Add cirq.SqrtIswapTargetGateset for (parameterized & non-parameterized) compilation to sqrt iswaps. (quantumlib#5025)

* Move parameterized decomposition to analytical decomposers and split PR to remove deprecations

* Add equality tests

Author: tanujkhattar

cirq/__init__.py

@@ -384,8 +384,10 @@
     merge_single_qubit_gates_to_phxz,
     merge_single_qubit_moments_to_phxz,
     optimize_for_target_gateset,
+    parameterized_2q_op_to_sqrt_iswap_operations,
     prepare_two_qubit_state_using_cz,
     prepare_two_qubit_state_using_sqrt_iswap,
+    SqrtIswapTargetGateset,
     single_qubit_matrix_to_gates,
     single_qubit_matrix_to_pauli_rotations,
     single_qubit_matrix_to_phased_x_z,

cirq/json_resolver_cache.py

@@ -156,6 +156,7 @@ def _parallel_gate_op(gate, qubits):
         'SingleQubitCliffordGate': cirq.SingleQubitCliffordGate,
         'SingleQubitPauliStringGateOperation': cirq.SingleQubitPauliStringGateOperation,
         'SingleQubitReadoutCalibrationResult': cirq.experiments.SingleQubitReadoutCalibrationResult,
+        'SqrtIswapTargetGateset': cirq.SqrtIswapTargetGateset,
         'StabilizerStateChForm': cirq.StabilizerStateChForm,
         'StatePreparationChannel': cirq.StatePreparationChannel,
         'SwapPowGate': cirq.SwapPowGate,

cirq/protocols/json_test_data/SqrtIswapTargetGateset.json

@@ -0,0 +1,20 @@
+[
+  {
+    "cirq_type": "SqrtIswapTargetGateset",
+    "atol": 1e-08,
+    "required_sqrt_iswap_count": null,
+    "use_sqrt_iswap_inv": false
+  },
+  {
+    "cirq_type": "SqrtIswapTargetGateset",
+    "atol": 1e-08,
+    "required_sqrt_iswap_count": 1,
+    "use_sqrt_iswap_inv": false
+  },
+  {
+    "cirq_type": "SqrtIswapTargetGateset",
+    "atol": 1e-06,
+    "required_sqrt_iswap_count": 2,
+    "use_sqrt_iswap_inv": true
+  }
+]

cirq/protocols/json_test_data/SqrtIswapTargetGateset.repr

@@ -0,0 +1,7 @@
+[
+    cirq.SqrtIswapTargetGateset(
+        atol=1e-08, required_sqrt_iswap_count=None, use_sqrt_iswap_inv=False
+    ),
+    cirq.SqrtIswapTargetGateset(atol=1e-08, required_sqrt_iswap_count=1, use_sqrt_iswap_inv=False),
+    cirq.SqrtIswapTargetGateset(atol=1e-06, required_sqrt_iswap_count=2, use_sqrt_iswap_inv=True),
+]

cirq/transformers/__init__.py

@@ -22,6 +22,7 @@
     decompose_multi_controlled_rotation,
     decompose_two_qubit_interaction_into_four_fsim_gates,
     is_negligible_turn,
+    parameterized_2q_op_to_sqrt_iswap_operations,
     prepare_two_qubit_state_using_cz,
     prepare_two_qubit_state_using_sqrt_iswap,
     single_qubit_matrix_to_gates,
@@ -44,6 +45,7 @@
 from cirq.transformers.target_gatesets import (
     CompilationTargetGateset,
     CZTargetGateset,
+    SqrtIswapTargetGateset,
     TwoQubitCompilationTargetGateset,
 )
 

cirq/transformers/analytical_decompositions/__init__.py

@@ -51,6 +51,7 @@
 )
 
 from cirq.transformers.analytical_decompositions.two_qubit_to_sqrt_iswap import (
+    parameterized_2q_op_to_sqrt_iswap_operations,
     two_qubit_matrix_to_sqrt_iswap_operations,
 )
 

cirq/transformers/analytical_decompositions/two_qubit_to_sqrt_iswap.py

@@ -23,6 +23,7 @@
 from typing import Optional, Sequence, Tuple, TYPE_CHECKING
 
 import numpy as np
+import sympy
 
 from cirq import circuits, ops, linalg, protocols
 from cirq.transformers.analytical_decompositions import single_qubit_decompositions
@@ -32,6 +33,201 @@
     import cirq
 
 
+def parameterized_2q_op_to_sqrt_iswap_operations(
+    op: 'cirq.Operation', *, use_sqrt_iswap_inv: bool = False
+) -> protocols.decompose_protocol.DecomposeResult:
+    """Tries to decompose a parameterized 2q operation into √iSWAP's + parameterized 1q rotations.
+
+    Currently only supports decomposing the following gates:
+        a) `cirq.CZPowGate`
+        b) `cirq.SwapPowGate`
+        c) `cirq.ISwapPowGate`
+        d) `cirq.FSimGate`
+
+    Args:
+        op: Parameterized two qubit operation to be decomposed into sqrt-iswaps.
+        use_sqrt_iswap_inv: If True, `cirq.SQRT_ISWAP_INV` is used as the target 2q gate, instead
+            of `cirq.SQRT_ISWAP`.
+
+    Returns:
+        A parameterized `cirq.OP_TREE` implementing `op` using only `cirq.SQRT_ISWAP`
+        (or `cirq.SQRT_ISWAP_INV`) and parameterized single qubit rotations OR
+        None or NotImplemented if decomposition of `op` is not known.
+    """
+    gate = op.gate
+    q0, q1 = op.qubits
+
+    if isinstance(gate, ops.CZPowGate):
+        return _cphase_symbols_to_sqrt_iswap(q0, q1, gate.exponent, use_sqrt_iswap_inv)
+    if isinstance(gate, ops.SwapPowGate):
+        return _swap_symbols_to_sqrt_iswap(q0, q1, gate.exponent, use_sqrt_iswap_inv)
+    if isinstance(gate, ops.ISwapPowGate):
+        return _iswap_symbols_to_sqrt_iswap(q0, q1, gate.exponent, use_sqrt_iswap_inv)
+    if isinstance(gate, ops.FSimGate):
+        return _fsim_symbols_to_sqrt_iswap(q0, q1, gate.theta, gate.phi, use_sqrt_iswap_inv)
+    return NotImplemented
+
+
+def _sqrt_iswap_inv(
+    a: 'cirq.Qid', b: 'cirq.Qid', use_sqrt_iswap_inv: bool = True
+) -> 'cirq.OP_TREE':
+    """Optree implementing `cirq.SQRT_ISWAP_INV(a, b)` using √iSWAPs.
+
+    Args:
+        a: The first qubit.
+        b: The second qubit.
+        use_sqrt_iswap_inv: If True, `cirq.SQRT_ISWAP_INV` is used instead of `cirq.SQRT_ISWAP`.
+
+    Returns:
+        `cirq.SQRT_ISWAP_INV(a, b)` or equivalent unitary implemented using `cirq.SQRT_ISWAP`.
+    """
+    return (
+        ops.SQRT_ISWAP_INV(a, b)
+        if use_sqrt_iswap_inv
+        else [ops.Z(a), ops.SQRT_ISWAP(a, b), ops.Z(a)]
+    )
+
+
+def _cphase_symbols_to_sqrt_iswap(
+    a: 'cirq.Qid', b: 'cirq.Qid', turns: 'cirq.TParamVal', use_sqrt_iswap_inv: bool = True
+):
+    """Implements `cirq.CZ(a, b) ** turns` using two √iSWAPs and single qubit rotations.
+
+    Output unitary:
+        [[1, 0, 0, 0],
+         [0, 1, 0, 0],
+         [0, 0, 1, 0],
+         [0, 0, 0, g]]
+    where:
+        g = exp(i·π·t).
+
+    Args:
+        a: The first qubit.
+        b: The second qubit.
+        turns: The rotational angle (t) that specifies the gate, where
+            g = exp(i·π·t/2).
+        use_sqrt_iswap_inv: If True, `cirq.SQRT_ISWAP_INV` is used instead of `cirq.SQRT_ISWAP`.
+
+    Yields:
+        A `cirq.OP_TREE` representing the decomposition.
+    """
+    theta = sympy.Mod(turns, 2.0) * sympy.pi
+
+    # -1 if theta > pi.  Adds a hacky fudge factor so theta=pi is not 0
+    sign = sympy.sign(sympy.pi - theta + 1e-9)
+
+    # For sign = 1: theta. For sign = -1, 2pi-theta
+    theta_prime = (sympy.pi - sign * sympy.pi) + sign * theta
+
+    phi = sympy.asin(np.sqrt(2) * sympy.sin(theta_prime / 4))
+    xi = sympy.atan(sympy.tan(phi) / np.sqrt(2))
+
+    yield ops.rz(sign * 0.5 * theta_prime).on(a)
+    yield ops.rz(sign * 0.5 * theta_prime).on(b)
+    yield ops.rx(xi).on(a)
+    yield ops.X(b) ** (-sign * 0.5)
+    yield _sqrt_iswap_inv(a, b, use_sqrt_iswap_inv)
+    yield ops.rx(-2 * phi).on(a)
+    yield ops.Z(a)
+    yield _sqrt_iswap_inv(a, b, use_sqrt_iswap_inv)
+    yield ops.Z(a)
+    yield ops.rx(xi).on(a)
+    yield ops.X(b) ** (sign * 0.5)
+
+
+def _swap_symbols_to_sqrt_iswap(
+    a: 'cirq.Qid', b: 'cirq.Qid', turns: 'cirq.TParamVal', use_sqrt_iswap_inv: bool = True
+):
+    """Implements `cirq.SWAP(a, b) ** turns` using two √iSWAPs and single qubit rotations.
+
+    Output unitary:
+        [[1, 0,        0,     0],
+         [0, g·c,    -i·g·s,  0],
+         [0, -i·g·s,  g·c,    0],
+         [0,   0,      0,     1]]
+    where:
+        c = cos(π·t/2), s = sin(π·t/2), g = exp(i·π·t/2).
+
+    Args:
+        a: The first qubit.
+        b: The second qubit.
+        turns: The rotational angle (t) that specifies the gate, where
+            c = cos(π·t/2), s = sin(π·t/2), g = exp(i·π·t/2).
+        use_sqrt_iswap_inv: If True, `cirq.SQRT_ISWAP_INV` is used instead of `cirq.SQRT_ISWAP`.
+
+    Yields:
+        A `cirq.OP_TREE` representing the decomposition.
+    """
+    yield ops.Z(a) ** 1.25
+    yield ops.Z(b) ** -0.25
+    yield _sqrt_iswap_inv(a, b, use_sqrt_iswap_inv)
+    yield ops.Z(a) ** (-turns / 2 + 1)
+    yield ops.Z(b) ** (turns / 2)
+    yield _sqrt_iswap_inv(a, b, use_sqrt_iswap_inv)
+    yield ops.Z(a) ** (turns / 2 - 0.25)
+    yield ops.Z(b) ** (turns / 2 + 0.25)
+    yield _cphase_symbols_to_sqrt_iswap(a, b, -turns, use_sqrt_iswap_inv)
+
+
+def _iswap_symbols_to_sqrt_iswap(
+    a: 'cirq.Qid', b: 'cirq.Qid', turns: 'cirq.TParamVal', use_sqrt_iswap_inv: bool = True
+):
+    """Implements `cirq.ISWAP(a, b) ** turns` using two √iSWAPs and single qubit rotations.
+
+    Output unitary:
+       [[1   0   0   0],
+        [0   c  is   0],
+        [0  is   c   0],
+        [0   0   0   1]]
+    where c = cos(π·t/2), s = sin(π·t/2).
+
+    Args:
+        a: The first qubit.
+        b: The second qubit.
+        turns: The rotational angle (t) that specifies the gate, where
+            c = cos(π·t/2), s = sin(π·t/2).
+        use_sqrt_iswap_inv: If True, `cirq.SQRT_ISWAP_INV` is used instead of `cirq.SQRT_ISWAP`.
+
+    Yields:
+        A `cirq.OP_TREE` representing the decomposition.
+    """
+    yield ops.Z(a) ** 0.75
+    yield ops.Z(b) ** 0.25
+    yield _sqrt_iswap_inv(a, b, use_sqrt_iswap_inv)
+    yield ops.Z(a) ** (-turns / 2 + 1)
+    yield ops.Z(b) ** (turns / 2)
+    yield _sqrt_iswap_inv(a, b, use_sqrt_iswap_inv)
+    yield ops.Z(a) ** 0.25
+    yield ops.Z(b) ** -0.25
+
+
+def _fsim_symbols_to_sqrt_iswap(
+    a: 'cirq.Qid',
+    b: 'cirq.Qid',
+    theta: 'cirq.TParamVal',
+    phi: 'cirq.TParamVal',
+    use_sqrt_iswap_inv: bool = True,
+):
+    """Implements `cirq.FSimGate(theta, phi)(a, b)` using two √iSWAPs and single qubit rotations.
+
+    FSimGate(θ, φ) = ISWAP**(-2θ/π) CZPowGate(exponent=-φ/π)
+
+    Args:
+        a: The first qubit.
+        b: The second qubit.
+        theta: Swap angle on the ``|01⟩`` ``|10⟩`` subspace, in radians.
+        phi: Controlled phase angle, in radians.
+        use_sqrt_iswap_inv: If True, `cirq.SQRT_ISWAP_INV` is used instead of `cirq.SQRT_ISWAP`.
+
+    Yields:
+        A `cirq.OP_TREE` representing the decomposition.
+    """
+    if theta != 0.0:
+        yield _iswap_symbols_to_sqrt_iswap(a, b, -2 * theta / np.pi, use_sqrt_iswap_inv)
+    if phi != 0.0:
+        yield _cphase_symbols_to_sqrt_iswap(a, b, -phi / np.pi, use_sqrt_iswap_inv)
+
+
 def two_qubit_matrix_to_sqrt_iswap_operations(
     q0: 'cirq.Qid',
     q1: 'cirq.Qid',

cirq/transformers/analytical_decompositions/two_qubit_to_sqrt_iswap_test.py

@@ -17,6 +17,7 @@
 import pytest
 
 import cirq
+import sympy
 
 ALLOW_DEPRECATION_IN_TEST = 'ALLOW_DEPRECATION_IN_TEST'
 
@@ -258,6 +259,61 @@ def assert_specific_sqrt_iswap_count(operations, count):
     assert actual == count, f'Incorrect sqrt-iSWAP count.  Expected {count} but got {actual}.'
 
 
+@pytest.mark.parametrize(
+    'gate',
+    [
+        cirq.ISwapPowGate(exponent=sympy.Symbol('t')),
+        cirq.SwapPowGate(exponent=sympy.Symbol('t')),
+        cirq.CZPowGate(exponent=sympy.Symbol('t')),
+    ],
+)
+def test_two_qubit_gates_with_symbols(gate: cirq.Gate):
+    op = gate(*cirq.LineQubit.range(2))
+    c_new_sqrt_iswap = cirq.Circuit(cirq.parameterized_2q_op_to_sqrt_iswap_operations(op))
+    c_new_sqrt_iswap_inv = cirq.Circuit(
+        cirq.parameterized_2q_op_to_sqrt_iswap_operations(op, use_sqrt_iswap_inv=True)
+    )
+    # Check if unitaries are the same
+    for val in np.linspace(0, 2 * np.pi, 12):
+        cirq.testing.assert_allclose_up_to_global_phase(
+            cirq.unitary(cirq.resolve_parameters(op, {'t': val})),
+            cirq.unitary(cirq.resolve_parameters(c_new_sqrt_iswap, {'t': val})),
+            atol=1e-6,
+        )
+        cirq.testing.assert_allclose_up_to_global_phase(
+            cirq.unitary(cirq.resolve_parameters(op, {'t': val})),
+            cirq.unitary(cirq.resolve_parameters(c_new_sqrt_iswap_inv, {'t': val})),
+            atol=1e-6,
+        )
+
+
+def test_fsim_gate_with_symbols():
+    theta, phi = sympy.symbols(['theta', 'phi'])
+    op = cirq.FSimGate(theta=theta, phi=phi).on(*cirq.LineQubit.range(2))
+    c_new_sqrt_iswap = cirq.Circuit(cirq.parameterized_2q_op_to_sqrt_iswap_operations(op))
+    c_new_sqrt_iswap_inv = cirq.Circuit(
+        cirq.parameterized_2q_op_to_sqrt_iswap_operations(op, use_sqrt_iswap_inv=True)
+    )
+    for theta_val in np.linspace(0, 2 * np.pi, 12):
+        for phi_val in np.linspace(0, 2 * np.pi, 12):
+            cirq.testing.assert_allclose_up_to_global_phase(
+                cirq.unitary(cirq.resolve_parameters(op, {'theta': theta_val, 'phi': phi_val})),
+                cirq.unitary(
+                    cirq.resolve_parameters(c_new_sqrt_iswap, {'theta': theta_val, 'phi': phi_val})
+                ),
+                atol=1e-6,
+            )
+            cirq.testing.assert_allclose_up_to_global_phase(
+                cirq.unitary(cirq.resolve_parameters(op, {'theta': theta_val, 'phi': phi_val})),
+                cirq.unitary(
+                    cirq.resolve_parameters(
+                        c_new_sqrt_iswap_inv, {'theta': theta_val, 'phi': phi_val}
+                    )
+                ),
+                atol=1e-6,
+            )
+
+
 @pytest.mark.parametrize('cnt', [-1, 4, 10])
 def test_invalid_required_sqrt_iswap_count(cnt):
     u = TWO_SQRT_ISWAP_UNITARIES[0]

cirq/transformers/target_gatesets/__init__.py

@@ -20,3 +20,5 @@
 )
 
 from cirq.transformers.target_gatesets.cz_gateset import CZTargetGateset
+
+from cirq.transformers.target_gatesets.sqrt_iswap_gateset import SqrtIswapTargetGateset