Add RemoveIdentityEquivalent transpiler pass (Qiskit#12384)

* add DropNegligible transpiler pass

* use math.isclose instead of numpy.allclose

* use pass directly instead of constructing pass manager

* lint

* allow passing additional gate types

* add docstrings to test classes

* Generalize the pass for all gates and rewrite in rust

This pivots the pass to work for all gates by checking all the
parameters in the standard gate library are within the specified
tolerance, and the matrix is equivalent to an identity for any other
gate defined in Python (with a matrix defined). To improve the
performance of the pass it is written in rust now. Additionally the
class is named to RemoveIdentityEquivalent to make the purpose of the
pass slightly more clear.

* Don't remove zero qubit gates

* Ensure computed gate error is always positive

* Pass docstring improvements

Co-authored-by: Julien Gacon <[email protected]>

* Move test file

* Fix lint

* Fix docs

* Add optimized identity check for Pauli gates

Co-authored-by: Julien Gacon <[email protected]>

* Mention zero qubit gates in the docstring

* Update approximation degree logic in absense of target

* Expand test coverage and fix bugs

---------

Co-authored-by: Matthew Treinish <[email protected]>
Co-authored-by: Julien Gacon <[email protected]>

crates/accelerate/src/lib.rs

@@ -37,6 +37,7 @@ pub mod nlayout;
 pub mod optimize_1q_gates;
 pub mod pauli_exp_val;
 pub mod remove_diagonal_gates_before_measure;
+pub mod remove_identity_equiv;
 pub mod results;
 pub mod sabre;
 pub mod sampled_exp_val;

crates/accelerate/src/remove_identity_equiv.rs

@@ -0,0 +1,149 @@
+// This code is part of Qiskit.
+//
+// (C) Copyright IBM 2024
+//
+// This code is licensed under the Apache License, Version 2.0. You may
+// obtain a copy of this license in the LICENSE.txt file in the root directory
+// of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+//
+// Any modifications or derivative works of this code must retain this
+// copyright notice, and modified files need to carry a notice indicating
+// that they have been altered from the originals.
+
+use num_complex::Complex64;
+use num_complex::ComplexFloat;
+use pyo3::prelude::*;
+use rustworkx_core::petgraph::stable_graph::NodeIndex;
+
+use crate::nlayout::PhysicalQubit;
+use crate::target_transpiler::Target;
+use qiskit_circuit::dag_circuit::DAGCircuit;
+use qiskit_circuit::operations::Operation;
+use qiskit_circuit::operations::OperationRef;
+use qiskit_circuit::operations::Param;
+use qiskit_circuit::operations::StandardGate;
+use qiskit_circuit::packed_instruction::PackedInstruction;
+
+#[pyfunction]
+#[pyo3(signature=(dag, approx_degree=Some(1.0), target=None))]
+fn remove_identity_equiv(
+    dag: &mut DAGCircuit,
+    approx_degree: Option<f64>,
+    target: Option<&Target>,
+) {
+    let mut remove_list: Vec<NodeIndex> = Vec::new();
+
+    let get_error_cutoff = |inst: &PackedInstruction| -> f64 {
+        match approx_degree {
+            Some(degree) => {
+                if degree == 1.0 {
+                    f64::EPSILON
+                } else {
+                    match target {
+                        Some(target) => {
+                            let qargs: Vec<PhysicalQubit> = dag
+                                .get_qargs(inst.qubits)
+                                .iter()
+                                .map(|x| PhysicalQubit::new(x.0))
+                                .collect();
+                            let error_rate = target.get_error(inst.op.name(), qargs.as_slice());
+                            match error_rate {
+                                Some(err) => err * degree,
+                                None => f64::EPSILON.max(1. - degree),
+                            }
+                        }
+                        None => f64::EPSILON.max(1. - degree),
+                    }
+                }
+            }
+            None => match target {
+                Some(target) => {
+                    let qargs: Vec<PhysicalQubit> = dag
+                        .get_qargs(inst.qubits)
+                        .iter()
+                        .map(|x| PhysicalQubit::new(x.0))
+                        .collect();
+                    let error_rate = target.get_error(inst.op.name(), qargs.as_slice());
+                    match error_rate {
+                        Some(err) => err,
+                        None => f64::EPSILON,
+                    }
+                }
+                None => f64::EPSILON,
+            },
+        }
+    };
+
+    for op_node in dag.op_nodes(false) {
+        let inst = dag.dag()[op_node].unwrap_operation();
+        match inst.op.view() {
+            OperationRef::Standard(gate) => {
+                let (dim, trace) = match gate {
+                    StandardGate::RXGate | StandardGate::RYGate | StandardGate::RZGate => {
+                        if let Param::Float(theta) = inst.params_view()[0] {
+                            let trace = (theta / 2.).cos() * 2.;
+                            (2., trace)
+                        } else {
+                            continue;
+                        }
+                    }
+                    StandardGate::RXXGate
+                    | StandardGate::RYYGate
+                    | StandardGate::RZZGate
+                    | StandardGate::RZXGate => {
+                        if let Param::Float(theta) = inst.params_view()[0] {
+                            let trace = (theta / 2.).cos() * 4.;
+                            (4., trace)
+                        } else {
+                            continue;
+                        }
+                    }
+                    _ => {
+                        // Skip global phase gate
+                        if gate.num_qubits() < 1 {
+                            continue;
+                        }
+                        if let Some(matrix) = gate.matrix(inst.params_view()) {
+                            let dim = matrix.shape()[0] as f64;
+                            let trace = matrix.diag().iter().sum::<Complex64>().abs();
+                            (dim, trace)
+                        } else {
+                            continue;
+                        }
+                    }
+                };
+                let error = get_error_cutoff(inst);
+                let f_pro = (trace / dim).powi(2);
+                let gate_fidelity = (dim * f_pro + 1.) / (dim + 1.);
+                if (1. - gate_fidelity).abs() < error {
+                    remove_list.push(op_node)
+                }
+            }
+            OperationRef::Gate(gate) => {
+                // Skip global phase like gate
+                if gate.num_qubits() < 1 {
+                    continue;
+                }
+                if let Some(matrix) = gate.matrix(inst.params_view()) {
+                    let error = get_error_cutoff(inst);
+                    let dim = matrix.shape()[0] as f64;
+                    let trace: Complex64 = matrix.diag().iter().sum();
+                    let f_pro = (trace / dim).abs().powi(2);
+                    let gate_fidelity = (dim * f_pro + 1.) / (dim + 1.);
+                    if (1. - gate_fidelity).abs() < error {
+                        remove_list.push(op_node)
+                    }
+                }
+            }
+            _ => continue,
+        }
+    }
+    for node in remove_list {
+        dag.remove_op_node(node);
+    }
+}
+
+pub fn remove_identity_equiv_mod(m: &Bound<PyModule>) -> PyResult<()> {
+    m.add_wrapped(wrap_pyfunction!(remove_identity_equiv))?;
+    Ok(())
+}

crates/pyext/src/lib.rs

@@ -50,6 +50,7 @@ fn _accelerate(m: &Bound<PyModule>) -> PyResult<()> {
     add_submodule(m, ::qiskit_accelerate::optimize_1q_gates::optimize_1q_gates, "optimize_1q_gates")?;
     add_submodule(m, ::qiskit_accelerate::pauli_exp_val::pauli_expval, "pauli_expval")?;
     add_submodule(m, ::qiskit_accelerate::remove_diagonal_gates_before_measure::remove_diagonal_gates_before_measure, "remove_diagonal_gates_before_measure")?;
+    add_submodule(m, ::qiskit_accelerate::remove_identity_equiv::remove_identity_equiv_mod, "remove_identity_equiv")?;
     add_submodule(m, ::qiskit_accelerate::results::results, "results")?;
     add_submodule(m, ::qiskit_accelerate::sabre::sabre, "sabre")?;
     add_submodule(m, ::qiskit_accelerate::sampled_exp_val::sampled_exp_val, "sampled_exp_val")?;

qiskit/__init__.py

@@ -106,6 +106,7 @@
 sys.modules["qiskit._accelerate.inverse_cancellation"] = _accelerate.inverse_cancellation
 sys.modules["qiskit._accelerate.check_map"] = _accelerate.check_map
 sys.modules["qiskit._accelerate.filter_op_nodes"] = _accelerate.filter_op_nodes
+sys.modules["qiskit._accelerate.remove_identity_equiv"] = _accelerate.remove_identity_equiv
 
 from qiskit.exceptions import QiskitError, MissingOptionalLibraryError
 

qiskit/transpiler/passes/__init__.py

@@ -93,6 +93,7 @@
    NormalizeRXAngle
    OptimizeAnnotated
    Split2QUnitaries
+   RemoveIdentityEquivalent
 
 Calibration
 =============
@@ -247,6 +248,7 @@
 from .optimization import ElidePermutations
 from .optimization import NormalizeRXAngle
 from .optimization import OptimizeAnnotated
+from .optimization import RemoveIdentityEquivalent
 from .optimization import Split2QUnitaries
 
 # circuit analysis

qiskit/transpiler/passes/optimization/__init__.py

@@ -38,5 +38,6 @@
 from .elide_permutations import ElidePermutations
 from .normalize_rx_angle import NormalizeRXAngle
 from .optimize_annotated import OptimizeAnnotated
+from .remove_identity_equiv import RemoveIdentityEquivalent
 from .split_2q_unitaries import Split2QUnitaries
 from .collect_and_collapse import CollectAndCollapse

qiskit/transpiler/passes/optimization/remove_identity_equiv.py

@@ -0,0 +1,69 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2024.
+#
+# This code is licensed under the Apache License, Version 2.0. You may
+# obtain a copy of this license in the LICENSE.txt file in the root directory
+# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# Any modifications or derivative works of this code must retain this
+# copyright notice, and modified files need to carry a notice indicating
+# that they have been altered from the originals.
+
+"""Transpiler pass to drop gates with negligible effects."""
+
+from __future__ import annotations
+
+from qiskit.dagcircuit import DAGCircuit
+from qiskit.transpiler.target import Target
+from qiskit.transpiler.basepasses import TransformationPass
+from qiskit._accelerate.remove_identity_equiv import remove_identity_equiv
+
+
+class RemoveIdentityEquivalent(TransformationPass):
+    r"""Remove gates with negligible effects.
+
+    Removes gates whose effect is close to an identity operation, up to the specified
+    tolerance. Zero qubit gates such as :class:`.GlobalPhaseGate` are not considered
+    by this pass.
+
+    For a cutoff fidelity :math:`f`, this pass removes gates whose average
+    gate fidelity with respect to the identity is below :math:`f`. Concretely,
+    a gate :math:`G` is removed if :math:`\bar F < f` where
+
+    .. math::
+
+        \bar{F} = \frac{1 + F_{\text{process}}{1 + d}
+
+        F_{\text{process}} = \frac{|\mathrm{Tr}(G)|^2}{d^2}
+
+    where :math:`d = 2^n` is the dimension of the gate for :math:`n` qubits.
+    """
+
+    def __init__(
+        self, *, approximation_degree: float | None = 1.0, target: None | Target = None
+    ) -> None:
+        """Initialize the transpiler pass.
+
+        Args:
+            approximation_degree: The degree to approximate for the equivalence check. This can be a
+                floating point value between 0 and 1, or ``None``. If the value is 1 this does not
+                approximate above floating point precision. For a value < 1 this is used as a scaling
+                factor for the cutoff fidelity. If the value is ``None`` this approximates up to the
+                fidelity for the gate specified in ``target``.
+
+            target: If ``approximation_degree`` is set to ``None`` and a :class:`.Target` is provided
+                for this field the tolerance for determining whether an operation is equivalent to
+                identity will be set to the reported error rate in the target. If
+                ``approximation_degree`` (the default) this has no effect, if
+                ``approximation_degree=None`` it uses the error rate specified in the ``Target`` for
+                the gate being evaluated, and a numeric value other than 1 with ``target`` set is
+                used as a scaling factor of the target's error rate.
+        """
+        super().__init__()
+        self._approximation_degree = approximation_degree
+        self._target = target
+
+    def run(self, dag: DAGCircuit) -> DAGCircuit:
+        remove_identity_equiv(dag, self._approximation_degree, self._target)
+        return dag

releasenotes/notes/remove_identity_equiv-9c627c8c35b2298a.yaml

@@ -0,0 +1,29 @@
+---
+features_transpiler:
+  - |
+    Added a new transpiler pass, :class:`.RemoveIdentityEquivalent` that is used
+    to remove gates that are equivalent to an identity up to some tolerance.
+    For example if you had a circuit like:
+
+        .. plot::
+
+           from qiskit.circuit import QuantumCircuit
+
+           qc = QuantumCircuit(2)
+           qc.cp(1e-20, 0, 1)
+           qc.draw("mpl")
+
+    running the pass would eliminate the :class:`.CPhaseGate`:
+
+        .. plot::
+           :include-source:
+
+           from qiskit.circuit import QuantumCircuit
+           from qiskit.transpiler.passes import RemoveIdentityEquivalent
+
+           qc = QuantumCircuit(2)
+           qc.cp(1e-20, 0, 1)
+
+           removal_pass = RemoveIdentityEquivalent()
+           result = removal_pass(qc)
+           result.draw("mpl")