From 666bb4f798306ff282404145d67962e409e4f163 Mon Sep 17 00:00:00 2001
From: John Lapeyre <jlapeyre@users.noreply.github.com>
Date: Tue, 25 Feb 2020 10:21:12 -0800
Subject: [PATCH] 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                |  1 +
 pyquil/paulis.py            | 20 +++++++++-----------
 pyquil/tests/test_paulis.py |  4 ++--
 3 files changed, 12 insertions(+), 13 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 64f3a9e50..87dd1aec4 100644
--- a/CHANGELOG.md
+++ b/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)
 ---------------------------------------------------------------------------------------
diff --git a/pyquil/paulis.py b/pyquil/paulis.py
index e2c868859..2de7642ec 100644
--- a/pyquil/paulis.py
+++ b/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
 
diff --git a/pyquil/tests/test_paulis.py b/pyquil/tests/test_paulis.py
index 2cc61dbd3..0bd3439f1 100644
--- a/pyquil/tests/test_paulis.py
+++ b/pyquil/tests/test_paulis.py
@@ -421,13 +421,13 @@ 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