QCut documentation

QCut is a quantum circuit knitting package capable of efficiently partitioning quantum circuits with wire and gate cuts using advanced LOCC and joint rotation decompositions along with the standard local decompositions. It is designed to be compatible with Qiskit and should be compatible with any Qiskit programmable backend but has been especially designed to be compatible with IQM’s qpus and the Finnish Quantum Computing Infrastructure (FiQCI).

QCut has been built at CSC - IT Center for Science (Finnish IT Center for Science)

Contents:

• Basic Usage
• Options
• Examples on using QCut
• Notebooks
• Theory
• Acknowledgements
• License
• QCut API reference
• Changelog

Installation

For installation a UNIX-like system is currently needed due to pymetis being used for automatic cut finding. On Windows use WSL

Pip

Installation should be done via uv

uv pip install QCut
#or
uv add QCut

If using other than the default Qiskit version (newest) it is recommended to install Qiskit first before installing QCut.

Using uv is the recommended install method.

Install from source

It is also possible to use QCut by cloning this repository and including it in your project folder.

IQM hardware and fake backends

QCut ships an extra for this, so the adapter comes with it:

uv pip install "QCut[iqm]"

That installs IQM client with its Qiskit adapter, which covers Qiskit 1.0 up to but not including 2.2. Installing the extra will therefore hold Qiskit below 2.2, put it in an environment whose Qiskit is already in range if you would rather the resolver left it alone.

After installation just import the backend you want to use:

from iqm.qiskit_iqm import IQMFakeAdonis
backend = IQMFakeAdonis()

Benchmarks

benchmarks/QCutVsAddon.ipynb in the repository compares QCut against IBM’s qiskit-addon-cutting given the same cuts: the gamma each achieves, how many subexperiments that comes to, how long they take to generate, and what the two cut finders settle on under the same qubit budget. The benchmarks show QCut consistently matching or beating the Qiskit Cutting Addon.

uv sync --group benchmark

Where one of QCut’s joint decompositions applies, the same cuts cost less:

	QCut	addon
three wires cut at the same point	gamma 15, 240 subexperiments	gamma 64, 1024
three rotations crossing one split	gamma 7.16, 264 subexperiments	gamma 10.5, 432
an rzz and an rxx on each of three crossing pairs	gamma 115	gamma 203

Where none applies, the QAOA layer at the sizes measured, the two agree exactly. Turning consolidation, joint rotation cutting and communicating wire cuts off reproduces the addon’s gamma in every case the notebook measures, which is what says the difference is those three and not something else.