Z gate implementation using cuStateVec. The Z gate applied to $\lvert 0\rangle$ is invisible at measurement (it leaves $\lvert 0\rangle$ unchanged), so this example prepares $\lvert +\rangle$ first to make the phase flip observable via interference.

// Compile: nvcc main.cu -o main -lcustatevec
// Run:     ./main
 
#include <stdio.h>
#include <math.h>
#include <cuda_runtime.h>
#include <custatevec.h>
 
int main() {
    const int nQubits = 1;
    const int dim = 1 << nQubits;
 
    cuDoubleComplex h_sv[2] = {{1,0},{0,0}};  // |0>
    cuDoubleComplex *d_sv;
    cudaMalloc(&d_sv, dim * sizeof(cuDoubleComplex));
    cudaMemcpy(d_sv, h_sv, dim * sizeof(cuDoubleComplex), cudaMemcpyHostToDevice);
 
    custatevecHandle_t handle;
    custatevecCreate(&handle);
 
    int32_t targets[] = {0};
 
    // Apply H: |0> -> |+>
    double s = M_SQRT1_2;
    cuDoubleComplex h_gate[4] = {{s,0},{s,0},{s,0},{-s,0}};
    custatevecApplyMatrix(
        handle, d_sv, CUDA_C_64F, nQubits,
        h_gate, CUDA_C_64F, CUSTATEVEC_MATRIX_LAYOUT_ROW, 0,
        targets, 1, NULL, NULL, 0,
        CUSTATEVEC_COMPUTE_64F, NULL, 0);
 
    // Apply Z: |+> -> |->
    cuDoubleComplex z_gate[4] = {{1,0},{0,0},{0,0},{-1,0}};
    custatevecApplyMatrix(
        handle, d_sv, CUDA_C_64F, nQubits,
        z_gate, CUDA_C_64F, CUSTATEVEC_MATRIX_LAYOUT_ROW, 0,
        targets, 1, NULL, NULL, 0,
        CUSTATEVEC_COMPUTE_64F, NULL, 0);
 
    cudaMemcpy(h_sv, d_sv, dim * sizeof(cuDoubleComplex), cudaMemcpyDeviceToHost);
    for (int i = 0; i < dim; i++)
        printf("|%d>: (%.4f, %.4f)\n", i, cuCreal(h_sv[i]), cuCimag(h_sv[i]));
    // |0>: ( 0.7071, 0.0000)
    // |1>: (-0.7071, 0.0000)  <- |->
 
    custatevecDestroy(handle);
    cudaFree(d_sv);
    return 0;
}