized(self):
              return None
          inner_matrix = cirq.unitary(cirq.rx(-2 * np.pi * self.exponent))
          a = args.subspace_index(0b0011)
          b = args.subspace_index(0b1100)
          return cirq.apply_matrix_to_slices(args.target_tensor,
                                             inner_matrix,
                                             slices=[a, b],
                                             out=args.available_buffer)
  
      def _with_exponent(self, exponent: Union[sympy.Symbol, float]
                        ) -> 'DoubleExcitationGate':
          return DoubleExcitationGate(exponent=exponent)
  
      def _decompose_(self, qubits):
          p, q, r, s = qubits
  
          rq_phase_block = [cirq.Z(q)**0.125, cirq.CNOT(r, q), cirq.Z(q)**-0.125]
  
          srq_parity_transform = [
              cirq.CNOT(s, r), cirq.CNOT(r, q),
              cirq.CNOT(s, r)
          ]
  
          phase_parity_block = [[
              rq_phase_block, srq_parity_transform, rq_phase_block
          ]]
  
          yield cirq.CNOT(r, s)
          yield cirq.CNOT(q, p)
          yield cirq.CNOT(q, r)
          yield cirq.X(q)**-self.exponent
          yield phase_parity_block
  
          yield cirq.CNOT(p, q)
          yield cirq.X(q)
          yield phase_parity_block
          yield cirq.X(q)**self.exponent
          yield phase_parity_block
          yield cirq.CNOT(p, q)
          yield cirq.X(q)
  
          yield phase_parity_block
          yield cirq.CNOT(q, p)
          yield cirq.CNOT(q, r)
          yield cirq.CNOT(r, s)
  
      def _circuit_diagram_info_(self, args: cirq.CircuitDiagramInfoArgs
                                ) -> cirq.CircuitDiagramInfo:
          if args.use_unicode_characters:
              wire_symbols = ('⇅', '⇅', '⇵', '⇵')
          else:
              # pylint: disable=anomalous-backslash-in-string
              wire_symbols = (r'/\ \/', r'/\ \/', '\/ /\\', '\/ /\\')
          return cirq.CircuitDiagramInfo(wire_symbols=wire_symbols,
                                         exponent=self._diagram_exponent(args))
  
      def __repr__(self):
          if self.exponent == 1:
              return 'openfermion.DoubleExcitation'
          return '(openfermion.DoubleExcitation**{})'.format(
              proper_repr(self.exponent))
  
  
  DoubleExcitation = DoubleExcitationGate()
  