Static For Loop

Since quantum expressions are evaluated at compile time, it is not possible to generate an expression with a runtime for-loop. However, in order to build larger quantum circuits, using a for loop will become a necessaty. Looping over quantum expressions can thus be done with the static_for() function. The generic interface of the static_for() function reads:

template<index_t for_start,
         index_t for_end,
         index_t for_step,
         template<index_t start, index_t end, index_t step, index_t index> class functor,
         typename functor_return_type,
         typename... functor_types>
inline auto
static_for(functor_return_type&& functor_return_arg,
           functor_types&&... functor_args)

Below is a small example on how to use the static_for() function. First, a functor needs to be created, which represents the loop’s body:

template<index_t start, index_t end, index_t step, index_t index>
struct ftor
    template<typename Expr>
    inline constexpr auto operator()(Expr&& expr) noexcept
        // Returns the controlled phase shift gate with angle
        // theta = pi/2^(index+1) between qubits index and index+1
        return crk<index+1>(sel<index>  (gototag<0>()),

To loop through this functor (at compile time) we call the utils::static_for<start, end, step, body>(...) function as follows. Note the usefulness of the tag/gototag mechanism to restore the original filter settings easily.

auto expr = utils::static_for<0,4,1,ftor>(tag<0>(init()));

This then generates the following 6 qubit circuit with only one line of code! Notice that the end index is set to 4 and is thus also included in the loop, similar to the regular for(int i=start; i<=end; i+=step){}

Figure made with TikZ

Generated circuit by the static_for loop

Computational offloading

LibKet’s computation offloading model is very similar to that of CUDA to ease the transition from GPU- to quantum-accelerated computing. The device.eval(...) is just one of three ways to run a quantum expression, which we will refer to as quantum kernel, on a quantum device.

LibKet::utils::json device.eval(...) :

This function offloads the quantum computation to the quantum device and returns the evaluated result as JSON object once the quantum computation has completed. Exceptions are the QuEST and QX simulators where a reference to the internal state vector is returned.

LibKet::QJob* device.execute(...)

Offloads the quantum computation to the quantum device and returns a QJob pointer once the quantum computation has completed.

LibKet::QJob* device.execute_async(...)

Offloads the quantum computation to the quantum device and returns a QJob pointer immediately.

The execute() and execute_async() have a similar interface as the eval() function:

QJob<QJobType::CXX>* execute(std::size_t shots                               = [default from ctor],
                             std::function<void(QDevice_QuEST*)> ftor_init   = NULL,
                             std::function<void(QDevice_QuEST*)> ftor_before = NULL,
                             std::function<void(QDevice_QuEST*)> ftor_after  = NULL,
                             QStream<QJobType::CXX>*             stream      = NULL)

Notice that different functors are optional parameters, which will be elaborated on in the next section. The QJob objects supports the following functionality

  • QObj* wait(): waits for the job to complete (blocking)

  • bool query(): returns true if the job completed and false otherwise (non-blocking)

  • utils::json get(): returns the result as JSON object after completion (blocking)

Let’s conclude these exection options with an example. Here an expression is executed asychronously for 20 shots on the Qiskit QASM simulator. The results are retreived with the get() function, which waits for the qpu to finish execution and return results.

QDevice<QDeviceType::qiskit_qasm_simulator, 2> qpu;
auto job = qpu.execute_async(20);
result = job->get();
std::cout << result.dump(2) << std::endl;

Execution Scripts

The optional hooks ftor_init, ftor_before, and ftor_after make it possible to inject user-defined code at three different locations of the execution process:


This functor is performed before any other code of the execution process. It can be used for importing additional Python modules.


This functor is performed just before sending the instructions to the quantum device. It can be used to pre-process the quantum circuit, e.g., to perform user-specific optimizations on the raw quantum circuit, before it runs through the backend-specific pipeline


Performed just after receiving the result from the quantum device. It can be used to post-process the raw results received from the quantum device, e.g., to generate histograms or other types of visualizations

Let’s inject a simple statement after the execution that collects the histogram data of the experiment using Qiskit’s get_count() function.

auto job = qiskit.execute_async(20,
                                /* init_script   */
                                /* before_script */
                                /* after_script */
                                "counts = result.get_counts(qc)\n"
                                "return json.dumps(counts)\n"
std::cout << job->get().dump(2) << std::endl;

It should be noted that the code injections are idented automatically and must not have trailing \t’s. Each line must end with \n.

Parameterized circuits [WIP]

The creating of parameteterized circuits is still under development. When finished, LibKet will be able to support platforms that use parameterised circuits, such as Qiskit Runtime.