Add ProgramNode

Author: ihincks

Cargo.lock

@@ -12,6 +12,7 @@ name = "qiskit_providers"
 rustworkx-core.workspace = true
 anyhow.workspace = true
 num-complex.workspace = true
+thiserror.workspace = true
 
 [dependencies.hashbrown]
 workspace = true

crates/providers/Cargo.toml

@@ -11,5 +11,10 @@
 // that they have been altered from the originals.
 
 mod data_tree;
+mod program_node;
+mod store;
 pub mod tensor;
+
 pub use data_tree::{DataTree, PathEntry};
+pub use program_node::ProgramNode;
+pub use store::Store;

crates/providers/src/lib.rs

@@ -0,0 +1,40 @@
+// This code is part of Qiskit.
+//
+// (C) Copyright IBM 2026
+//
+// 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 https://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 crate::data_tree::DataTree;
+use crate::tensor::{Tensor, TensorType};
+
+/// A node in a quantum program graph that transforms tensors.
+pub trait ProgramNode {
+    /// The name of this program node.
+    fn name(&self) -> &'static str;
+
+    /// The namespace this program node belongs to.
+    fn namespace(&self) -> &'static str;
+
+    /// The namespace and name as one string.
+    fn full_name(&self) -> String {
+        format_args!("{}.{}", self.namespace(), self.name()).to_string()
+    }
+
+    /// The inputs expected at `call` time.
+    fn input_types(&self) -> &DataTree<TensorType>;
+
+    /// The outputs promised on `call` return.
+    fn output_types(&self) -> &DataTree<TensorType>;
+
+    /// Whether this program node implements the call method.
+    fn implements_call(&self) -> bool;
+
+    /// The action of this program node.
+    fn call(&self, args: &DataTree<Tensor>) -> anyhow::Result<DataTree<Tensor>>;
+}