Allow to apply unitary qubit operators to qubits (#263)

Author: damiansteiger

projectq/backends/_sim/_simulator.py

@@ -303,8 +303,8 @@ def collapse_wavefunction(self, qureg, values):
 
         Args:
             qureg (Qureg|list[Qubit]): Qubits to collapse.
-            values (list[bool|int]|string[0|1]): Measurement outcome for each of the qubits
-                in `qureg`.
+            values (list[bool|int]|string[0|1]): Measurement outcome for each
+                                                 of the qubits in `qureg`.
 
         Raises:
             RuntimeError: If an outcome has probability (approximately) 0 or

projectq/libs/revkit/_control_function_test.py

@@ -41,6 +41,7 @@ def test_control_function_majority():
 
     assert len(saving_backend.received_commands) == 7
 
+
 def test_control_function_majority_from_python():
     dormouse = pytest.importorskip('dormouse')
 

projectq/ops/_qubit_operator.py

@@ -13,11 +13,16 @@
 #   limitations under the License.
 
 """QubitOperator stores a sum of Pauli operators acting on qubits."""
+import cmath
 import copy
 import itertools
 
 import numpy
 
+from ._basics import BasicGate, NotInvertible, NotMergeable
+from ._command import apply_command
+from ._gates import Ph, X, Y, Z
+
 
 EQ_TOLERANCE = 1e-12
 
@@ -45,7 +50,7 @@ class QubitOperatorError(Exception):
     pass
 
 
-class QubitOperator(object):
+class QubitOperator(BasicGate):
     """
     A sum of terms acting on qubits, e.g., 0.5 * 'X0 X5' + 0.3 * 'Z1 Z2'.
 
@@ -68,6 +73,25 @@ class QubitOperator(object):
 
         hamiltonian = 0.5 * QubitOperator('X0 X5') + 0.3 * QubitOperator('Z0')
 
+    Our Simulator takes a hermitian QubitOperator to directly calculate the
+    expectation value (see Simulator.get_expectation_value) of this observable.
+
+    A hermitian QubitOperator can also be used as input for the
+    TimeEvolution gate.
+
+    If the QubitOperator is unitary, i.e., it contains only one term with a
+    coefficient, whose absolute value is 1, then one can apply it directly to
+    qubits:
+
+    .. code-block:: python
+
+        eng = projectq.MainEngine()
+        qureg = eng.allocate_qureg(6)
+        QubitOperator('X0 X5', 1.j) | qureg  # Applies X to qubit 0 and 5
+                                             # with an additional global phase
+                                             # of 1.j
+
+
     Attributes:
         terms (dict): **key**: A term represented by a tuple containing all
                       non-trivial local Pauli operators ('X', 'Y', or 'Z').
@@ -128,6 +152,7 @@ def __init__(self, term=None, coefficient=1.):
         Raises:
             QubitOperatorError: Invalid operators provided to QubitOperator.
         """
+        BasicGate.__init__(self)
         if not isinstance(coefficient, (int, float, complex)):
             raise ValueError('Coefficient must be a numeric type.')
         self.terms = {}
@@ -226,6 +251,143 @@ def isclose(self, other, rel_tol=1e-12, abs_tol=1e-12):
                 return False
         return True
 
+    def __or__(self, qubits):
+        """
+        Operator| overload which enables the following syntax:
+
+        .. code-block:: python
+
+            QubitOperator(...) | qureg
+            QubitOperator(...) | (qureg,)
+            QubitOperator(...) | qubit
+            QubitOperator(...) | (qubit,)
+
+        Unlike other gates, this gate is only allowed to be applied to one
+        quantum register or one qubit and only if the QubitOperator is
+        unitary, i.e., consists of one term with a coefficient whose absolute
+        values is 1.
+
+        Example:
+
+        .. code-block:: python
+
+            eng = projectq.MainEngine()
+            qureg = eng.allocate_qureg(6)
+            QubitOperator('X0 X5', 1.j) | qureg  # Applies X to qubit 0 and 5
+                                                 # with an additional global
+                                                 # phase of 1.j
+
+        While in the above example the QubitOperator gate is applied to 6
+        qubits, it only acts non-trivially on the two qubits qureg[0] and
+        qureg[5]. Therefore, the operator| will create a new rescaled
+        QubitOperator, i.e, it sends the equivalent of the following new gate
+        to the MainEngine:
+
+        .. code-block:: python
+
+            QubitOperator('X0 X1', 1.j) | [qureg[0], qureg[5]]
+
+        which is only a two qubit gate.
+
+        Args:
+            qubits: one Qubit object, one list of Qubit objects, one Qureg
+                    object, or a tuple of the former three cases.
+
+        Raises:
+            TypeError: If QubitOperator is not unitary or applied to more than
+                       one quantum register.
+            ValueError: If quantum register does not have enough qubits
+        """
+        # Check that input is only one qureg or one qubit
+        qubits = self.make_tuple_of_qureg(qubits)
+        if len(qubits) != 1:
+            raise TypeError("Only one qubit or qureg allowed.")
+        # Check that operator is unitary
+        if not len(self.terms) == 1:
+            raise TypeError("Too many terms. Only QubitOperators consisting "
+                            "of a single term (single n-qubit Pauli operator) "
+                            "with a coefficient of unit length can be applied "
+                            "to qubits with this function.")
+        (term, coefficient), = self.terms.items()
+        phase = cmath.phase(coefficient)
+        if (abs(coefficient) < 1 - EQ_TOLERANCE or
+                abs(coefficient) > 1 + EQ_TOLERANCE):
+            raise TypeError("abs(coefficient) != 1. Only QubitOperators "
+                            "consisting of a single term (single n-qubit "
+                            "Pauli operator) with a coefficient of unit "
+                            "length can be applied to qubits with this "
+                            "function.")
+        # Test if we need to apply only Ph
+        if term == ():
+            Ph(phase) | qubits[0][0]
+            return
+        # Check that Qureg has enough qubits:
+        num_qubits = len(qubits[0])
+        non_trivial_qubits = set()
+        for index, action in term:
+            non_trivial_qubits.add(index)
+        if max(non_trivial_qubits) >= num_qubits:
+            raise ValueError("QubitOperator acts on more qubits than the gate "
+                             "is applied to.")
+        # Apply X, Y, Z, if QubitOperator acts only on one qubit
+        if len(term) == 1:
+            if term[0][1] == "X":
+                X | qubits[0][term[0][0]]
+            elif term[0][1] == "Y":
+                Y | qubits[0][term[0][0]]
+            elif term[0][1] == "Z":
+                Z | qubits[0][term[0][0]]
+            Ph(phase) | qubits[0][term[0][0]]
+            return
+        # Create new QubitOperator gate with rescaled qubit indices in
+        # 0,..., len(non_trivial_qubits) - 1
+        new_index = dict()
+        non_trivial_qubits = sorted(list(non_trivial_qubits))
+        for i in range(len(non_trivial_qubits)):
+            new_index[non_trivial_qubits[i]] = i
+        new_qubitoperator = QubitOperator()
+        assert len(new_qubitoperator.terms) == 0
+        new_term = tuple([(new_index[index], action)
+                          for index, action in term])
+        new_qubitoperator.terms[new_term] = coefficient
+        new_qubits = [qubits[0][i] for i in non_trivial_qubits]
+        # Apply new gate
+        cmd = new_qubitoperator.generate_command(new_qubits)
+        apply_command(cmd)
+
+    def get_inverse(self):
+        """
+        Return the inverse gate of a QubitOperator if applied as a gate.
+
+        Raises:
+            NotInvertible: Not implemented for QubitOperators which have
+                           multiple terms or a coefficient with absolute value
+                           not equal to 1. 
+        """
+        
+        if len(self.terms) == 1:
+            (term, coefficient), = self.terms.items()
+            if (not abs(coefficient) < 1 - EQ_TOLERANCE and not
+                    abs(coefficient) > 1 + EQ_TOLERANCE):
+                return QubitOperator(term, coefficient**(-1))
+        raise NotInvertible("BasicGate: No get_inverse() implemented.")
+
+    def get_merged(self, other):
+        """
+        Return this gate merged with another gate.
+
+        Standard implementation of get_merged:
+
+        Raises:
+            NotMergeable: merging is not possible
+        """
+        if (isinstance(other, self.__class__) and
+                len(other.terms) == 1 and
+                len(self.terms) == 1):
+            return self * other
+        else:
+            raise NotMergeable()
+
     def __imul__(self, multiplier):
         """
         In-place multiply (*=) terms with scalar or QubitOperator.
@@ -463,3 +625,6 @@ def __str__(self):
 
     def __repr__(self):
         return str(self)
+
+    def __hash__(self):
+        return hash(str(self))

projectq/ops/_qubit_operator_test.py

@@ -13,11 +13,18 @@
 #   limitations under the License.
 
 """Tests for _qubit_operator.py."""
+import cmath
 import copy
+import math
 
 import numpy
 import pytest
 
+from projectq import MainEngine
+from projectq.cengines import DummyEngine
+from ._basics import NotInvertible, NotMergeable
+from ._gates import Ph, T, X, Y, Z
+
 from projectq.ops import _qubit_operator as qo
 
 
@@ -184,6 +191,98 @@ def test_isclose_different_num_terms():
     assert not a.isclose(b, rel_tol=1e-12, abs_tol=0.05)
 
 
+def test_get_inverse():
+    qo0 = qo.QubitOperator("X1 Z2", cmath.exp(0.6j))
+    qo1 = qo.QubitOperator("", 1j)
+    assert qo0.get_inverse().isclose(
+        qo.QubitOperator("X1 Z2", cmath.exp(-0.6j)))
+    assert qo1.get_inverse().isclose(qo.QubitOperator("", -1j))
+    qo0 += qo1
+    with pytest.raises(NotInvertible):
+        qo0.get_inverse()
+
+
+def test_get_merged():
+    qo0 = qo.QubitOperator("X1 Z2", 1j)
+    qo1 = qo.QubitOperator("Y3", 1j)
+    merged = qo0.get_merged(qo1)
+    assert qo0.isclose(qo.QubitOperator("X1 Z2", 1j))
+    assert qo1.isclose(qo.QubitOperator("Y3", 1j))
+    assert qo0.get_merged(qo1).isclose(qo.QubitOperator("X1 Z2 Y3", -1))
+    with pytest.raises(NotMergeable):
+        qo1.get_merged(T)
+    qo2 = qo0 + qo1
+    with pytest.raises(NotMergeable):
+        qo2.get_merged(qo0)
+    with pytest.raises(NotMergeable):
+        qo0.get_merged(qo2)
+
+
+def test_or_one_qubit():
+    saving_backend = DummyEngine(save_commands=True)
+    eng = MainEngine(backend=saving_backend, engine_list=[])
+    qureg = eng.allocate_qureg(3)
+    eng.flush()
+    identity = qo.QubitOperator("", 1j)
+    x = qo.QubitOperator("X1", cmath.exp(0.5j))
+    y = qo.QubitOperator("Y2", cmath.exp(0.6j))
+    z = qo.QubitOperator("Z0", cmath.exp(4.5j))
+    identity | qureg
+    eng.flush()
+    x | qureg
+    eng.flush()
+    y | qureg
+    eng.flush()
+    z | qureg
+    eng.flush()
+    assert saving_backend.received_commands[4].gate == Ph(math.pi/2.)
+
+    assert saving_backend.received_commands[6].gate == X
+    assert saving_backend.received_commands[6].qubits == ([qureg[1]],)
+    assert saving_backend.received_commands[7].gate == Ph(0.5)
+    assert saving_backend.received_commands[7].qubits == ([qureg[1]],)
+
+    assert saving_backend.received_commands[9].gate == Y
+    assert saving_backend.received_commands[9].qubits == ([qureg[2]],)
+    assert saving_backend.received_commands[10].gate == Ph(0.6)
+    assert saving_backend.received_commands[10].qubits == ([qureg[2]],)
+
+    assert saving_backend.received_commands[12].gate == Z
+    assert saving_backend.received_commands[12].qubits == ([qureg[0]],)
+    assert saving_backend.received_commands[13].gate == Ph(4.5)
+    assert saving_backend.received_commands[13].qubits == ([qureg[0]],)
+
+
+def test_wrong_input():
+    eng = MainEngine()
+    qureg = eng.allocate_qureg(3)
+    op0 = qo.QubitOperator("X1", 0.99)
+    with pytest.raises(TypeError):
+        op0 | qureg
+    op1 = qo.QubitOperator("X2", 1)
+    with pytest.raises(ValueError):
+        op1 | qureg[1]
+    with pytest.raises(TypeError):
+        op0 | (qureg[1], qureg[2])
+    op2 = op0 + op1
+    with pytest.raises(TypeError):
+        op2 | qureg
+
+
+def test_rescaling_of_indices():
+    saving_backend = DummyEngine(save_commands=True)
+    eng = MainEngine(backend=saving_backend, engine_list=[])
+    qureg = eng.allocate_qureg(4)
+    eng.flush()
+    op = qo.QubitOperator("X0 Y1 Z3", 1j)
+    op | qureg
+    eng.flush()
+    assert saving_backend.received_commands[5].gate.isclose(
+        qo.QubitOperator("X0 Y1 Z2", 1j))
+    # test that gate creates a new QubitOperator
+    assert op.isclose(qo.QubitOperator("X0 Y1 Z3", 1j))
+
+
 def test_imul_inplace():
     qubit_op = qo.QubitOperator("X1")
     prev_id = id(qubit_op)
@@ -444,6 +543,11 @@ def test_str():
     assert str(op2) == "2 I"
 
 
+def test_hash():
+    op = qo.QubitOperator(((1, 'X'), (3, 'Y'), (8, 'Z')), 0.5)
+    assert hash(op) == hash("0.5 X1 Y3 Z8")
+
+
 def test_str_empty():
     op = qo.QubitOperator()
     assert str(op) == '0'

projectq/setups/decompositions/__init__.py

@@ -21,6 +21,7 @@
                entangle,
                globalphase,
                ph2r,
+               qubitop2onequbit,
                qft2crandhadamard,
                r2rzandph,
                rx2rz,
@@ -43,6 +44,7 @@
                    entangle,
                    globalphase,
                    ph2r,
+                   qubitop2onequbit,
                    qft2crandhadamard,
                    r2rzandph,
                    rx2rz,