7. Sub-circuit Synthesis

Execution of pure device quantum kernels can be modified via explicit intrinsic library calls that provide multi-qubit controlled application and adjoint, or circuit reversal, semantics. CUDA Quantum defines the following library functions for enabling multi-controlled and adjoint operations on a general pure device kernel

template <typename QuantumKernel, typename... Args>
void control(QuantumKernel &&kernel,
             cudaq::qubit& ctrl_qubit, Args &... args) { ... }

template <typename QuantumKernel, typename QuantumRegister, typename... Args>
requires(std::ranges::range<QuantumRegister>)
void control(QuantumKernel &&kernel,
             QuantumRegister& ctrl_qubits, Args &... args) { ... }

template <typename QuantumKernel, typename... Args>
void adjoint(QuantumKernel &&kernel, Args &... args) { ... }

These functions can be leveraged in quantum kernel code in the following way

struct x_gate {
  void operator()(cudaq::qubit& q) __qpu__ { x(q); }
};
struct kernel {
  void operator() () __qpu__ {
    cudaq::qreg q(3);
    ...
    // Create Toffoli gate
    auto ctrl_bits = q.front(2);
    cudaq::control(x_gate{}, ctrl_bits, q[2]);
    ...
  }
};
struct rx_and_h_gate {
  void operator()(double x, cudaq::qubit& q) __qpu__ { rx(x,q); h(q); }
};
struct kernel {
  void operator(int N) () __qpu__ {
    cudaq::qreg q(N);
    ...
    // apply h(q[2]); rx(-pi, q[2]);
    cudaq::adjoint(rx_and_h_gate{}, M_PI, q[2]);
    ...
  }
};

The cudaq::control(...) function takes as input an instantiated pure device quantum kernel, an std::range of control qubits (cudaq::qreg or cudaq::qspan), and the remaining arguments for the kernel itself. Compiler implementations are free to synthesize multi-controlled operations using any pertinent synthesis strategy available. Qubits may be aggregated into a range of control qubits with or without the use of the operator! negated polarity operator.

cudaq::control(kernel{}, {qubit0, !qubit1}, kernel_arg);

The cudaq::adjoint(...) function takes as input an instantiated pure device quantum kernel (or specified template type) and the remaining arguments for the kernel.