Prepare PauliEvolutionGate for Rustiq & port it to Rust

crates/accelerate/src/circuit_library/mod.rs

@@ -13,11 +13,13 @@
 use pyo3::prelude::*;
 
 mod entanglement;
+mod pauli_evolution;
 mod iqp;
 mod pauli_feature_map;
 mod quantum_volume;
 
 pub fn circuit_library(m: &Bound<PyModule>) -> PyResult<()> {
+    m.add_wrapped(wrap_pyfunction!(pauli_evolution::py_pauli_evolution))?;
     m.add_wrapped(wrap_pyfunction!(pauli_feature_map::pauli_feature_map))?;
     m.add_wrapped(wrap_pyfunction!(entanglement::get_entangler_map))?;
     m.add_wrapped(wrap_pyfunction!(iqp::py_iqp))?;

crates/accelerate/src/circuit_library/pauli_evolution.rs

@@ -0,0 +1,330 @@
+// 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 pyo3::prelude::*;
+use pyo3::types::{PyList, PyString, PyTuple};
+use qiskit_circuit::circuit_data::CircuitData;
+use qiskit_circuit::operations::{multiply_param, radd_param, Param, PyInstruction, StandardGate};
+use qiskit_circuit::packed_instruction::PackedOperation;
+use qiskit_circuit::{imports, Clbit, Qubit};
+use smallvec::{smallvec, SmallVec};
+
+// custom types for a more readable code
+type StandardInstruction = (StandardGate, SmallVec<[Param; 3]>, SmallVec<[Qubit; 2]>);
+type Instruction = (
+    PackedOperation,
+    SmallVec<[Param; 3]>,
+    Vec<Qubit>,
+    Vec<Clbit>,
+);
+
+/// Return instructions (using only StandardGate operations) to implement a Pauli evolution
+/// of a given Pauli string over a given time (as Param).
+///
+/// Args:
+///     pauli: The Pauli string, e.g. "IXYZ".
+///     indices: The qubit indices the Pauli acts on, e.g. if given as [0, 1, 2, 3] with the
+///         Pauli "IXYZ", then the correspondence is I_0 X_1 Y_2 Z_3.
+///     time: The rotation angle. Note that this will directly be used as input of the
+///         rotation gate and not be multiplied by a factor of 2 (that should be done before so
+///         that this function can remain Rust-only).
+///     phase_gate: If ``true``, use the ``PhaseGate`` instead of ``RZGate`` as single-qubit rotation.
+///     do_fountain: If ``true``, implement the CX propagation as "fountain" shape, where each
+///         CX uses the top qubit as target. If ``false``, uses a "chain" shape, where CX in between
+///         neighboring qubits are used.
+///
+/// Returns:
+///     A pointer to an iterator over standard instructions.
+pub fn pauli_evolution<'a>(
+    pauli: &'a str,
+    indices: Vec<u32>,
+    time: Param,
+    phase_gate: bool,
+    do_fountain: bool,
+) -> Box<dyn Iterator<Item = StandardInstruction> + 'a> {
+    // ensure the Pauli has no identity terms
+    let binding = pauli.to_lowercase(); // lowercase for convenience
+    let active = binding
+        .as_str()
+        .chars()
+        .zip(indices)
+        .filter(|(pauli, _)| *pauli != 'i');
+    let (paulis, indices): (Vec<char>, Vec<u32>) = active.unzip();
+
+    match (phase_gate, indices.len()) {
+        (_, 0) => Box::new(std::iter::empty()),
+        (false, 1) => Box::new(single_qubit_evolution(paulis[0], indices[0], time)),
+        (false, 2) => two_qubit_evolution(paulis, indices, time),
+        _ => Box::new(multi_qubit_evolution(
+            paulis,
+            indices,
+            time,
+            phase_gate,
+            do_fountain,
+        )),
+    }
+}
+
+/// Implement a single-qubit Pauli evolution of a Pauli given as char, on a given index and
+/// for given time. Note that the time here equals the angle of the rotation and is not
+/// multiplied by a factor of 2.
+fn single_qubit_evolution(
+    pauli: char,
+    index: u32,
+    time: Param,
+) -> impl Iterator<Item = StandardInstruction> {
+    let qubit: SmallVec<[Qubit; 2]> = smallvec![Qubit(index)];
+    let param: SmallVec<[Param; 3]> = smallvec![time];
+
+    std::iter::once(match pauli {
+        'x' => (StandardGate::RXGate, param, qubit),
+        'y' => (StandardGate::RYGate, param, qubit),
+        'z' => (StandardGate::RZGate, param, qubit),
+        _ => unreachable!("Unsupported Pauli, at this point we expected one of x, y, z."),
+    })
+}
+
+/// Implement a 2-qubit Pauli evolution of a Pauli string, on a given indices and
+/// for given time. Note that the time here equals the angle of the rotation and is not
+/// multiplied by a factor of 2.
+///
+/// If possible, Qiskit's native 2-qubit Pauli rotations are used. Otherwise, the general
+/// multi-qubit evolution is called.
+fn two_qubit_evolution<'a>(
+    pauli: Vec<char>,
+    indices: Vec<u32>,
+    time: Param,
+) -> Box<dyn Iterator<Item = StandardInstruction> + 'a> {
+    let qubits: SmallVec<[Qubit; 2]> = smallvec![Qubit(indices[0]), Qubit(indices[1])];
+    let param: SmallVec<[Param; 3]> = smallvec![time.clone()];
+    let paulistring: String = pauli.iter().collect();
+
+    match paulistring.as_str() {
+        "xx" => Box::new(std::iter::once((StandardGate::RXXGate, param, qubits))),
+        "zx" => Box::new(std::iter::once((StandardGate::RZXGate, param, qubits))),
+        "yy" => Box::new(std::iter::once((StandardGate::RYYGate, param, qubits))),
+        "zz" => Box::new(std::iter::once((StandardGate::RZZGate, param, qubits))),
+        // Note: the CX modes (do_fountain=true/false) give the same circuit for a 2-qubit
+        // Pauli, so we just set it to false here
+        _ => Box::new(multi_qubit_evolution(pauli, indices, time, false, false)),
+    }
+}
+
+/// Implement a multi-qubit Pauli evolution. See ``pauli_evolution`` detailed docs.
+fn multi_qubit_evolution(
+    pauli: Vec<char>,
+    indices: Vec<u32>,
+    time: Param,
+    phase_gate: bool,
+    do_fountain: bool,
+) -> impl Iterator<Item = StandardInstruction> {
+    let active_paulis: Vec<(char, Qubit)> = pauli
+        .into_iter()
+        .zip(indices.into_iter().map(Qubit))
+        .collect();
+
+    // get the basis change: x -> HGate, y -> SXdgGate, z -> nothing
+    let basis_change: Vec<StandardInstruction> = active_paulis
+        .iter()
+        .filter(|(p, _)| *p != 'z')
+        .map(|(p, q)| match p {
+            'x' => (StandardGate::HGate, smallvec![], smallvec![*q]),
+            'y' => (StandardGate::SXGate, smallvec![], smallvec![*q]),
+            _ => unreachable!("Invalid Pauli string."), // "z" and "i" have been filtered out
+        })
+        .collect();
+
+    // get the inverse basis change
+    let inverse_basis_change: Vec<StandardInstruction> = basis_change
+        .iter()
+        .map(|(gate, _, qubit)| match gate {
+            StandardGate::HGate => (StandardGate::HGate, smallvec![], qubit.clone()),
+            StandardGate::SXGate => (StandardGate::SXdgGate, smallvec![], qubit.clone()),
+            _ => unreachable!("Invalid basis-changing Clifford."),
+        })
+        .collect();
+
+    // get the CX propagation up to the first qubit, and down
+    let (chain_up, chain_down) = match do_fountain {
+        true => (
+            cx_fountain(active_paulis.clone()),
+            cx_fountain(active_paulis.clone()).rev(),
+        ),
+        false => (
+            cx_chain(active_paulis.clone()),
+            cx_chain(active_paulis.clone()).rev(),
+        ),
+    };
+
+    // get the RZ gate on the first qubit
+    let first_qubit = active_paulis.first().unwrap().1;
+    let z_rotation = std::iter::once((
+        if phase_gate {
+            StandardGate::PhaseGate
+        } else {
+            StandardGate::RZGate
+        },
+        smallvec![time],
+        smallvec![first_qubit],
+    ));
+
+    // and finally chain everything together
+    basis_change
+        .into_iter()
+        .chain(chain_down)
+        .chain(z_rotation)
+        .chain(chain_up)
+        .chain(inverse_basis_change)
+}
+
+/// Implement a Pauli evolution circuit.
+///
+/// The Pauli evolution is implemented as a basis transformation to the Pauli-Z basis,
+/// followed by a CX-chain and then a single Pauli-Z rotation on the last qubit. Then the CX-chain
+/// is uncomputed and the inverse basis transformation applied. E.g. for the evolution under the
+/// Pauli string XIYZ we have the circuit
+///                 ┌───┐┌───────┐┌───┐
+/// 0: ─────────────┤ X ├┤ Rz(2) ├┤ X ├───────────
+///    ┌──────┐┌───┐└─┬─┘└───────┘└─┬─┘┌───┐┌────┐
+/// 1: ┤ √Xdg ├┤ X ├──■─────────────■──┤ X ├┤ √X ├
+///    └──────┘└─┬─┘                   └─┬─┘└────┘
+/// 2: ──────────┼───────────────────────┼────────
+///     ┌───┐    │                       │  ┌───┐
+/// 3: ─┤ H ├────■───────────────────────■──┤ H ├─
+///     └───┘                               └───┘
+///
+/// Args:
+///     num_qubits: The number of qubits in the Hamiltonian.
+///     sparse_paulis: The Paulis to implement. Given in a sparse-list format with elements
+///         ``(pauli_string, qubit_indices, coefficient)``. An element of the form
+///         ``("IXYZ", [0,1,2,3], 0.2)``, for example, is interpreted in terms of qubit indices as
+///          I_q0 X_q1 Y_q2 Z_q3 and will use a RZ rotation angle of 0.4.
+///     insert_barriers: If ``true``, insert a barrier in between the evolution of individual
+///         Pauli terms.
+///     do_fountain: If ``true``, implement the CX propagation as "fountain" shape, where each
+///         CX uses the top qubit as target. If ``false``, uses a "chain" shape, where CX in between
+///         neighboring qubits are used.
+///
+/// Returns:
+///     Circuit data for to implement the evolution.
+#[pyfunction]
+#[pyo3(name = "pauli_evolution", signature = (num_qubits, sparse_paulis, insert_barriers=false, do_fountain=false))]
+pub fn py_pauli_evolution(
+    num_qubits: i64,
+    sparse_paulis: &Bound<PyList>,
+    insert_barriers: bool,
+    do_fountain: bool,
+) -> PyResult<CircuitData> {
+    let py = sparse_paulis.py();
+    let num_paulis = sparse_paulis.len();
+    let mut paulis: Vec<String> = Vec::with_capacity(num_paulis);
+    let mut indices: Vec<Vec<u32>> = Vec::with_capacity(num_paulis);
+    let mut times: Vec<Param> = Vec::with_capacity(num_paulis);
+    let mut global_phase = Param::Float(0.0);
+    let mut modified_phase = false; // keep track of whether we modified the phase
+
+    for el in sparse_paulis.iter() {
+        let tuple = el.downcast::<PyTuple>()?;
+        let pauli = tuple.get_item(0)?.downcast::<PyString>()?.to_string();
+        let time = Param::extract_no_coerce(&tuple.get_item(2)?)?;
+
+        if pauli.as_str().chars().all(|p| p == 'i') {
+            global_phase = radd_param(global_phase, time, py);
+            modified_phase = true;
+            continue;
+        }
+
+        paulis.push(pauli);
+        times.push(time); // note we do not multiply by 2 here, this is done Python side!
+        indices.push(tuple.get_item(1)?.extract::<Vec<u32>>()?)
+    }
+
+    let barrier = get_barrier(py, num_qubits as u32);
+
+    let evos = paulis.iter().enumerate().zip(indices).zip(times).flat_map(
+        |(((i, pauli), qubits), time)| {
+            let as_packed = pauli_evolution(pauli, qubits, time, false, do_fountain).map(
+                |(gate, params, qubits)| -> PyResult<Instruction> {
+                    Ok((
+                        gate.into(),
+                        params,
+                        Vec::from_iter(qubits.into_iter()),
+                        Vec::new(),
+                    ))
+                },
+            );
+
+            // this creates an iterator containing a barrier only if required, otherwise it is empty
+            let maybe_barrier = (insert_barriers && i < (num_paulis - 1))
+                .then_some(Ok(barrier.clone()))
+                .into_iter();
+            as_packed.chain(maybe_barrier)
+        },
+    );
+
+    // When handling all-identity Paulis above, we added the time as global phase.
+    // However, the all-identity Paulis should add a negative phase, as they implement
+    // exp(-i t I). We apply the negative sign here, to only do a single (-1) multiplication,
+    // instead of doing it every time we find an all-identity Pauli.
+    if modified_phase {
+        global_phase = multiply_param(&global_phase, -1.0, py);
+    }
+
+    CircuitData::from_packed_operations(py, num_qubits as u32, 0, evos, global_phase)
+}
+
+fn cx_chain(
+    active_paulis: Vec<(char, Qubit)>,
+) -> Box<dyn DoubleEndedIterator<Item = StandardInstruction>> {
+    let num_terms = active_paulis.len();
+    Box::new(
+        (0..num_terms - 1)
+            .map(move |i| (active_paulis[i].1, active_paulis[i + 1].1))
+            .map(|(target, ctrl)| (StandardGate::CXGate, smallvec![], smallvec![ctrl, target])),
+    )
+}
+
+fn cx_fountain(
+    active_paulis: Vec<(char, Qubit)>,
+) -> Box<dyn DoubleEndedIterator<Item = StandardInstruction>> {
+    let num_terms = active_paulis.len();
+    let first_qubit = active_paulis[0].1;
+    Box::new((1..num_terms).rev().map(move |i| {
+        let ctrl = active_paulis[i].1;
+        (
+            StandardGate::CXGate,
+            smallvec![],
+            smallvec![ctrl, first_qubit],
+        )
+    }))
+}
+
+fn get_barrier(py: Python, num_qubits: u32) -> Instruction {
+    let barrier_cls = imports::BARRIER.get_bound(py);
+    let barrier = barrier_cls
+        .call1((num_qubits,))
+        .expect("Could not create Barrier Python-side");
+    let barrier_inst = PyInstruction {
+        qubits: num_qubits,
+        clbits: 0,
+        params: 0,
+        op_name: "barrier".to_string(),
+        control_flow: false,
+        instruction: barrier.into(),
+    };
+    (
+        barrier_inst.into(),
+        smallvec![],
+        (0..num_qubits).map(Qubit).collect(),
+        vec![],
+    )
+}