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Ket (|⟩) is a quantum computing library for C++20, with first-class Python bindings. You build a circuit, simulate it on a built-in state-vector simulator, and sample measurement outcomes — from C++, from Python, the command line, a desktop GUI, or right in your browser.

▶  Try the debugger in your browser
No install — step a circuit gate by gate and watch every amplitude and Bloch vector.

Internally a circuit is a directed acyclic graph (DAG) of gates: each node is a gate and edges connect gates that share a qubit. This makes gate dependencies explicit and leaves room for future analysis and optimization passes.

Features

• A broad gate set — Paulis, phase and T gates, parameterized rotations (Rx/Ry/Rz), the general single-qubit U, their controlled forms (including controlled rotations and controlled-phase), SWAP, and the three-qubit Toffoli (CCX) and Fredkin (CSWAP).
• Exact state-vector simulation of the full 2ⁿ amplitude vector, multithreaded across a persistent std:: thread pool (set_num_threads), with gate fusion.
• Automatic backend selection — Clifford circuits are routed to an O(n²) stabilizer simulator (scaling to thousands of qubits), everything else to the state vector. The choice is automatic but overridable.
• Measurement and sampling into a classical register, for shot experiments.
• Composite gates — package a sub-circuit into a reusable, labeled block, and decompose() it back into primitives.
• Qiskit-style ASCII diagrams via circuit.print().
• OpenQASM 2.0 import and export, including user-defined gate blocks.
• Python bindings (built with [pybind11]) that mirror the C++ API.
• Tooling — a ket-cli command-line tool and a ket-gui circuit viewer.

The complete list of gate methods lives in include/ket/circuit.hpp.

Building

Ket uses CMake and a C++20 compiler. By default build.sh builds everything — the library, the example programs, and the ket-cli and ket-gui executables; --help lists the rest (focused builds, the test runners, and the Python bindings).

./build.sh          # build everything
./build.sh --help   # all options

./build.sh --install installs ket under /usr/local: the ket-cli and ket-gui executables, plus the library, headers, and a CMake package config so other projects can find_package(ket) and link ket::ket. --uninstall removes it.

Examples

The examples/ directory has runnable programs — Bell and GHZ states, the Deutsch–Jozsa and Bernstein–Vazirani algorithms, a bit-flip code, and Grover's search. Once built, run them from build/examples/:

./build/examples/bell
./build/examples/grover

CLI

ket-cli operates on OpenQASM files (or stdin, so it composes in pipelines):

./build/cli/ket-cli draw   examples/bell.qasm                 # ASCII diagram
./build/cli/ket-cli run    examples/bell.qasm                 # final state vector
./build/cli/ket-cli sample examples/bell.qasm --shots 1000    # measurement histogram

GUI

ket-gui is a step-through debugger: an editable QASM panel, a circuit view, the live state vector, and per-qubit Bloch spheres (GLFW, Dear ImGui, ImPlot). The fastest way to try it is the live demo — it's the desktop GUI compiled to WebAssembly, no install required.

To run it natively:

./build/gui/ket-gui examples/grover.qasm

C++

To use ket as a library, link the ket target, add include/ to your include path, and include the umbrella header <ket/ket.hpp>.

#include <ket/ket.hpp>
#include <iostream>

int main() {
  // A Bell state: H on q0, then CNOT(q0 -> q1).
  ket::Circuit c{2};
  c.h(0);
  c.cx(0, 1);

  std::cout << c.print();            // ASCII circuit diagram
  std::cout << ket::run(c).print();  // final state vector
}

     ┌───┐
q_0: ┤ H ├──■──
     └───┘┌─┴─┐
q_1: ─────┤ X ├
          └───┘
|00⟩: 0.707107
|01⟩: 0
|10⟩: 0
|11⟩: 0.707107

Python

Install from PyPI — prebuilt wheels, so no compiler is needed:

pip install ketpy

The distribution is named ketpy (ket is taken on PyPI), imported as such (commonly import ketpy as ket):

import ketpy as ket

c = ket.Circuit(2)
c.h(0)
c.cx(0, 1)
c.measure_all()

print(c)                       # circuit diagram
print(ket.run(c))              # state vector
print(ket.sample(c, seed=0))   # one shot, e.g. [0, 0] or [1, 1]

The Python API mirrors the C++ one — Circuit, run, measure, sample — with a few Pythonic touches: Circuit and State render through print()/str(), State supports len() and indexing (returning a Python complex), and measure/sample take an optional seed.

For development, build the bindings from source instead — this needs a C++20 compiler and CMake (via [scikit-build-core]):

pip install --editable ".[test]"

How it works

A register of n qubits is a vector of 2ⁿ complex amplitudes. Each gate is applied as an in-place linear transformation over that vector — qubit i is bit i of the basis-state index (little-endian). The simulator walks the DAG in topological order (which matches insertion order) and applies each gate in turn. Measurement samples a basis state by the Born rule (outcome i with probability |amplitudeᵢ|²), with an optional seed for reproducibility.

Because the state vector stores all 2ⁿ amplitudes, simulation is exact but bounded by memory — practical up to roughly 25 qubits. There is no noise modeling.

Backends. sample and expval go through simulate(circuit, method), which picks an engine: a Clifford circuit (only H/S/Sdg/X/Y/Z/ CX/CY/CZ/SWAP) is run on a stabilizer tableau in O(n²) time and memory — thousands of qubits — while anything else uses the state vector. method defaults to auto but can be forced to statevector or stabilizer (is_clifford(circuit) / chosen_method(circuit) report the decision). The stabilizer engine never forms a 2ⁿ vector, so run() — which returns the full state vector — is always the dense path.

The state-vector backend applies each gate's 2ⁿ-amplitude update across a persistent std::thread pool (the pairs are independent, so the split needs no locks). set_num_threads(n) controls it — 0 selects the hardware concurrency; it defaults to 1 (or KET_NUM_THREADS). Small states stay serial, since the synchronization would cost more than the work. Consecutive gates on the same qubits are fused into one combined gate, so a deep circuit makes fewer sweeps over the state vector.

Roadmap

• DAG optimization passes (gate cancellation, commutation, fusion) and a scheduler that no longer relies on insertion order.
• A state panel in the GUI (amplitudes / Bloch-style plots via ImPlot3D).
• More backends behind simulate() — e.g. a tensor-network (MPS) simulator.
• Cache-blocking with qubit reordering, to fuse across non-adjacent qubits without thrashing the cache (the remaining gap to Aer on deep circuits).

Benchmark

Generated by the benchmark script.

License

MIT © 2026 Breno Cunha Queiroz