IBM broadens Qiskit SDK 2.6 C API for compiled-language workflows

IBM expanded Qiskit SDK 2.6 with a C API pass-manager framework, QPY circuit serialization and support for custom operations, giving developers in compiled languages more direct access to compiler and circuit infrastructure. IBM published its detailed release summary on October 22; the signed Qiskit 2.6.0 repository release and package distribution had been available since October 16.

Programs using Qiskit through C or a language’s C foreign-function interface now have greater control over compilation. Developers can write individual compiler passes, combine them into custom pipelines and declare the intermediate representation each pass consumes and produces. The pass manager checks those declarations, allowing a program to move through built-in circuit and directed-acyclic-graph representations or custom representations while catching incompatible stages.

QPY support also provides a path for moving circuits between languages and runs. The C API can read or write multiple native circuits in a QPY stream through memory buffers or files, and the output works with Python’s qiskit.qpy interface. IBM says this lets C, C++, Julia and Fortran workflows save a compiled circuit and reload it later without starting a Python interpreter, a workflow intended to make Qiskit easier to integrate into high-performance-computing systems.

That does not yet make Python entirely optional. IBM says standalone C programs do not start the Python interpreter, but Python still must be installed because parts of Qiskit’s Rust data model refer to Python types at link time. The company says version 2.6 advances work to remove that restriction and that it hopes to complete the work in Qiskit 2.7.

Developers can also define custom operations and gates from C using virtual function tables, then add them to circuits or DAGs without relying on Python’s object model. Qiskit 2.6 introduces QPY format version 18 as the default, with a more compact encoding for registers. Files written in that format may not load in older Qiskit releases, though callers can request an earlier QPY version when writing a file.

IBM says the C API additions from Qiskit 2.5 to 2.6 are additive, but the C API documentation continues to label the interface experimental and warns that it may change between minor releases. Qiskit 2.6 keeps Python 3.10 as its minimum supported version, while source builds now require Rust 1.89; the Rust requirement does not affect users installing prebuilt wheels.