Initial draft of ConvertToSycamore (#2516)

* Initial draft of ConvertToSycamore

- Port of ConvertToSycamore from cirq_internal
- Cleaned up format, mypy, deprecated commands
- Refactored various functions to try to clean them up.
- Got rid of half-pi only rotations
- Combines textbook_gates into convert_to_sycamore

cirq/google/optimizers/convert_to_sycamore_gates.py

@@ -0,0 +1,386 @@
+# Copyright 2019 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 List, cast
+
+import math
+import numpy as np
+import scipy.linalg
+from cirq import circuits, google, linalg, ops, optimizers, protocols
+from cirq.google import SycamoreGate
+
+UNITARY_ZZ = np.kron(protocols.unitary(ops.Z), protocols.unitary(ops.Z))
+PAULI_OPS = [
+    np.eye(2),
+    protocols.unitary(ops.X),
+    protocols.unitary(ops.Y),
+    protocols.unitary(ops.Z)
+]
+
+
+class ConvertToSycamoreGates(circuits.PointOptimizer):
+    """Attempts to convert non-native gates into SycamoreGates.
+
+    First, checks if the given operation is already a native sycamore operation.
+
+    Second, checks if the operation has a known unitary. If so, and the gate
+        is a 1-qubit or 2-qubit gate, then performs circuit synthesis of the
+        operation.
+
+    Third, attempts to `cirq.decompose` to the operation.
+
+    Fourth, if ignore_failures is set, gives up and returns the gate unchanged.
+        Otherwise raises a TypeError.
+    """
+
+    def __init__(self, ignore_failures=False) -> None:
+        """
+        Args:
+            ignore_failures: If set, gates that fail to convert are forwarded
+                unchanged. If not set, conversion failures raise a TypeError.
+        """
+        super().__init__()
+        self.ignore_failures = ignore_failures
+
+    def _is_native_sycamore_op(self, op: ops.Operation) -> bool:
+        """Check if the given operation is native to a Sycamore device.
+
+        Args:
+            op: Input operation.
+
+        Returns:
+            True if the operation is native to the gmon, false otherwise.
+        """
+        return (isinstance(op, ops.GateOperation) and isinstance(
+            cast(ops.GateOperation, op).gate,
+            (SycamoreGate, ops.MeasurementGate, ops.PhasedXPowGate,
+             ops.XPowGate, ops.YPowGate, ops.ZPowGate)))
+
+    def _convert_one(self, op: ops.Operation) -> ops.OP_TREE:
+        """
+        Decomposer intercept:  Upon cirq.protocols.decompose catch and
+        return new OP_Tree
+
+        This should decompose based on number of qubits.
+        """
+        if len(op.qubits) == 1:
+            mat = protocols.unitary(op, None)
+            gates = optimizers.single_qubit_matrix_to_phased_x_z(mat)
+            return [g.on(op.qubits[0]) for g in gates]
+        elif len(op.qubits) == 2 and isinstance(op, ops.GateOperation):
+            return self.known_two_q_operations_to_sycamore_operations(
+                op.qubits[0], op.qubits[1], op)
+        return NotImplemented
+
+    def convert(self, op: ops.Operation) -> List[ops.Operation]:
+
+        def on_stuck_raise(bad):
+            return TypeError("Don't know how to work with {!r}. "
+                             "It isn't a native xmon operation, "
+                             "a 1 or 2 qubit gate with a known unitary, "
+                             "or composite.".format(bad))
+
+        return protocols.decompose(
+            op,
+            keep=self._is_native_sycamore_op,
+            intercepting_decomposer=self._convert_one,
+            on_stuck_raise=None if self.ignore_failures else on_stuck_raise)
+
+    def optimization_at(self, circuit, index, op):
+
+        if not isinstance(op, ops.GateOperation):
+            return None
+
+        gate = op.gate
+
+        # Check for a SWAP and ZZPowGate together
+        if isinstance(gate, ops.ZZPowGate) or gate == ops.SWAP:
+            gate2 = None
+            rads = None
+            next_index = circuit.next_moment_operating_on(op.qubits, index + 1)
+            if next_index is not None:
+                ops_in_front = list(
+                    {circuit.operation_at(q, next_index) for q in op.qubits})
+                if len(ops_in_front) == 1 and isinstance(
+                        ops_in_front[0], ops.GateOperation):
+                    gate2 = ops_in_front[0].gate
+
+            if (isinstance(gate, ops.SwapPowGate) and
+                    isinstance(gate2, ops.ZZPowGate)):
+                rads = gate2.exponent * np.pi / 2
+            if (isinstance(gate, ops.ZZPowGate) and gate2 == ops.SWAP):
+                rads = gate.exponent * np.pi / 2
+            if rads is not None:
+                return circuits.PointOptimizationSummary(
+                    clear_span=next_index - index + 1,
+                    clear_qubits=op.qubits,
+                    new_operations=swap_rzz(rads, op.qubits[0], op.qubits[1]))
+
+        converted = self.convert(op)
+        if len(converted) == 1 and converted[0] is op:
+            return None
+
+        return circuits.PointOptimizationSummary(clear_span=1,
+                                                 new_operations=converted,
+                                                 clear_qubits=op.qubits)
+
+    def known_two_q_operations_to_sycamore_operations(self, qubit_a: ops.Qid,
+                                                      qubit_b: ops.Qid,
+                                                      op: ops.GateOperation
+                                                     ) -> ops.OP_TREE:
+        """
+        Synthesize a known gate operation to a sycamore operation
+
+        This function dispatches based on gate type
+
+        Args:
+            qubit_a: first qubit of GateOperation
+            qubit_b: second qubit of GateOperation
+            op: operation to decompose
+        Returns:
+            New operations iterable object
+        """
+        gate = op.gate
+        if isinstance(gate, ops.CNotPowGate):
+            return [
+                ops.Y(qubit_b)**-0.5,
+                cphase(
+                    cast(ops.CNotPowGate, gate).exponent * np.pi, qubit_a,
+                    qubit_b),
+                ops.Y(qubit_b)**0.5,
+            ]
+        elif isinstance(gate, ops.CZPowGate):
+            gate = cast(ops.CZPowGate, gate)
+            if math.isclose(gate.exponent, 1.0):  # check if CZ or CPHASE
+                return decompose_cz_into_syc(qubit_a, qubit_b)
+            else:
+                # because CZPowGate == diag([1, 1, 1, e^{i pi phi}])
+                return cphase(gate.exponent * np.pi, qubit_a, qubit_b)
+        elif isinstance(gate, ops.SwapPowGate) and math.isclose(
+                cast(ops.SwapPowGate, gate).exponent, 1.0):
+            return decompose_swap_into_syc(qubit_a, qubit_b)
+        elif isinstance(gate, ops.ISwapPowGate) and math.isclose(
+                cast(ops.ISwapPowGate, gate).exponent, 1.0):
+            return decompose_iswap_into_syc(qubit_a, qubit_b)
+        elif isinstance(gate, ops.ZZPowGate):
+            return rzz(
+                cast(ops.ZZPowGate, gate).exponent * np.pi / 2, *op.qubits)
+        elif isinstance(gate, ops.MatrixGate) and len(op.qubits) == 2:
+            new_ops = optimizers.two_qubit_matrix_to_operations(
+                op.qubits[0], op.qubits[1], op, allow_partial_czs=True)
+            gate_ops = []
+            for new_op in new_ops:
+                num_qubits = len(new_op.qubits)
+                if num_qubits == 1:
+                    gate_ops.extend([
+                        term.on(new_op.qubits[0])
+                        for term in optimizers.single_qubit_matrix_to_gates(
+                            protocols.unitary(new_op))
+                    ])
+                elif num_qubits == 2:
+                    gate_ops.extend(
+                        ops.flatten_to_ops(
+                            self.known_two_q_operations_to_sycamore_operations(
+                                new_op.qubits[0], new_op.qubits[1],
+                                cast(ops.GateOperation, new_op))))
+            return gate_ops
+        else:
+            raise ValueError("Unrecognized gate: {!r}".format(op))
+
+
+def decompose_cz_into_syc(a: ops.Qid, b: ops.Qid):
+    """Decompose CZ into sycamore gates using precomputed coefficients"""
+    yield ops.PhasedXPowGate(phase_exponent=0.5678998743900456,
+                             exponent=0.5863459345743176).on(a)
+    yield ops.PhasedXPowGate(phase_exponent=0.3549946157441739).on(b)
+    yield google.SYC.on(a, b)
+    yield ops.PhasedXPowGate(phase_exponent=-0.5154334589432878,
+                             exponent=0.5228733015013345).on(b)
+    yield ops.PhasedXPowGate(phase_exponent=0.06774925307475355).on(a)
+    yield google.SYC.on(a, b),
+    yield ops.PhasedXPowGate(phase_exponent=-0.5987667922766213,
+                             exponent=0.4136540654256824).on(a),
+    yield (ops.Z**-0.9255092746611595).on(b),
+    yield (ops.Z**-1.333333333333333).on(a),
+
+
+def decompose_iswap_into_syc(a: ops.Qid, b: ops.Qid):
+    """Decompose ISWAP into sycamore gates using precomputed coefficients"""
+    yield ops.PhasedXPowGate(phase_exponent=-0.27250925776964596,
+                             exponent=0.2893438375555899).on(a)
+    yield google.SYC.on(a, b)
+    yield ops.PhasedXPowGate(phase_exponent=0.8487591858680898,
+                             exponent=0.9749387200813147).on(b),
+    yield ops.PhasedXPowGate(phase_exponent=-0.3582574564210601).on(a),
+    yield google.SYC.on(a, b)
+    yield ops.PhasedXPowGate(phase_exponent=0.9675022326694225,
+                             exponent=0.6908986856555526).on(a),
+    yield google.SYC.on(a, b),
+    yield ops.PhasedXPowGate(phase_exponent=0.9161706861686068,
+                             exponent=0.14818318325264102).on(b),
+    yield ops.PhasedXPowGate(phase_exponent=0.9408341606787907).on(a),
+    yield (ops.Z**-1.1551880579397293).on(b),
+    yield (ops.Z**0.31848343246696365).on(a),
+
+
+def decompose_swap_into_syc(a: ops.Qid, b: ops.Qid):
+    """Decompose SWAP into sycamore gates using precomputed coefficients"""
+    yield ops.PhasedXPowGate(phase_exponent=0.44650378384076217,
+                             exponent=0.8817921214052824).on(a)
+    yield ops.PhasedXPowGate(phase_exponent=-0.7656774060816165,
+                             exponent=0.6628666504604785).on(b)
+    yield google.SYC.on(a, b)
+    yield ops.PhasedXPowGate(phase_exponent=-0.6277589946716742,
+                             exponent=0.5659160932099687).on(a)
+    yield google.SYC.on(a, b)
+    yield ops.PhasedXPowGate(phase_exponent=0.28890767199499257,
+                             exponent=0.4340839067900317).on(b)
+    yield ops.PhasedXPowGate(phase_exponent=-0.22592784059288928).on(a)
+    yield google.SYC.on(a, b)
+    yield ops.PhasedXPowGate(phase_exponent=-0.4691261557936808,
+                             exponent=0.7728525693920243).on(a)
+    yield ops.PhasedXPowGate(phase_exponent=-0.8150261316932077,
+                             exponent=0.11820787859471782).on(b)
+    yield (ops.Z**-0.7384700844660306).on(b)
+    yield (ops.Z**-0.7034535141382525).on(a)
+
+
+def find_local_equivalents(unitary1: np.ndarray, unitary2: np.ndarray):
+    """
+        Given two unitaries with the same interaction coefficients but different
+        local unitary rotations determine the local unitaries that turns
+        one type of gate into another.
+
+        1) perform the kak decomp on each unitary and confirm interaction terms
+           are equivalent
+        2) identify the elements of SU(2) to transform unitary2 into unitary1
+
+        Args:
+            unitary1: unitary that we target
+            unitary2: unitary that we transform the local gates to the target
+        Returns:
+            four 2x2 unitaries.  first two are pre-rotations last two are post
+            rotations.
+        """
+    kak_u1 = linalg.kak_decomposition(unitary1)
+    kak_u2 = linalg.kak_decomposition(unitary2)
+
+    u_0 = (kak_u1.single_qubit_operations_after[0]
+           @ kak_u2.single_qubit_operations_after[0].conj().T)
+    u_1 = (kak_u1.single_qubit_operations_after[1]
+           @ kak_u2.single_qubit_operations_after[1].conj().T)
+
+    v_0 = (kak_u2.single_qubit_operations_before[0].conj().T
+           @ kak_u1.single_qubit_operations_before[0])
+    v_1 = (kak_u2.single_qubit_operations_before[1].conj().T
+           @ kak_u1.single_qubit_operations_before[1])
+
+    return v_0, v_1, u_0, u_1
+
+
+def create_corrected_circuit(target_unitary: np.ndarray,
+                             program: circuits.Circuit, q0: ops.Qid,
+                             q1: ops.Qid):
+    # Get the local equivalents
+    b_0, b_1, a_0, a_1 = find_local_equivalents(
+        target_unitary,
+        program.unitary(qubit_order=ops.QubitOrder.explicit([q0, q1])))
+
+    # Apply initial corrections
+    yield (gate(q0) for gate in optimizers.single_qubit_matrix_to_gates(b_0))
+    yield (gate(q1) for gate in optimizers.single_qubit_matrix_to_gates(b_1))
+
+    # Apply interaction part
+    yield program
+
+    # Apply final corrections
+    yield (gate(q0) for gate in optimizers.single_qubit_matrix_to_gates(a_0))
+    yield (gate(q1) for gate in optimizers.single_qubit_matrix_to_gates(a_1))
+
+
+def rzz(theta: float, q0: ops.Qid, q1: ops.Qid) -> ops.OP_TREE:
+    """Generate exp(-1j * theta * zz) from Sycamore gates.
+
+        Args:
+            theta: rotation parameter
+            q0: First qubit id to operate on
+            q1: Second qubit id to operate on
+        Returns:
+            a Cirq program implementing the Ising gate
+        rtype:
+            cirq.OP_Tree
+        """
+    phi = -np.pi / 24
+    c_phi = np.cos(2 * phi)
+    target_unitary = scipy.linalg.expm(-1j * theta * UNITARY_ZZ)
+
+    if abs(np.cos(theta)) > c_phi:
+        c2 = abs(np.sin(theta)) / c_phi
+    else:
+        c2 = abs(np.cos(theta)) / c_phi
+
+    # Prepare program that has same Schmidt coeffs as exp(1j theta ZZ)
+    program = circuits.Circuit(google.SYC.on(q0, q1),
+                               ops.rx(2 * np.arccos(c2)).on(q1),
+                               google.SYC.on(q0, q1))
+
+    yield create_corrected_circuit(target_unitary, program, q0, q1)
+
+
+def cphase(theta: float, q0: ops.Qid, q1: ops.Qid) -> ops.OP_TREE:
+    """
+        Implement a cphase using the Ising gate generated from 2 Sycamore gates
+
+        A CPHASE gate has the matrix diag([1, 1, 1, exp(1j * theta)]) and can
+        be mapped to the Ising gate by prep and post rotations of Z-pi/4.
+        We drop the global phase shift of theta/4.
+
+        Args:
+            theta: exp(1j * theta )
+            q0: First qubit id to operate on
+            q1: Second qubit id to operate on
+        returns:
+            a cirq program implementating cphase
+        """
+    yield rzz(-theta / 4, q0, q1)
+    yield ops.rz(theta / 2).on(q0)
+    yield ops.rz(theta / 2).on(q1)
+
+
+def swap_rzz(theta: float, q0: ops.Qid, q1: ops.Qid) -> ops.OP_TREE:
+    """
+        An implementation of SWAP * EXP(1j theta ZZ) using three sycamore gates.
+
+        This builds off of the zztheta method.  A sycamore gate following the
+        zz-gate is a SWAP EXP(1j (THETA - pi/24) ZZ).
+
+        Args:
+            theta: exp(1j * theta )
+            q0: First qubit id to operate on
+            q1: Second qubit id to operate on
+        Returns:
+            The circuit that implements ZZ followed by a swap
+        """
+
+    # Set interaction part.
+    circuit = circuits.Circuit()
+    angle_offset = np.pi / 24 - np.pi / 4
+    circuit.append(google.SYC(q0, q1))
+    circuit.append(rzz(theta - angle_offset, q0, q1))
+
+    # Get the intended circuit.
+    intended_circuit = circuits.Circuit(
+        ops.SWAP(q0, q1),
+        ops.ZZPowGate(exponent=2 * theta / np.pi, global_shift=-0.5).on(q0, q1))
+
+    yield create_corrected_circuit(intended_circuit, circuit, q0, q1)