shots=None will not execute any measurement (#682)

* impl+test [sf] - `shots=None`

* changelog entry

* fix typo

Co-authored-by: Theodor <theodor@xanadu.ai>

* test [tdmprogram] - test_shots_passed and test_shots_run_options

Co-authored-by: Theodor <theodor@xanadu.ai>

.github/CHANGELOG.md

@@ -9,6 +9,8 @@
 * A `Device.certificate` method is added which returns the hardware device certificate.
   [(#679)](https://github.com/XanaduAI/strawberryfields/pull/679)
 
+* Setting `shots=None` in the engine or program run options will not execute any measurements applied on the circuit. [(#682)](https://github.com/XanaduAI/strawberryfields/pull/682)
+
 <h3>Breaking Changes</h3>
 
 * `DeviceSpec` is renamed to `Device`.
@@ -33,7 +35,7 @@
 
 This release contains contributions from (in alphabetical order):
 
-Theodor Isacsson, Jon Schlipf, Hossein Seifoory
+Sebastian Duque, Theodor Isacsson, Jon Schlipf, Hossein Seifoory
 
 # Release 0.21.0 (current release)
 

strawberryfields/engine.py

@@ -451,12 +451,14 @@ def run(self, program, *, args=None, compile_options=None, **kwargs):
             # if a tdm program is input in a rolled state, then unroll it
             if not program.is_unrolled:
                 received_rolled_circuit = True
-                program.unroll(shots=shots)
+                # if shots = None, then set shots=1 here only to unroll the program
+                program.unroll(shots=shots if shots is not None else 1)
 
             # Shots >1 for a TDM program simply corresponds to creating
             # multiple copies of the program, and appending them to run sequentially.
-            # As a result, we set the backend shots to 1 for the Gaussian backend.
-            kwargs["shots"] = 1
+            # As a result, we set the backend shots to 1 for the Gaussian backend
+            # unless shots was set to `None`.
+            kwargs["shots"] = 1 if shots else None
 
         args = args or {}
         compile_options = compile_options or {}

strawberryfields/ops.py

@@ -316,6 +316,12 @@ def apply(self, reg, backend, **kwargs):
             shots (int): Number of independent evaluations to perform.
                 Only applies to Measurements.
         """
+        # Do not apply any measurement operation in the circuit
+        # if `shots` is set to None
+        _shots = kwargs.get("shots", 1)
+        if _shots is None:
+            return None
+
         values = super().apply(reg, backend, **kwargs)
 
         # store the results in the register reference objects

tests/frontend/test_engine.py

@@ -84,7 +84,8 @@ def test_shots_default(self):
         results = eng.run(prog)
         assert len(results.samples) == 1
 
-    def test_shots_run_options(self):
+    @pytest.mark.parametrize("shots,len_samples", [(None, 0), (5, 5)])
+    def test_shots_run_options(self, shots, len_samples):
         """Test that run_options takes precedence over default"""
         prog = sf.Program(1)
         eng = sf.Engine("gaussian")
@@ -93,23 +94,23 @@ def test_shots_run_options(self):
             ops.Sgate(0.5) | q[0]
             ops.MeasureFock() | q
 
-        prog.run_options = {"shots": 5}
+        prog.run_options = {"shots": shots}
         results = eng.run(prog)
-        assert len(results.samples) == 5
+        assert len(results.samples) == len_samples
 
-    def test_shots_passed(self):
+    @pytest.mark.parametrize("shots,len_samples", [(None, 0), (2, 2)])
+    def test_shots_passed(self, shots, len_samples):
         """Test that shots supplied via eng.run takes precedence over
         run_options and that run_options isn't changed"""
         prog = sf.Program(1)
         eng = sf.Engine("gaussian")
-
         with prog.context as q:
             ops.Sgate(0.5) | q[0]
             ops.MeasureFock() | q
 
         prog.run_options = {"shots": 5}
-        results = eng.run(prog, shots=2)
-        assert len(results.samples) == 2
+        results = eng.run(prog, shots=shots)
+        assert len(results.samples) == len_samples
         assert prog.run_options["shots"] == 5
 
     def test_history(self, eng, prog):

tests/frontend/test_tdmprogram.py

@@ -702,7 +702,8 @@ def test_shots_default(self):
         results = eng.run(prog)
         assert len(results.samples) == 1
 
-    def test_shots_run_options(self):
+    @pytest.mark.parametrize("shots,len_samples", [(None, 0), (5, 5)])
+    def test_shots_run_options(self, shots, len_samples):
         """Test that run_options takes precedence over default"""
         prog = sf.TDMProgram(2)
         eng = sf.Engine("gaussian")
@@ -711,11 +712,12 @@ def test_shots_run_options(self):
             ops.Sgate(p[0]) | q[0]
             ops.MeasureHomodyne(p[1]) | q[0]
 
-        prog.run_options = {"shots": 5}
+        prog.run_options = {"shots": shots}
         results = eng.run(prog)
-        assert len(results.samples) == 5
+        assert len(results.samples) == len_samples
 
-    def test_shots_passed(self):
+    @pytest.mark.parametrize("shots,len_samples", [(None, 0), (2, 2)])
+    def test_shots_passed(self, shots, len_samples):
         """Test that shots supplied via eng.run takes precedence over
         run_options and that run_options isn't changed"""
         prog = sf.TDMProgram(2)
@@ -726,8 +728,8 @@ def test_shots_passed(self):
             ops.MeasureHomodyne(p[1]) | q[0]
 
         prog.run_options = {"shots": 5}
-        results = eng.run(prog, shots=2)
-        assert len(results.samples) == 2
+        results = eng.run(prog, shots=shots)
+        assert len(results.samples) == len_samples
         assert prog.run_options["shots"] == 5
 
     def test_shots_with_timebins_non_multiple_of_concurrent_modes(self):