Support sqrt(iSWAP) and Sycamore gates in Floquet calibration (#4248)

Goal of this PR is that circuits having any of these 2-qubit gates (sqrt(iSWAP), Sycamore) can be calibrated on real hardware. In #4164 I ensured that any FSim-compatible gate can be handled on compensation phase. But on characterization step we can support only these 2 gates, because only they are implemented in hardware (so far).

For that:
* Added method `try_convert_syc_or_sqrt_iswap_to_fsim` which is restriction of `try_convert_gate_to_fsim` on gates supported by hardware.
* Used that method in `workflow.py` as gate translator everywhere where `try_convert_sqrt_iswap_to_fsim` was used.
* Fixed a bug in `_merge_into_calibrations` so if there are calibrations for 2 different gates, it wouldn't fail with assertion error, but would append new calibration to the list of calibration.
* Modified a test `test_run_zeta_chi_gamma_calibration_for_moments` so the circuit under test contains gates of 2 types.

cirq-google/cirq_google/calibration/__init__.py

@@ -35,6 +35,7 @@
     WITHOUT_CHI_FLOQUET_PHASED_FSIM_CHARACTERIZATION,
     merge_matching_results,
     try_convert_sqrt_iswap_to_fsim,
+    try_convert_syc_or_sqrt_iswap_to_fsim,
 )
 
 from cirq_google.calibration.workflow import (

cirq-google/cirq_google/calibration/phased_fsim.py

@@ -15,6 +15,7 @@
 import collections
 import dataclasses
 import functools
+import math
 import re
 from typing import (
     Any,
@@ -1016,35 +1017,12 @@ def try_convert_sqrt_iswap_to_fsim(gate: cirq.Gate) -> Optional[PhaseCalibratedF
         either FSimGate, ISWapPowGate, PhasedFSimGate or PhasedISwapPowGate that is equivalent to
         FSimGate(theta=±π/4, phi=0). None otherwise.
     """
-    if isinstance(gate, cirq.FSimGate):
-        if not np.isclose(gate.phi, 0.0):
-            return None
-        angle = gate.theta
-    elif isinstance(gate, cirq.ISwapPowGate):
-        angle = -gate.exponent * np.pi / 2
-    elif isinstance(gate, cirq.PhasedFSimGate):
-        if (
-            not np.isclose(gate.zeta, 0.0)
-            or not np.isclose(gate.chi, 0.0)
-            or not np.isclose(gate.gamma, 0.0)
-            or not np.isclose(gate.phi, 0.0)
-        ):
-            return None
-        angle = gate.theta
-    elif isinstance(gate, cirq.PhasedISwapPowGate):
-        if not np.isclose(-gate.phase_exponent - 0.5, 0.0):
-            return None
-        angle = gate.exponent * np.pi / 2
-    else:
+    result = try_convert_gate_to_fsim(gate)
+    if result is None:
         return None
-
-    angle_canonical = angle % (2 * np.pi)
-
-    if np.isclose(angle_canonical, np.pi / 4):
-        return PhaseCalibratedFSimGate(cirq.FSimGate(theta=np.pi / 4, phi=0.0), 0.0)
-    elif np.isclose(angle_canonical, 7 * np.pi / 4):
-        return PhaseCalibratedFSimGate(cirq.FSimGate(theta=np.pi / 4, phi=0.0), 0.5)
-
+    engine_gate = result.engine_gate
+    if math.isclose(engine_gate.theta, np.pi / 4) and math.isclose(engine_gate.phi, 0.0):
+        return result
     return None
 
 
@@ -1097,3 +1075,28 @@ def try_convert_gate_to_fsim(gate: cirq.Gate) -> Optional[PhaseCalibratedFSimGat
         phase_exponent = phase_exponent + 0.5
     phase_exponent %= 1
     return PhaseCalibratedFSimGate(cirq.FSimGate(theta=theta, phi=phi), phase_exponent)
+
+
+def try_convert_syc_or_sqrt_iswap_to_fsim(
+    gate: cirq.Gate,
+) -> Optional[PhaseCalibratedFSimGate]:
+    """Converts a gate to equivalent PhaseCalibratedFSimGate if possible.
+
+    Args:
+        gate: Gate to convert.
+
+    Returns:
+        Equivalent PhaseCalibratedFSimGate if its `engine_gate` is Sycamore or inverse sqrt(iSWAP)
+        gate. Otherwise returns None.
+    """
+    result = try_convert_gate_to_fsim(gate)
+    if result is None:
+        return None
+    engine_gate = result.engine_gate
+    if math.isclose(engine_gate.theta, np.pi / 2) and math.isclose(engine_gate.phi, np.pi / 6):
+        # Sycamore gate.
+        return result
+    if math.isclose(engine_gate.theta, np.pi / 4) and math.isclose(engine_gate.phi, 0.0):
+        # Inverse sqrt(iSWAP) gate.
+        return result
+    return None

cirq-google/cirq_google/calibration/phased_fsim_test.py

@@ -37,6 +37,7 @@
     WITHOUT_CHI_FLOQUET_PHASED_FSIM_CHARACTERIZATION,
     merge_matching_results,
     try_convert_gate_to_fsim,
+    try_convert_syc_or_sqrt_iswap_to_fsim,
     try_convert_sqrt_iswap_to_fsim,
     XEBPhasedFSimCalibrationRequest,
     XEBPhasedFSimCalibrationOptions,
@@ -767,7 +768,7 @@ def test_try_convert_sqrt_iswap_to_fsim_converts_correctly():
         expected, 0.0
     )
     assert (
-        try_convert_sqrt_iswap_to_fsim(cirq.PhasedISwapPowGate(exponent=-0.5, phase_exponent=0.1))
+        try_convert_sqrt_iswap_to_fsim(cirq.PhasedISwapPowGate(exponent=-0.6, phase_exponent=0.1))
         is None
     )
 
@@ -949,11 +950,33 @@ def check(gate: cirq.Gate, expected: PhaseCalibratedFSimGate):
     assert try_convert_gate_to_fsim(cirq.CZPowGate(exponent=x)) is None
 
 
+def test_try_convert_syc_or_sqrt_iswap_to_fsim():
+    def check_converts(gate: cirq.Gate):
+        result = try_convert_syc_or_sqrt_iswap_to_fsim(gate)
+        assert np.allclose(cirq.unitary(gate), cirq.unitary(result))
+
+    def check_none(gate: cirq.Gate):
+        assert try_convert_syc_or_sqrt_iswap_to_fsim(gate) is None
+
+    check_converts(cirq_google.ops.SYC)
+    check_converts(cirq.FSimGate(np.pi / 2, np.pi / 6))
+    check_none(cirq.FSimGate(0, np.pi))
+    check_converts(cirq.FSimGate(np.pi / 4, 0.0))
+    check_none(cirq.FSimGate(0.2, 0.3))
+    check_converts(cirq.ISwapPowGate(exponent=0.5))
+    check_converts(cirq.ISwapPowGate(exponent=-0.5))
+    check_none(cirq.ISwapPowGate(exponent=0.3))
+    check_converts(cirq.PhasedFSimGate(theta=np.pi / 4, phi=0.0, chi=0.7))
+    check_none(cirq.PhasedFSimGate(theta=0.3, phi=0.4))
+    check_converts(cirq.PhasedISwapPowGate(exponent=0.5, phase_exponent=0.75))
+    check_none(cirq.PhasedISwapPowGate(exponent=0.4, phase_exponent=0.75))
+    check_none(cirq.ops.CZPowGate(exponent=1.0))
+    check_none(cirq.CX)
+
+
 # Test that try_convert_gate_to_fsim is extension of try_convert_sqrt_iswap_to_fsim.
 # In other words, that both function return the same result for all gates on which
 # try_convert_sqrt_iswap_to_fsim is defined.
-# TODO: instead of having multiple gate translators, we should have one, the most general, and
-# restrict it to different gate sets.
 def test_gate_translators_are_consistent():
     def check(gate):
         result1 = try_convert_gate_to_fsim(gate)

cirq-google/cirq_google/calibration/workflow.py

@@ -41,7 +41,7 @@
     THETA_ZETA_GAMMA_FLOQUET_PHASED_FSIM_CHARACTERIZATION,
     merge_matching_results,
     try_convert_gate_to_fsim,
-    try_convert_sqrt_iswap_to_fsim,
+    try_convert_syc_or_sqrt_iswap_to_fsim,
     PhasedFSimCalibrationOptions,
     RequestT,
     LocalXEBPhasedFSimCalibrationRequest,
@@ -73,7 +73,7 @@ def prepare_characterization_for_moment(
     *,
     gates_translator: Callable[
         [cirq.Gate], Optional[PhaseCalibratedFSimGate]
-    ] = try_convert_sqrt_iswap_to_fsim,
+    ] = try_convert_syc_or_sqrt_iswap_to_fsim,
     canonicalize_pairs: bool = False,
     sort_pairs: bool = False,
 ) -> Optional[RequestT]:
@@ -116,7 +116,7 @@ def prepare_floquet_characterization_for_moment(
     options: FloquetPhasedFSimCalibrationOptions,
     gates_translator: Callable[
         [cirq.Gate], Optional[PhaseCalibratedFSimGate]
-    ] = try_convert_sqrt_iswap_to_fsim,
+    ] = try_convert_syc_or_sqrt_iswap_to_fsim,
     canonicalize_pairs: bool = False,
     sort_pairs: bool = False,
 ) -> Optional[FloquetPhasedFSimCalibrationRequest]:
@@ -271,7 +271,7 @@ def prepare_characterization_for_moments(
     *,
     gates_translator: Callable[
         [cirq.Gate], Optional[PhaseCalibratedFSimGate]
-    ] = try_convert_sqrt_iswap_to_fsim,
+    ] = try_convert_syc_or_sqrt_iswap_to_fsim,
     merge_subsets: bool = True,
     initial: Optional[Sequence[RequestT]] = None,
 ) -> Tuple[CircuitWithCalibration, List[RequestT]]:
@@ -345,7 +345,7 @@ def prepare_characterization_for_circuits_moments(
     *,
     gates_translator: Callable[
         [cirq.Gate], Optional[PhaseCalibratedFSimGate]
-    ] = try_convert_sqrt_iswap_to_fsim,
+    ] = try_convert_syc_or_sqrt_iswap_to_fsim,
     merge_subsets: bool = True,
     initial: Optional[Sequence[RequestT]] = None,
 ) -> Tuple[List[CircuitWithCalibration], List[RequestT]]:
@@ -406,7 +406,7 @@ def prepare_floquet_characterization_for_moments(
     options: FloquetPhasedFSimCalibrationOptions = WITHOUT_CHI_FLOQUET_PHASED_FSIM_CHARACTERIZATION,
     gates_translator: Callable[
         [cirq.Gate], Optional[PhaseCalibratedFSimGate]
-    ] = try_convert_sqrt_iswap_to_fsim,
+    ] = try_convert_syc_or_sqrt_iswap_to_fsim,
     merge_subsets: bool = True,
     initial: Optional[Sequence[FloquetPhasedFSimCalibrationRequest]] = None,
 ) -> Tuple[CircuitWithCalibration, List[FloquetPhasedFSimCalibrationRequest]]:
@@ -466,7 +466,7 @@ def prepare_characterization_for_operations(
     *,
     gates_translator: Callable[
         [cirq.Gate], Optional[PhaseCalibratedFSimGate]
-    ] = try_convert_sqrt_iswap_to_fsim,
+    ] = try_convert_syc_or_sqrt_iswap_to_fsim,
     permit_mixed_moments: bool = False,
 ) -> List[RequestT]:
     """Extracts a minimal set of characterization requests necessary to characterize all the
@@ -531,7 +531,7 @@ def prepare_floquet_characterization_for_operations(
     options: FloquetPhasedFSimCalibrationOptions = WITHOUT_CHI_FLOQUET_PHASED_FSIM_CHARACTERIZATION,
     gates_translator: Callable[
         [cirq.Gate], Optional[PhaseCalibratedFSimGate]
-    ] = try_convert_sqrt_iswap_to_fsim,
+    ] = try_convert_syc_or_sqrt_iswap_to_fsim,
     permit_mixed_moments: bool = False,
 ) -> List[FloquetPhasedFSimCalibrationRequest]:
     """Extracts a minimal set of Floquet characterization requests necessary to characterize all the
@@ -679,8 +679,12 @@ def _merge_into_calibrations(
     """
     new_pairs = set(calibration.pairs)
     for index in pairs_map.values():
-        assert calibration.gate == calibrations[index].gate
-        assert calibration.options == calibrations[index].options
+        can_merge = (
+            calibration.gate == calibrations[index].gate
+            and calibration.options == calibrations[index].options
+        )
+        if not can_merge:
+            continue
         existing_pairs = calibrations[index].pairs
         if new_pairs.issubset(existing_pairs):
             return index
@@ -1057,6 +1061,7 @@ def from_characterization(
         """Creates an operation that compensates for zeta, chi and gamma angles of the supplied
         gate and characterization.
 
+        Args:
         Args:
             qubits: Qubits that the gate should act on.
             gate_calibration: Original, imperfect gate that is supposed to run on the hardware
@@ -1081,7 +1086,7 @@ def run_floquet_characterization_for_moments(
     options: FloquetPhasedFSimCalibrationOptions = WITHOUT_CHI_FLOQUET_PHASED_FSIM_CHARACTERIZATION,
     gates_translator: Callable[
         [cirq.Gate], Optional[PhaseCalibratedFSimGate]
-    ] = try_convert_sqrt_iswap_to_fsim,
+    ] = try_convert_syc_or_sqrt_iswap_to_fsim,
     merge_subsets: bool = True,
     max_layers_per_request: int = 1,
     progress_func: Optional[Callable[[int, int], None]] = None,
@@ -1147,7 +1152,7 @@ def run_zeta_chi_gamma_compensation_for_moments(
     ),
     gates_translator: Callable[
         [cirq.Gate], Optional[PhaseCalibratedFSimGate]
-    ] = try_convert_sqrt_iswap_to_fsim,
+    ] = try_convert_syc_or_sqrt_iswap_to_fsim,
     merge_subsets: bool = True,
     max_layers_per_request: int = 1,
     progress_func: Optional[Callable[[int, int], None]] = None,

cirq-google/cirq_google/calibration/workflow_test.py

@@ -45,6 +45,9 @@
 SQRT_ISWAP_INV_PARAMETERS = cirq_google.PhasedFSimCharacterization(
     theta=np.pi / 4, zeta=0.0, chi=0.0, gamma=0.0, phi=0.0
 )
+SYCAMORE_PARAMETERS = cirq_google.PhasedFSimCharacterization(
+    theta=np.pi / 2, zeta=0.0, chi=0.0, gamma=0.0, phi=np.pi / 6
+)
 SQRT_ISWAP_GATE = cirq.FSimGate(7 * np.pi / 4, 0.0)
 SQRT_ISWAP_INV_GATE = cirq.FSimGate(np.pi / 4, 0.0)
 
@@ -1462,19 +1465,19 @@ def test_run_zeta_chi_gamma_calibration_for_moments() -> None:
     parameters_cd = cirq_google.PhasedFSimCharacterization(zeta=0.2, chi=0.3, gamma=0.4)
 
     a, b, c, d = cirq.LineQubit.range(4)
-    engine_simulator = cirq_google.PhasedFSimEngineSimulator.create_from_dictionary_sqrt_iswap(
+    engine_simulator = cirq_google.PhasedFSimEngineSimulator.create_from_dictionary(
         parameters={
-            (a, b): parameters_ab.merge_with(SQRT_ISWAP_INV_PARAMETERS),
-            (b, c): parameters_bc.merge_with(SQRT_ISWAP_INV_PARAMETERS),
-            (c, d): parameters_cd.merge_with(SQRT_ISWAP_INV_PARAMETERS),
+            (a, b): {SQRT_ISWAP_INV_GATE: parameters_ab.merge_with(SQRT_ISWAP_INV_PARAMETERS)},
+            (b, c): {cirq_google.ops.SYC: parameters_bc.merge_with(SYCAMORE_PARAMETERS)},
+            (c, d): {SQRT_ISWAP_INV_GATE: parameters_cd.merge_with(SQRT_ISWAP_INV_PARAMETERS)},
         }
     )
 
     circuit = cirq.Circuit(
         [
             [cirq.X(a), cirq.Y(c)],
             [SQRT_ISWAP_INV_GATE.on(a, b), SQRT_ISWAP_INV_GATE.on(c, d)],
-            [SQRT_ISWAP_INV_GATE.on(b, c)],
+            [cirq_google.ops.SYC.on(b, c)],
         ]
     )
 
@@ -1490,7 +1493,7 @@ def test_run_zeta_chi_gamma_calibration_for_moments() -> None:
         circuit,
         engine_simulator,
         processor_id=None,
-        gate_set=cirq_google.SQRT_ISWAP_GATESET,
+        gate_set=cirq_google.FSIM_GATESET,
         options=options,
     )
 
@@ -1505,7 +1508,7 @@ def test_run_zeta_chi_gamma_calibration_for_moments() -> None:
             options=options,
         ),
         cirq_google.PhasedFSimCalibrationResult(
-            gate=SQRT_ISWAP_INV_GATE, parameters={(b, c): parameters_bc}, options=options
+            gate=cirq_google.ops.SYC, parameters={(b, c): parameters_bc}, options=options
         ),
     ]
     assert calibrated_circuit.moment_to_calibration == [None, None, 0, None, None, 1, None]