Add cirq.StabilizerSampler (#3355)

- Minimal interface
- Supports some random operations (depolarize, probabilistic gates)
- This is a potential replacement for cirq.CliffordSimulator (which is in a bad state right now, tracking two redundant state objects in a fashion observable to callers)

Author: Strilanc

cirq/__init__.py

@@ -365,6 +365,7 @@
     SimulationTrialResult,
     Simulator,
     SparseSimulatorStep,
+    StabilizerSampler,
     StateVectorMixin,
     StateVectorSimulatorState,
     StateVectorStepResult,

cirq/ops/common_channels.py

@@ -308,6 +308,16 @@ def __str__(self) -> str:
             return f"depolarize(p={self._p})"
         return f"depolarize(p={self._p},n_qubits={self._n_qubits})"
 
+    def _act_on_(self, args: Any) -> bool:
+        from cirq.sim import clifford
+        if isinstance(args, clifford.ActOnCliffordTableauArgs):
+            if args.prng.random() < self._p:
+                gate = args.prng.choice(
+                    [pauli_gates.X, pauli_gates.Y, pauli_gates.Z])
+                protocols.act_on(gate, args)
+            return True
+        return NotImplemented
+
     def _circuit_diagram_info_(self,
                                args: 'protocols.CircuitDiagramInfoArgs') -> str:
         if args.precision is not None:

cirq/ops/common_channels_test.py

@@ -571,7 +571,6 @@ def test_bit_flip_channel_invalid_probability():
         cirq.bit_flip(1.1)
 
 
-
 def test_bit_flip_channel_text_diagram():
     bf = cirq.bit_flip(0.1234567)
     assert (cirq.circuit_diagram_info(
@@ -582,6 +581,17 @@ def test_bit_flip_channel_text_diagram():
             wire_symbols=('BF(0.12)',)))
 
 
+def test_stabilizer_supports_depolarize():
+    with pytest.raises(TypeError, match="act_on"):
+        for _ in range(100):
+            cirq.act_on(cirq.depolarize(3 / 4), object())
+
+    q = cirq.LineQubit(0)
+    c = cirq.Circuit(cirq.depolarize(3 / 4).on(q), cirq.measure(q, key='m'))
+    m = np.sum(cirq.StabilizerSampler().sample(c, repetitions=100)['m'])
+    assert 5 < m < 95
+
+
 def test_default_asymmetric_depolarizing_channel():
     d = cirq.asymmetric_depolarize()
     assert d.p_x == 0.0

cirq/ops/random_gate_channel.py

@@ -109,6 +109,19 @@ def _trace_distance_bound_(self) -> float:
             result *= float(self.probability)
         return result
 
+    def _act_on_(self, args):
+        from cirq.sim import clifford
+        if self._is_parameterized_():
+            return NotImplemented
+        if isinstance(args, clifford.ActOnCliffordTableauArgs):
+            if args.prng.random() < self.probability:
+                # Note: because we're doing this probabilistically, it's not
+                # safe to fallback to other strategies if act_on fails. Those
+                # strategies could double-count the probability.
+                protocols.act_on(self.sub_gate, args)
+            return True
+        return NotImplemented
+
     def _json_dict_(self) -> Dict[str, Any]:
         return protocols.obj_to_dict_helper(self, ['sub_gate', 'probability'])
 

cirq/ops/random_gate_channel_test.py

@@ -240,3 +240,24 @@ def test_trace_distance():
 
 def test_str():
     assert str(cirq.X.with_probability(0.5)) == 'X[prob=0.5]'
+
+
+def test_stabilizer_supports_probability():
+    q = cirq.LineQubit(0)
+    c = cirq.Circuit(cirq.X(q).with_probability(0.5), cirq.measure(q, key='m'))
+    m = np.sum(cirq.StabilizerSampler().sample(c, repetitions=100)['m'])
+    assert 5 < m < 95
+
+
+def test_unsupported_stabilizer_safety():
+    with pytest.raises(TypeError, match="act_on"):
+        for _ in range(100):
+            cirq.act_on(cirq.X.with_probability(0.5), object())
+    with pytest.raises(TypeError, match="act_on"):
+        cirq.act_on(cirq.X.with_probability(sympy.Symbol('x')), object())
+
+    q = cirq.LineQubit(0)
+    c = cirq.Circuit((cirq.X(q)**0.25).with_probability(0.5),
+                     cirq.measure(q, key='m'))
+    with pytest.raises(TypeError, match='Failed to act'):
+        cirq.StabilizerSampler().sample(c, repetitions=100)

cirq/protocols/json_serialization_test.py

@@ -197,11 +197,14 @@ def test_fail_to_resolve():
 
     # utility:
     'AnnealSequenceSearchStrategy',
+    'CliffordSimulator',
     'DeserializingArg',
     'GateOpDeserializer',
     'GateOpSerializer',
     'GreedySequenceSearchStrategy',
     'SerializingArg',
+    'Simulator',
+    'StabilizerSampler',
     'Unique',
     'DEFAULT_RESOLVERS',
 
@@ -278,7 +281,6 @@ def test_mutually_exclusive_blacklist():
     'CircuitDiagramInfo',
     'CircuitDiagramInfoArgs',
     'CircuitSampleJob',
-    'CliffordSimulator',
     'CliffordSimulatorStepResult',
     'CliffordState',
     'CliffordTrialResult',
@@ -317,7 +319,6 @@ def test_mutually_exclusive_blacklist():
     'SerializableDevice',
     'SerializableGateSet',
     'SimulationTrialResult',
-    'Simulator',
     'SingleQubitCliffordGate',
     'SparseSimulatorStep',
     'SQRT_ISWAP_GATESET',

cirq/sim/__init__.py

@@ -79,6 +79,7 @@
 from cirq.sim.clifford import (
     ActOnCliffordTableauArgs,
     ActOnStabilizerCHFormArgs,
+    StabilizerSampler,
     StabilizerStateChForm,
     CliffordSimulator,
     CliffordState,

cirq/sim/clifford/__init__.py

@@ -14,5 +14,8 @@
 from cirq.sim.clifford.act_on_stabilizer_ch_form_args import (
     ActOnStabilizerCHFormArgs,)
 
+from cirq.sim.clifford.stabilizer_sampler import (
+    StabilizerSampler,)
+
 from cirq.sim.clifford.stabilizer_state_ch_form import (
     StabilizerStateChForm,)

cirq/sim/clifford/stabilizer_sampler.py

@@ -0,0 +1,77 @@
+# Copyright 2020 The Cirq Developers
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from typing import Dict, List
+
+import numpy as np
+
+import cirq
+from cirq import circuits, protocols, value
+from cirq.sim.clifford.act_on_clifford_tableau_args import \
+    ActOnCliffordTableauArgs
+from cirq.sim.clifford.clifford_tableau import CliffordTableau
+from cirq.work import sampler
+
+
+class StabilizerSampler(sampler.Sampler):
+    """An efficient sampler for stabilizer circuits."""
+
+    def __init__(self, *, seed: 'cirq.RANDOM_STATE_OR_SEED_LIKE' = None):
+        """
+        Args:
+            seed: The random seed or generator to use when sampling.
+        """
+        self.init = True
+        self._prng = value.parse_random_state(seed)
+
+    def run_sweep(
+            self,
+            program: 'cirq.Circuit',
+            params: 'cirq.Sweepable',
+            repetitions: int = 1,
+    ) -> List['cirq.Result']:
+        results: List[cirq.Result] = []
+        for param_resolver in cirq.to_resolvers(params):
+            resolved_circuit = cirq.resolve_parameters(program, param_resolver)
+            measurements = self._run(
+                resolved_circuit,
+                repetitions=repetitions,
+            )
+            results.append(
+                cirq.Result(params=param_resolver, measurements=measurements))
+        return results
+
+    def _run(self, circuit: circuits.Circuit,
+             repetitions: int) -> Dict[str, np.ndarray]:
+
+        measurements: Dict[str, List[int]] = {
+            key: [] for key in protocols.measurement_keys(circuit)
+        }
+        axes_map = {q: i for i, q in enumerate(circuit.all_qubits())}
+
+        for _ in range(repetitions):
+            state = ActOnCliffordTableauArgs(
+                CliffordTableau(num_qubits=len(axes_map)),
+                axes=(),
+                prng=self._prng,
+                log_of_measurement_results={},
+            )
+            for op in circuit.all_operations():
+                state.axes = tuple(axes_map[q] for q in op.qubits)
+                protocols.act_on(op, state)
+
+            for k, v in state.log_of_measurement_results.items():
+                measurements[k].append(v)
+
+        return {k: np.array(v) for k, v in measurements.items()}

cirq/sim/clifford/stabilizer_sampler_test.py

@@ -0,0 +1,17 @@
+import cirq
+
+import numpy as np
+
+
+def test_produces_samples():
+    a, b = cirq.LineQubit.range(2)
+    c = cirq.Circuit(
+        cirq.H(a),
+        cirq.CNOT(a, b),
+        cirq.measure(a, key='a'),
+        cirq.measure(b, key='b'),
+    )
+
+    result = cirq.StabilizerSampler().sample(c, repetitions=100)
+    assert 5 < sum(result['a']) < 95
+    assert np.all(result['a'] ^ result['b'] == 0)