Remove lone identity gates in Optimize 1q decomposition pass (#5292) (#…

…5385)

* Remove lone identity gates in Optimize 1q decomposition pass

When running the optimize 1q decomposition pass for basis sets other
than u1, u2, u3 there was an edge case where it wouldn't remove lone
identity gates. This was because normally the pass skips runs of length
1 because the Euler decomposer will normally not do better than that.
But, in the case of identity gates we know that they can just be
removed, and the simplify=True flag on the decomposer will normally do
this for us. But, since we don't run the decomposer this commit fixes
this edge case by explicitly checking the single gate for an identity
and removing it if it is.

Fixes #5287

* Use append instead of deprecated u3 method in test

* Compare to identity matrix instead of checking all parameters are 0

(cherry picked from commit 1578fb4)

Co-authored-by: Matthew Treinish <[email protected]>

qiskit/transpiler/passes/optimization/optimize_1q_decomposition.py

@@ -15,6 +15,8 @@
 from itertools import groupby
 import logging
 
+import numpy as np
+
 from qiskit.circuit import QuantumCircuit, QuantumRegister
 from qiskit.quantum_info import Operator
 from qiskit.transpiler.basepasses import TransformationPass
@@ -71,8 +73,14 @@ def run(self, dag):
         runs = dag.collect_runs(self.euler_basis_names[self.basis])
         runs = _split_runs_on_parameters(runs)
         for run in runs:
-            # Don't try to optimize a single 1q gate
             if len(run) <= 1:
+                params = run[0].op.params
+                # Remove single identity gates
+                if run[0].op.name in self.euler_basis_names[self.basis] and len(
+                        params) > 0 and np.array_equal(run[0].op.to_matrix(),
+                                                       np.eye(2)):
+                    dag.remove_op_node(run[0])
+                # Don't try to optimize a single 1q gate
                 continue
             q = QuantumRegister(1, "q")
             qc = QuantumCircuit(1)

test/python/transpiler/test_optimize_1q_decomposition.py

@@ -18,6 +18,7 @@
 import numpy as np
 
 from qiskit.circuit import QuantumRegister, QuantumCircuit, ClassicalRegister
+from qiskit.circuit.library.standard_gates import U3Gate
 from qiskit.transpiler import PassManager
 from qiskit.transpiler.passes import Optimize1qGatesDecomposition
 from qiskit.transpiler.passes import BasisTranslator
@@ -235,6 +236,86 @@ def test_parameterized_expressions_in_circuits(self, basis):
             Operator(qc.bind_parameters({theta: 3.14, phi: 10})).equiv(
                 Operator(result.bind_parameters({theta: 3.14, phi: 10}))))
 
+    def test_identity_xyx(self):
+        """Test lone identity gates in rx ry basis are removed."""
+        circuit = QuantumCircuit(2)
+        circuit.rx(0, 1)
+        circuit.ry(0, 0)
+        basis = ['rxx', 'rx', 'ry']
+        passmanager = PassManager()
+        passmanager.append(BasisTranslator(sel, basis))
+        passmanager.append(Optimize1qGatesDecomposition(basis))
+        result = passmanager.run(circuit)
+        self.assertEqual([], result.data)
+
+    def test_identity_zxz(self):
+        """Test lone identity gates in rx rz basis are removed."""
+        circuit = QuantumCircuit(2)
+        circuit.rx(0, 1)
+        circuit.rz(0, 0)
+        basis = ['cz', 'rx', 'rz']
+        passmanager = PassManager()
+        passmanager.append(BasisTranslator(sel, basis))
+        passmanager.append(Optimize1qGatesDecomposition(basis))
+        result = passmanager.run(circuit)
+        self.assertEqual([], result.data)
+
+    def test_identity_psx(self):
+        """Test lone identity gates in p sx basis are removed."""
+        circuit = QuantumCircuit(1)
+        circuit.p(0, 0)
+        basis = ['cx', 'p', 'sx']
+        passmanager = PassManager()
+        passmanager.append(BasisTranslator(sel, basis))
+        passmanager.append(Optimize1qGatesDecomposition(basis))
+        result = passmanager.run(circuit)
+        self.assertEqual([], result.data)
+
+    def test_identity_u(self):
+        """Test lone identity gates in u basis are removed."""
+        circuit = QuantumCircuit(1)
+        circuit.u(0, 0, 0, 0)
+        basis = ['cx', 'u']
+        passmanager = PassManager()
+        passmanager.append(BasisTranslator(sel, basis))
+        passmanager.append(Optimize1qGatesDecomposition(basis))
+        result = passmanager.run(circuit)
+        self.assertEqual([], result.data)
+
+    def test_identity_u3(self):
+        """Test lone identity gates in u3 basis are removed."""
+        circuit = QuantumCircuit(1)
+        circuit.append(U3Gate(0, 0, 0), [0])
+        basis = ['cx', 'u3']
+        passmanager = PassManager()
+        passmanager.append(BasisTranslator(sel, basis))
+        passmanager.append(Optimize1qGatesDecomposition(basis))
+        result = passmanager.run(circuit)
+        self.assertEqual([], result.data)
+
+    def test_identity_r(self):
+        """Test lone identity gates in r basis are removed."""
+        circuit = QuantumCircuit(1)
+        circuit.r(0, 0, 0)
+        basis = ['r']
+        passmanager = PassManager()
+        passmanager.append(BasisTranslator(sel, basis))
+        passmanager.append(Optimize1qGatesDecomposition(basis))
+        result = passmanager.run(circuit)
+        self.assertEqual([], result.data)
+
+    def test_identity_u1x(self):
+        """Test lone identity gates in u1 rx basis are removed."""
+        circuit = QuantumCircuit(2)
+        circuit.u1(0, 0)
+        circuit.rx(0, 1)
+        basis = ['cx', 'u1', 'rx']
+        passmanager = PassManager()
+        passmanager.append(BasisTranslator(sel, basis))
+        passmanager.append(Optimize1qGatesDecomposition(basis))
+        result = passmanager.run(circuit)
+        self.assertEqual([], result.data)
+
 
 if __name__ == '__main__':
     unittest.main()