Support multi moment gauge compiling

cirq-core/cirq/transformers/__init__.py

@@ -150,6 +150,7 @@
     SpinInversionGaugeTransformer as SpinInversionGaugeTransformer,
     SqrtCZGaugeTransformer as SqrtCZGaugeTransformer,
     SqrtISWAPGaugeTransformer as SqrtISWAPGaugeTransformer,
+    CPhaseGaugeTransformerMM as CPhaseGaugeTransformerMM,
 )
 
 from cirq.transformers.randomized_measurements import (

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

@@ -47,3 +47,11 @@
 from cirq.transformers.gauge_compiling.idle_moments_gauge import (
     IdleMomentsGauge as IdleMomentsGauge,
 )
+
+from cirq.transformers.gauge_compiling.multi_moment_gauge_compiling import (
+    MultiMomentGaugeTransformer as MultiMomentGaugeTransformer,
+)
+
+from cirq.transformers.gauge_compiling.multi_moment_cphase_gauge import (
+    CPhaseGaugeTransformerMM as CPhaseGaugeTransformerMM,
+)

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

@@ -0,0 +1,245 @@
+# 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.
+
+"""A Multi-Moment Gauge Transformer for the cphase gate."""
+
+from __future__ import annotations
+
+from typing import cast
+
+import numpy as np
+from attrs import field, frozen
+
+from cirq import circuits, ops
+from cirq.transformers.gauge_compiling.multi_moment_gauge_compiling import (
+    MultiMomentGaugeTransformer,
+)
+
+_PAULIS: np.ndarray = np.array((ops.I, ops.X, ops.Y, ops.Z), dtype=object)
+_COMMUTING_GATES = {ops.I, ops.Z}  # I,Z Commute with ZPowGate and CZPowGate; X,Y anti-commute.
+
+
+def _merge_pauliandzpow(left: _PauliAndZPow, right: _PauliAndZPow) -> _PauliAndZPow:
+    # 1. Commute left.zpow and right.pauli:
+    #        ─left.pauli─left.zpow─right.pauli─right.zpow─
+    #    ==> ─left.pauli─right.pauli─(+/-left.zpow─right.zpow)─
+    if right.pauli in _COMMUTING_GATES:
+        new_zpow_exp = left.zpow.exponent + right.zpow.exponent
+    else:
+        new_zpow_exp = -left.zpow.exponent + right.zpow.exponent
+
+    # 2. Merge left.pauli and right.pauli
+    new_pauli = left.pauli
+    if right.pauli is not ops.I:
+        if new_pauli is ops.I:
+            new_pauli = right.pauli
+        else:
+            # left.pauli * right.pauli
+            new_pauli = cast(ops.Pauli, new_pauli).phased_pauli_product(
+                cast(ops.Pauli, right.pauli)
+            )[1]
+
+    return _PauliAndZPow(pauli=new_pauli, zpow=ops.ZPowGate(exponent=new_zpow_exp))
+
+
+@frozen
+class _PauliAndZPow:
+    """A gate represented by a Pauli followed by a ZPowGate.
+
+    The order is ─Pauli──ZPowGate─.
+
+    Attributes:
+        pauli: The Pauli gate.
+        zpow: The ZPowGate.
+    """
+
+    pauli: ops.Pauli | ops.IdentityGate = ops.I
+    zpow: ops.ZPowGate = ops.ZPowGate(exponent=0)
+
+    def merge_left(self, left: _PauliAndZPow) -> _PauliAndZPow:
+        """Merges another `_PauliAndZPow` from the left.
+
+        Calculates `─left─self─` and returns a new `_PauliAndZPow` instance.
+        """
+        return _merge_pauliandzpow(left, self)
+
+    def merge_right(self, right: _PauliAndZPow) -> _PauliAndZPow:
+        """Merges another `_PauliAndZPow` from the right.
+
+        Calculates `─self─right─` and returns a new `_PauliAndZPow` instance.
+        """
+        return _merge_pauliandzpow(self, right)
+
+    def after_cphase(
+        self, cphase: ops.CZPowGate
+    ) -> tuple[ops.CZPowGate, _PauliAndZPow, _PauliAndZPow]:
+        """Pull self through cphase.
+
+        Returns:
+            A tuple of
+                (updated cphase gate, pull_through of this qubit, pull_through of the other qubit).
+        """
+        if self.pauli in _COMMUTING_GATES:
+            return cphase, _PauliAndZPow(self.pauli, self.zpow), _PauliAndZPow()
+        else:
+            # Taking self.pauli==X gate as an example:
+            # 0: ─X─Z^t──@──────      0: ─X──@─────Z^t─       0: ─@──────X──Z^t──
+            #            │       ==>         │           ==>      │
+            # 1: ────────@^exp──      1: ────@^exp─────       1: ─@^-exp─Z^exp───
+            # Similarly for X|Y on qubit 0/1, the result is always flipping cphase and
+            # add an extra Rz rotation on the other qubit.
+            return (
+                cast(ops.CZPowGate, cphase**-1),
+                _PauliAndZPow(self.pauli, self.zpow),
+                _PauliAndZPow(zpow=ops.ZPowGate(exponent=cphase.exponent)),
+            )
+
+    def after_pauli(self, pauli: ops.Pauli | ops.IdentityGate) -> _PauliAndZPow:
+        """Calculates ─self─pauli─  ==>  ─pauli─output─."""
+        if pauli in _COMMUTING_GATES:
+            return _PauliAndZPow(self.pauli, self.zpow)
+        else:
+            return _PauliAndZPow(self.pauli, ops.ZPowGate(exponent=-self.zpow.exponent))
+
+    def after_zpow(self, zpow: ops.ZPowGate) -> tuple[ops.ZPowGate, _PauliAndZPow]:
+        """Calculates ─self─zpow─  ==>  ─+/-zpow─output─."""
+        if self.pauli in _COMMUTING_GATES:
+            return zpow, _PauliAndZPow(self.pauli, self.zpow)
+        else:
+            return ops.ZPowGate(exponent=-zpow.exponent), self
+
+    def __str__(self) -> str:
+        return f"─{self.pauli}──{self.zpow}─"
+
+    def to_single_qubit_gate(self) -> ops.PhasedXZGate | ops.ZPowGate | ops.IdentityGate:
+        """Converts the _PauliAndZPow to a single-qubit gate."""
+        exp = self.zpow.exponent
+        match self.pauli:
+            case ops.I:
+                if exp % 2 == 0:
+                    return ops.I
+                return self.zpow
+            case ops.X:
+                return ops.PhasedXZGate(x_exponent=1, z_exponent=exp, axis_phase_exponent=0)
+            case ops.Y:
+                return ops.PhasedXZGate(x_exponent=1, z_exponent=exp - 1, axis_phase_exponent=0)
+            case _:  # ops.Z
+                return ops.ZPowGate(exponent=1 + exp)
+
+
+def _pull_through_single_cphase(
+    cphase: ops.CZPowGate, input0: _PauliAndZPow, input1: _PauliAndZPow
+) -> tuple[ops.CZPowGate, _PauliAndZPow, _PauliAndZPow]:
+    """Pulls input0 and input1 through a CZPowGate.
+    Input:                      Output:
+    0: ─(input0)─@─────         0: ─@────────(output0)─
+                 │        ==>       │
+    1: ─(input1)─@^exp─         1: ─@^+/-exp─(output1)─
+    """
+
+    # Step 1; pull input0 through CZPowGate.
+    # 0: ─input0─@─────     0: ────────@─────────output0─
+    #            │      ==>            │
+    # 1: ─input1─@^exp─     1: ─input1─@^+/-exp──output1─
+    output_cphase, output0, output1 = input0.after_cphase(cphase)
+
+    # Step 2; similar to step 1, pull input1 through CZPowGate.
+    #      0: ─@──────────pulled0────output0─     0: ─@────────output0─
+    #  ==>     │                              ==>     │
+    #      1: ─@^+/-exp───pulled1────output1─     1: ─@^+/-exp─output1─
+    output_cphase, pulled1, pulled0 = input1.after_cphase(output_cphase)
+    output0 = output0.merge_left(pulled0)
+    output1 = output1.merge_left(pulled1)
+
+    return output_cphase, output0, output1
+
+
+_TARGET_GATESET: ops.Gateset = ops.Gateset(ops.CZPowGate)
+_SUPPORTED_GATESET: ops.Gateset = ops.Gateset(ops.Pauli, ops.IdentityGate, ops.ZPowGate)
+
+
+@frozen
+class CPhaseGaugeTransformerMM(MultiMomentGaugeTransformer):
+    """A gauge transformer for the cphase gate."""
+
+    target: ops.GateFamily | ops.Gateset = field(default=_TARGET_GATESET, init=False)
+    supported_gates: ops.GateFamily | ops.Gateset = field(default=_SUPPORTED_GATESET)
+
+    def sample_left_moment(
+        self, active_qubits: frozenset[ops.Qid], rng: np.random.Generator
+    ) -> circuits.Moment:
+        """Samples a random single-qubit moment to be inserted before the target block."""
+        return circuits.Moment([cast(ops.Gate, rng.choice(_PAULIS)).on(q) for q in active_qubits])
+
+    def gauge_on_moments(
+        self,
+        moments_to_gauge: list[circuits.Moment],
+        prng: np.random.Generator = np.random.default_rng(),
+    ) -> list[circuits.Moment]:
+        """Gauges a block of moments that contains at least a cphase gate in each of the moment.
+
+        Args:
+            moments_to_gauge: A list of moments to be gauged.
+            prng: A pseudorandom number generator.
+
+        Returns:
+            A list of moments after gauging.
+        """
+        active_qubits = circuits.Circuit.from_moments(*moments_to_gauge).all_qubits()
+        left_moment = self.sample_left_moment(active_qubits, prng)
+        pulled: dict[ops.Qid, _PauliAndZPow] = {
+            op.qubits[0]: _PauliAndZPow(pauli=cast(ops.Pauli | ops.IdentityGate, op.gate))
+            for op in left_moment
+            if op.gate
+        }
+        ret: list[circuits.Moment] = [left_moment]
+        # The loop iterates through each moment of the target block, propagating
+        # the `pulled` gauge from left to right. In each iteration, `prev` holds
+        # the gauge to the left of the current `moment`, and the loop computes
+        # the transformed `moment` and the new `pulled` gauge to its right.
+        for moment in moments_to_gauge:
+            # Calculate --prev--moment-- ==> --updated_momment--pulled--
+            prev = pulled
+            pulled = {}
+            ops_at_updated_moment: list[ops.Operation] = []
+            for op in moment:
+                # Pull prev through ops at the moment.
+                if op.gate:
+                    match op.gate:
+                        case ops.CZPowGate():
+                            q0, q1 = op.qubits
+                            new_gate, pulled[q0], pulled[q1] = _pull_through_single_cphase(
+                                op.gate, prev[q0], prev[q1]
+                            )
+                            ops_at_updated_moment.append(new_gate.on(q0, q1))
+                        case ops.Pauli() | ops.IdentityGate():
+                            q = op.qubits[0]
+                            ops_at_updated_moment.append(op)
+                            pulled[q] = prev[q].after_pauli(op.gate)
+                        case ops.ZPowGate():
+                            q = op.qubits[0]
+                            new_zpow, pulled[q] = prev[q].after_zpow(op.gate)
+                            ops_at_updated_moment.append(new_zpow.on(q))
+                        case _:
+                            raise ValueError(f"Gate type {type(op.gate)} is not supported.")
+            # Keep the other ops of prev
+            for q, gate in prev.items():
+                if q not in pulled:
+                    pulled[q] = gate
+            ret.append(circuits.Moment(ops_at_updated_moment))
+        last_moment = circuits.Moment(
+            [gate.to_single_qubit_gate().on(q) for q, gate in pulled.items()]
+        )
+        ret.append(last_moment)
+        return ret