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Qiskit C++ Tutorial

Qiskit C++ Overview

Qiskit C++ is an opensource interface library for Quantum Centric Super Computing applications. Qiskit C++ provides similar interface classes to Python interface of Qiskit through Qiskit C-API, can be used from C++ applications. You do not have to bind your applications to Python to make and run the quantum circuits. Qiskit C++ is easy to use library, consists of header files only, so you do not have to build any binaries and install to the system, but you only have to include Qiskit C++ header files from your applicaitons.

Qiskit C++ supports:

  • Building quantum circuits using standard gates
  • Transpiling quantum circuits to ISA circuits for a target quantum hardware
  • Running quantum circuits through sampler interface on a quantum hardware

alt text This example shows building GHZ circuit in Qiskit (Python) and Qiksit C++. As you can see, only the difference between Python and C++ is type definitions and end of line semicorons in C++. Most of classes and functions implemented in Qiskit C++ are as same as that of Qiskit Python. So it is easy to port Qiskit codes into your C++ applications using Qiskit C++.

alt C-API and Qiskit C++ interface structure This chart shows how Qiskit C++ is implemented by using Qiskit C-API. Qiskit C++ accesses Qiskit's core functions written in Rust through C-API. To access quantum resources, we use one of QRMI https://github.com/qiskit-community/qrmi, Qiskit IBM Runtime C https://github.com/Qiskit/qiskit-ibm-runtime-c or SQC https://github.com/jhpc-quantum/SQC (for ibm_kobe) through runtime class of Qiskit C++.

Getting started

Before coding with Qiskit C++, install dependencies.

Bulding Qiskit C-API Library

git clone git@github.com:Qiskit/qiskit.git
cd qiskit
make c

To build Qiskit C-API library, you will need Rust compiler. See also https://github.com/Qiskit/qiskit/tree/main/crates/cext

Make sure you will need the path to the Qiskit later to include and link from Qiskit C++.

Building Runtime Service Interface Library

To access quantum hardware from Qiskit C++, install one of the runtime service API. This example installs QRMI as a runtiem service API.

git clone git@github.com:qiskit-community/qrmi.git
cd qrmi
cargo build --release

Make sure you will need the path to the QRMI later to include and link from Qiskit C++.

Other Software

Qiskit C++ uses nlohmann_json https://github.com/nlohmann/json to read/write JSON format. Install and set include path so that Qiskit C++ can find it.

Building Sample of Qiskit C++

Now you can use Qiskit C++ from the applications. This example builds examples of Qiskit C++.

cd samples
mkdir build
cd build
cmake -DQISKIT_ROOT=(put a path to the root of Qiskit you cloned) -DQRMI_ROOT=(put a path to the root of QRMI you cloned) ..
make

Set QISKIT_ROOT and QRMI_ROOT to cmake to set include path and library path referred from Qiskit C++.

In your applications, set include path and library path to compiler options:

-I${QISKIT_ROOT}/dist/c/include -I${QRMI}/
-L${QISKIT_ROOT}/dist/c/lib -L${QRMI}/target/Release

See CMakeLists.txt in samples.

Getting Ready for Qiskit C++ Tutorial

Qiskit C++ tutorial uses Jupyter notebook to learn interactively. Before go into the tutorial, install Qiskit C-API and QRMI shown above, and install xeus-cling https://github.com/jupyter-xeus/xeus-cling to run C++ code interactively on Jupyter notebook.

Tutorials

  1. Making Quantum Circuits
  2. Runnning Quantum Circuits