9. Just-in-Time Kernel Creation

CUDA Quantum provides a set of programming abstractions for dynamically constructing quantum kernel code at runtime. The callable cudaq::kernel_builder abstraction facilitates the dynamic definition of quantum kernels that can optionally be parameterized by user-defined input arguments. The builder enables an API for constructing parameterized quantum kernels but is also itself callable. The kernel_builder takes the following structure

namespace cudaq {
  // Type wrapping a value in the kernel IR
  struct Value;
  template <typename... Args>
  class kernel_builder {
    private:
      std::vector<Value> arguments;

    public:
      std::vector<Value>& getArguments();
      std::string name() const;
      std::size_t getNumParams() const;

      Value qalloc(std::size_t nQubits = 1);
      Value qalloc(Value size);

      void h(Value& qubit);
      //... all other quantum operations ...

      // if (mz(q)) thenFunctor()
      void c_if(Value& result, std::function<void()>&& thenFunctor);

      // Invoke a predefined kernel
      template<typename OtherKernelBuilder, typename... Values>
      void call(OtherKernelBuilder&& kernelToCall, Values&... args);

      // General multi-control on a predefined kernel
      // models cudaq::control(...)
      template<typename OtherKernelBuilder, typename... Values>
      void control(OtherKernelBuilder&& kernelToControl, Value& ctrl, Values&... values);

      // General adjoint on a predefined kernel
      // models cudaq::adjoint(...)
      template<typename OtherKernelBuilder, typename... Values>
      void adjoint(OtherKernelBuilder&& kernelToAdjoint, Values&... values);

      // The constructed kernel is callable
      void operator()(Args... args);
      void operator()(void** argsArray);

      // Enable structured bindings
      template <std::size_t N>
      decltype(auto) get() {
        if constexpr (N == 0)
          return *this;
        else
          return arguments[N - 1];
      }
  };
}

/// Enable structured bindings on the kernel_builder type.
/// auto [kernel, theta, phi] = std::make_kernel<double,double>();
namespace std {
  template <typename... Args> // std::size_t NArgs>
  struct tuple_size<cudaq::kernel_builder<Args...>>
    : std::integral_constant<std::size_t, sizeof...(Args) + 1> {};

  template <std::size_t N, typename... Args>
  struct tuple_element<N, cudaq::kernel_builder<Args...>> {
    using type = std::conditional_t<N == 0, cudaq::kernel_builder<Args...>,
                              cudaq::QuakeValue>;
  };
} // namespace std

namespace cudaq {
  kernel_builder<> make_kernel();
  template<typename... Args>
  kernel_builder<Args...> make_kernel();
}

The structure above allows one to leverage the provided factory functions (make_kernel) to construct an empty CUDA Quantum kernel with defined argument signature.

For each type in the signature, the programmer is returned a new Value instance which can be leveraged in the construction of the kernel. The intended mechanism for kernel creation and argument value extraction is via standard C++ structured bindings.

Once the kernel is created, the programmer is free to build up the kernel expression using the exposed API. There are methods for qubit allocation, quantum operation invocation, control and adjoint synthesis, and conditional branching based on boolean values.

Here is a simple example how one might build a CUDA Quantum kernel dynamically.

auto kernel = cudaq::make_kernel();
auto qubits = kernel.qalloc(2);
kernel.h(qubits[0]);
kernel.x<cudaq::ctrl>(qubits[0], qubits[1]);
kernel.mz();

// See algorithmic primitives section for more on sample
auto counts = cudaq::sample(kernel);

Here is an example demonstrating how one may build a dynamic set of CUDA Quantum kernels for executing the standard Hadamard test.

auto [xPrep, qubitIn] = cudaq::make_kernel<cudaq::qubit>();
xPrep.x(qubitIn);

// Compute <1|X|1> = 0
auto hadamardTest = cudaq::make_kernel();
auto q = hadamardTest.qalloc();
auto ancilla = hadamardTest.qalloc();
hadamardTest.call(xPrep, q);
hadamardTest.h(ancilla);
hadamardTest.control(xPrep, ancilla, q);
hadamardTest.h(ancilla);
hadamardTest.mz(ancilla);

// See algorithmic primitives section for more on sample
auto counts = cudaq::sample(hadamardTest);