Fix spurious PHASE and conditional in exponential_map

* exponential_map of the identity applies a PHASE gate and
its inverse. This was apparently for debugging purposes. This
PR replaces the PHASE gates with an empty Program.

* This PR moves conditionals on data that is closed over from
inside the closure to outside. That is, the gate that is exponentiated
is checked when the closure is created, but not when it is executed.

Closes #1055. This PR obsoletes PR #373.

CHANGELOG.md

@@ -13,6 +13,7 @@ Changelog
 -   Fixed the QCS access request link in the README (@amyfbrown, gh-1171).
 -   Fix the SDK download link and instructions in the docs (@amyfbrown, gh-1173).
 -   Removed HALT from valid Protoquil / supported Quil. (@kilimanjaro, gh-1176).
+-   Fix spurious PHASE and conditional in exponential_map (@jlapeyre, gh-1182).
 
 [v2.17](https://github.com/rigetti/pyquil/compare/v2.16.0...v2.17.0) (January 30, 2020)
 ---------------------------------------------------------------------------------------

pyquil/paulis.py

@@ -923,18 +923,16 @@ def exponential_map(term: PauliTerm) -> Callable[[float], Program]:
     coeff = term.coefficient.real
     term.coefficient = term.coefficient.real
 
-    def exp_wrap(param: float) -> Program:
-        prog = Program()
-        if is_identity(term):
-            prog.inst(X(0))
-            prog.inst(PHASE(-param * coeff, 0))
-            prog.inst(X(0))
-            prog.inst(PHASE(-param * coeff, 0))
-        elif is_zero(term):
-            pass
-        else:
+    if is_zero(term) or is_identity(term):
+        def exp_wrap(param: float) -> Program:
+            prog = Program()
+            return prog
+
+    else:
+        def exp_wrap(param: float) -> Program:
+            prog = Program()
             prog += _exponentiate_general_case(term, param)
-        return prog
+            return prog
 
     return exp_wrap
 

pyquil/tests/test_paulis.py

@@ -421,15 +421,21 @@ def test_exponentiate_identity():
     generator = PauliTerm("I", 1, 1.0)
     para_prog = exponential_map(generator)
     prog = para_prog(1)
-    result_prog = Program().inst([X(0), PHASE(-1.0, 0), X(0), PHASE(-1.0, 0)])
+    result_prog = Program()
     assert prog == result_prog
 
     generator = PauliTerm("I", 10, 0.08)
     para_prog = exponential_map(generator)
     prog = para_prog(1)
-    result_prog = Program().inst([X(0), PHASE(-0.08, 0), X(0), PHASE(-0.08, 0)])
+    result_prog = Program()
     assert prog == result_prog
 
+    pop = 2.1 * sI(0)
+    expprog = exponential_map(pop)
+    assert expprog(.5) == Program()
+    pop.__dict__['_ops'][0] = 'Y'
+    assert expprog(.5) == Program()
+
 
 def test_trotterize():
     term_one = PauliTerm("X", 0, 1.0)

pyquil/tests/test_paulis_with_placeholders.py

@@ -398,14 +398,14 @@ def test_exponentiate_identity():
     generator = PauliTerm("I", q[1], 1.0)
     para_prog = exponential_map(generator)
     prog = para_prog(1)
-    result_prog = Program().inst([X(q[0]), PHASE(-1.0, q[0]), X(q[0]), PHASE(-1.0, q[0])])
-    assert address_qubits(prog) == address_qubits(result_prog)
+    result_prog = Program()
+    assert prog == result_prog
 
     generator = PauliTerm("I", q[10], 0.08)
     para_prog = exponential_map(generator)
     prog = para_prog(1)
-    result_prog = Program().inst([X(q[0]), PHASE(-0.08, q[0]), X(q[0]), PHASE(-0.08, q[0])])
-    assert address_qubits(prog) == address_qubits(result_prog)
+    result_prog = Program()
+    assert prog == result_prog
 
 
 def test_trotterize():