10. Common Quantum Programming Patterns

10.1. Compute-Action-Uncompute

[1] CUDA-Q specifies syntax and semantics to support the compute-action-uncompute pattern, \(W = U V U \dagger\) (here \(U\) is the compute block, and \(V\) is the action block).

[2] Via special syntax for this pattern CUDA-Q enables compiler implementations to key on circuit optimizations via programmer intent. Specifically, controlled versions of \(W\) do not produce controlled operations of all instructions in \(U\), \(V\), and \(U \dagger\), but instead we are free to only control \(V\).

[3] The CUDA-Q specification requires the following syntax for this pattern:

C++

// Will invoke U V U^dag
cudaq::compute_action (
    [&](){
      /*U_code*/
    },
    [&]() {
      /*V_code*/
    }
);

Python

def computeF():
   ...

def actionF():
   ...

# Can take user-defined functions
cudaq.compute_action(computeF, actionF)

# Can take Pythonic CUDA-Q lambda kernels
computeL = lambda : (h(q), x(q), ry(-np.pi, q[0]))
cudaq.compute_action(computeL, actionF)

[4] Compiler implementations must add the uncompute segment and optimize on any controlled versions of this block of code.