Create a gauge for idle moments

cirq-core/cirq/transformers/gauge_compiling/__init__.py

@@ -42,3 +42,8 @@
 from cirq.transformers.gauge_compiling.cphase_gauge import (
     CPhaseGaugeTransformer as CPhaseGaugeTransformer,
 )
+
+
+from cirq.transformers.gauge_compiling.idle_moments_gauge import (
+    IdleMomentsGauge as IdleMomentsGauge,
+)

cirq-core/cirq/transformers/gauge_compiling/idle_moments_gauge.py

@@ -0,0 +1,248 @@
+# Copyright 2025 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 __future__ import annotations
+
+from typing import Iterator, Sequence, TYPE_CHECKING
+
+import attrs
+import numpy as np
+
+import cirq.circuits as circuits
+import cirq.ops as ops
+import cirq.protocols as protocols
+import cirq.transformers.transformer_api as transformer_api
+
+if TYPE_CHECKING:
+    import cirq
+
+_PAULIS: tuple[cirq.Gate, ...] = (ops.I, ops.X, ops.Y, ops.Z)  # type: ignore[has-type]
+_CLIFFORDS = tuple(ops.SingleQubitCliffordGate.all_single_qubit_cliffords)
+_INV_CLIFFORDS = tuple(c**-1 for c in ops.SingleQubitCliffordGate.all_single_qubit_cliffords)
+
+_NAME_TO_GATES = {'pauli': _PAULIS, 'clifford': _CLIFFORDS, 'inv_clifford': _INV_CLIFFORDS}
+
+
+def _gauges_arg_converter(gauges: str | Sequence[cirq.Gate] = 'clifford') -> tuple[cirq.Gate, ...]:
+    if isinstance(gauges, str):
+        return _NAME_TO_GATES[gauges]
+    return tuple(gauges)
+
+
+def _repr_fn(gauges: tuple[cirq.Gate, ...]) -> str:
+    if gauges is _PAULIS:
+        return '"pauli"'
+    if gauges is _CLIFFORDS:
+        return '"clifford"'
+    if gauges is _INV_CLIFFORDS:
+        return '"inv_clifford"'
+    return repr(gauges)
+
+
+def _get_structure(
+    active: list[tuple[int, bool]],
+    min_length: int,
+    n: int,
+    gauge_beginning: bool,
+    gauge_ending: bool,
+) -> Iterator[tuple[int, int]]:
+    assert active
+    if gauge_beginning:
+        stop, is_mergable = active[0]
+        if min_length <= stop:
+            if is_mergable:
+                yield (0, stop)
+            else:
+                yield (0, stop - 1)
+
+    for i in range(len(active) - 1):
+        left_pos, left_is_mergable = active[i]
+        right_pos, right_is_mergable = active[i + 1]
+        if right_pos - left_pos - 1 >= min_length:
+            yield (left_pos + 1 - left_is_mergable, right_pos - 1 + right_is_mergable)
+
+    if gauge_ending:
+        stop, is_mergable = active[-1]
+        if min_length <= n - stop - 1:
+            if is_mergable:
+                yield (stop, n - 1)
+            else:
+                yield (stop + 1, n - 1)
+
+
+def _merge(g1: cirq.Gate, g2: cirq.Gate, q: cirq.Qid, tags: Sequence) -> cirq.Operation:
+    u1 = protocols.unitary(g1)
+    u2 = protocols.unitary(g2)
+    return ops.PhasedXZGate.from_matrix(u2 @ u1)(q).with_tags(*tags)
+
+
+@transformer_api.transformer
+@attrs.frozen
+class IdleMomentsGauge:
+    r"""A transformer that inserts identity-preserving "gauge" gates around idle qubit moments.
+
+    This transformer identifies sequences of consecutive idle moments on a single qubit
+    that meet a `min_length` threshold. For each such sequence, it inserts a randomly
+    selected gate `G` from `gauges` at the start of the idle period and its inverse `G^-1`
+    from `gauges_inverse` at the end. This ensures the logical circuit behavior remains
+    unchanged ($G \cdot G^{-1} = I$).
+
+    The primary goal is to introduce specific structure into idle periods, which is
+    useful for experiments.
+
+    Attributes:
+        min_length: Minimum number of consecutive idle moments for a gauge to be applied (>= 1).
+
+        gauges: A sequence of `cirq.Gate` objects to randomly select from.
+            Can be a custom tuple or a string alias:
+            - `"pauli"`: Uses single-qubit Pauli gates (I, X, Y, Z).
+            - `"clifford"`: Uses all 24 single-qubit Clifford gates.
+
+        gauges_inverse: An optional sequence of `cirq.Gate` objects representing
+            the inverses of gates in `gauges`. The `k`-th gate in `gauges_inverse`
+            must be the inverse of the `k`-th gate in `gauges`. If not provided,
+            it's automatically computed:
+            - `"pauli"` defaults to `"pauli"`.
+            - `"clifford"` defaults to `_INV_CLIFFORDS` (inverses of Clifford gates).
+            - Custom gate sequences have their inverses computed.
+            This positional correspondence is enforced by an internal assertion to
+            ensure $G \cdot G^{-1} = I$.
+
+        gauge_beginning: If `True`, applies a gauge to idle moments at the circuit's start,
+            before any other qubit operation. Defaults to `False`.
+
+        gauge_ending: If `True`, applies a gauge to idle moments at the circuit's end,
+            after the last qubit operation. Defaults to `False`.
+    """
+
+    min_length: int = attrs.field(
+        validator=(attrs.validators.instance_of(int), attrs.validators.ge(1))
+    )
+    gauges: tuple[cirq.Gate, ...] = attrs.field(
+        default='clifford', converter=_gauges_arg_converter, repr=_repr_fn
+    )
+    gauges_inverse: tuple[cirq.Gate, ...] = attrs.field(
+        converter=_gauges_arg_converter, repr=_repr_fn
+    )
+    gauge_beginning: bool = False
+    gauge_ending: bool = False
+
+    @gauges_inverse.default
+    def _gauges_inverse_default(self) -> tuple[cirq.Gate, ...]:
+        if self.gauges is _PAULIS:
+            return _PAULIS
+        if self.gauges is _CLIFFORDS:
+            return _INV_CLIFFORDS
+        if self.gauges is _INV_CLIFFORDS:
+            return _CLIFFORDS
+        return tuple(g**-1 for g in self.gauges)
+
+    def __attrs_post_init__(self):
+        if self.gauges is _PAULIS:
+            assert self.gauges_inverse is _PAULIS
+        elif self.gauges is _CLIFFORDS:
+            assert self.gauges_inverse is _INV_CLIFFORDS
+        elif self.gauges is _INV_CLIFFORDS:
+            assert self.gauges_inverse is _CLIFFORDS
+        else:
+            identity = np.eye(2)
+            assert np.all(
+                np.all(protocols.unitary(g * g_adj), identity)
+                for g, g_adj in zip(self.gauges, self.gauges_inverse, strict=True)
+            )
+
+    def __call__(
+        self,
+        circuit: cirq.AbstractCircuit,
+        *,
+        context: transformer_api.TransformerContext | None = None,
+        rng_or_seed: np.random.Generator | int | None = None,
+    ):
+        """Apply the IdleMomentGauge transformer.
+
+        Args:
+            circuit: The circuit to process.
+            context: The TransformerContext.
+            rng_or_seed: The source of randomness.
+
+        Returns:
+            A transformed circuit.
+
+        Raises:
+            ValueError: if the TransformerContext has deep=True.
+        """
+        rng = (
+            rng_or_seed
+            if isinstance(rng_or_seed, np.random.Generator)
+            else np.random.default_rng(rng_or_seed)
+        )
+        context = (
+            context
+            if isinstance(context, transformer_api.TransformerContext)
+            else transformer_api.TransformerContext(deep=False)
+        )
+        if context.deep:
+            raise ValueError("IdleMomentsGauge doesn't support deep TransformerContext")
+
+        tags_to_ignore = frozenset(context.tags_to_ignore)
+        all_qubits = circuit.all_qubits()
+
+        active_moments: dict[cirq.Qid, list[tuple[int, bool]]] = {q: [] for q in all_qubits}
+        for m_id, moment in enumerate(circuit):
+            if tags_to_ignore & frozenset(moment.tags):
+                for q in all_qubits:
+                    active_moments[q].append((m_id, False))
+            else:
+                for op in moment:
+                    if len(op.qubits) == 1:
+                        is_mergable = True
+                        if tags_to_ignore & frozenset(op.tags):
+                            is_mergable = False
+                    else:
+                        is_mergable = False
+                    for q in op.qubits:
+                        active_moments[q].append((m_id, is_mergable))
+
+        single_qubit_moments = [{q: op for op in m if len(op.qubits) == 1} for m in circuit]
+        non_single_qubit_moments = [[op for op in m if len(op.qubits) != 1] for m in circuit]
+
+        for q, active in active_moments.items():
+            for s, e in _get_structure(
+                active, self.min_length, len(circuit), self.gauge_beginning, self.gauge_ending
+            ):
+                gate_index = rng.choice(len(self.gauges))
+                gate = self.gauges[gate_index]
+                gate_inv = self.gauges_inverse[gate_index]
+
+                if existing_op := single_qubit_moments[s].get(q, None):
+                    existing_gate = existing_op.gate
+                    assert existing_gate is not None
+                    single_qubit_moments[s][q] = _merge(existing_gate, gate, q, existing_op.tags)
+                else:
+                    single_qubit_moments[s][q] = gate(q)
+
+                if existing_op := single_qubit_moments[e].get(q, None):
+                    existing_gate = existing_op.gate
+                    assert existing_gate is not None
+                    single_qubit_moments[e][q] = _merge(
+                        gate_inv, existing_gate, q, existing_op.tags
+                    )
+                else:
+                    single_qubit_moments[e][q] = gate_inv(q)
+
+        return circuits.Circuit.from_moments(
+            *(
+                [op for op in sq.values()] + nsq
+                for sq, nsq in zip(single_qubit_moments, non_single_qubit_moments, strict=True)
+            )
+        )