Add FlipBits gate (#289)

docs/projectq.ops.rst

@@ -52,6 +52,7 @@ The operations collection consists of various default gates and is a work-in-pro
 	projectq.ops.UniformlyControlledRy
 	projectq.ops.UniformlyControlledRz
 	projectq.ops.StatePreparation
+	projectq.ops.FlipBits
 
 
 Module contents

projectq/ops/_gates.py

@@ -44,7 +44,6 @@
                       FastForwardingGate,
                       BasicMathGate)
 from ._command import apply_command
-from projectq.types import BasicQubit
 
 
 class HGate(SelfInverseGate):
@@ -338,3 +337,56 @@ def get_inverse(self):
 
 #: Shortcut (instance of) :class:`projectq.ops.BarrierGate`
 Barrier = BarrierGate()
+
+
+class FlipBits(SelfInverseGate):
+    """ Gate for flipping qubits by means of XGates """
+    def __init__(self, bits_to_flip):
+        """
+        Initialize FlipBits gate.
+
+        Example:
+            .. code-block:: python
+
+                qureg = eng.allocate_qureg(2)
+                FlipBits([0, 1]) | qureg
+
+        Args:
+            bits_to_flip(list[int]|list[bool]|str|int): int or array of 0/1,
+               True/False, or string of 0/1 identifying the qubits to flip.
+               In case of int, the bits to flip are determined from the
+               binary digits, with the least significant bit corresponding
+               to qureg[0]. If bits_to_flip is negative, exactly all qubits
+               which would not be flipped for the input -bits_to_flip-1 are
+               flipped, i.e., bits_to_flip=-1 flips all qubits.
+        """
+        SelfInverseGate.__init__(self)
+        if isinstance(bits_to_flip, int):
+            self.bits_to_flip = bits_to_flip
+        else:
+            self.bits_to_flip = 0
+            for i in reversed(list(bits_to_flip)):
+                bit = 0b1 if i == '1' or i == 1 or i is True else 0b0
+                self.bits_to_flip = (self.bits_to_flip << 1) | bit
+
+    def __str__(self):
+        return "FlipBits("+str(self.bits_to_flip)+")"
+
+    def __or__(self, qubits):
+        quregs_tuple = self.make_tuple_of_qureg(qubits)
+        if len(quregs_tuple) > 1:
+            raise ValueError(self.__str__()+' can only be applied to qubits,'
+                             'quregs, arrays of qubits, and tuples with one'
+                             'individual qubit')
+        for qureg in quregs_tuple:
+            for i, qubit in enumerate(qureg):
+                if (self.bits_to_flip >> i) & 1:
+                    XGate() | qubit
+
+    def __eq__(self, other):
+        if isinstance(other, self.__class__):
+            return self.bits_to_flip == other.bits_to_flip
+        return False
+
+    def __hash__(self):
+        return hash(self.__str__())

projectq/ops/_gates_test.py

@@ -19,9 +19,10 @@
 import numpy as np
 import pytest
 
-from projectq.ops import (get_inverse, SelfInverseGate, BasicRotationGate,
-                          ClassicalInstructionGate, FastForwardingGate,
-                          BasicGate)
+from projectq import MainEngine
+from projectq.ops import (All, FlipBits, get_inverse, SelfInverseGate,
+                          BasicRotationGate, ClassicalInstructionGate,
+                          FastForwardingGate, BasicGate, Measure)
 
 from projectq.ops import _gates
 
@@ -217,3 +218,78 @@ def test_barrier_gate():
     assert str(gate) == "Barrier"
     assert gate.get_inverse() == _gates.BarrierGate()
     assert isinstance(_gates.Barrier, _gates.BarrierGate)
+
+
+def test_flip_bits_equality_and_hash():
+    gate1 = _gates.FlipBits([1, 0, 0, 1])
+    gate2 = _gates.FlipBits([1, 0, 0, 1])
+    gate3 = _gates.FlipBits([0, 1, 0, 1])
+    assert gate1 == gate2
+    assert hash(gate1) == hash(gate2)
+    assert gate1 != gate3
+    assert gate1 != _gates.X
+
+
+def test_flip_bits_str():
+    gate1 = _gates.FlipBits([0, 0, 1])
+    assert str(gate1) == "FlipBits(4)"
+
+
+def test_error_on_tuple_input():
+    with pytest.raises(ValueError):
+        _gates.FlipBits(2) | (None, None)
+
+
+flip_bits_testdata = [
+    ([0, 1, 0, 1], '0101'),
+    ([1, 0, 1, 0], '1010'),
+    ([False, True, False, True], '0101'),
+    ('0101', '0101'),
+    ('1111', '1111'),
+    ('0000', '0000'),
+    (8, '0001'),
+    (11, '1101'),
+    (1, '1000'),
+    (-1, '1111'),
+    (-2, '0111'),
+    (-3, '1011'),
+]
+
+
+@pytest.mark.parametrize("bits_to_flip, result", flip_bits_testdata)
+def test_simulator_flip_bits(bits_to_flip, result):
+    eng = MainEngine()
+    qubits = eng.allocate_qureg(4)
+    FlipBits(bits_to_flip) | qubits
+    eng.flush()
+    assert pytest.approx(eng.backend.get_probability(result, qubits)) == 1.
+    All(Measure) | qubits
+
+
+def test_flip_bits_can_be_applied_to_various_qubit_qureg_formats():
+    eng = MainEngine()
+    qubits = eng.allocate_qureg(4)
+    eng.flush()
+    assert pytest.approx(eng.backend.get_probability('0000', qubits)) == 1.
+    FlipBits([0, 1, 1, 0]) | qubits
+    eng.flush()
+    assert pytest.approx(eng.backend.get_probability('0110', qubits)) == 1.
+    FlipBits([1]) | qubits[0]
+    eng.flush()
+    assert pytest.approx(eng.backend.get_probability('1110', qubits)) == 1.
+    FlipBits([1]) | (qubits[0], )
+    eng.flush()
+    assert pytest.approx(eng.backend.get_probability('0110', qubits)) == 1.
+    FlipBits([1, 1]) | [qubits[0], qubits[1]]
+    eng.flush()
+    assert pytest.approx(eng.backend.get_probability('1010', qubits)) == 1.
+    FlipBits(-1) | qubits
+    eng.flush()
+    assert pytest.approx(eng.backend.get_probability('0101', qubits)) == 1.
+    FlipBits(-4) | [qubits[0], qubits[1], qubits[2], qubits[3]]
+    eng.flush()
+    assert pytest.approx(eng.backend.get_probability('0110', qubits)) == 1.
+    FlipBits(2) | [qubits[0]] + [qubits[1], qubits[2]]
+    eng.flush()
+    assert pytest.approx(eng.backend.get_probability('0010', qubits)) == 1.
+    All(Measure) | qubits

projectq/ops/_qubit_operator.py

@@ -362,9 +362,9 @@ def get_inverse(self):
         Raises:
             NotInvertible: Not implemented for QubitOperators which have
                            multiple terms or a coefficient with absolute value
-                           not equal to 1. 
+                           not equal to 1.
         """
-        
+
         if len(self.terms) == 1:
             (term, coefficient), = self.terms.items()
             if (not abs(coefficient) < 1 - EQ_TOLERANCE and not

projectq/ops/_state_prep.py

@@ -30,9 +30,9 @@ def __init__(self, final_state):
                 StatePreparation([0.5, -0.5j, -0.5, 0.5]) | qureg
 
         Note:
-            The amplitude of state k is final_state[k]. When the state k is
-            written in binary notation, then qureg[0] denotes the qubit
-            whose state corresponds to the least significant bit of k.
+            final_state[k] is taken to be the amplitude of the computational
+            basis state whose string is equal to the binary representation
+            of k.
 
         Args:
             final_state(list[complex]): wavefunction of the desired