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  {
   "cell_type": "markdown",
   "id": "4b1ce9ef-6970-49bf-bae4-332d3eef0423",
   "metadata": {},
   "source": [
    "# Control-Flow\n",
    "\n",
    "**Download this notebook - {nb-download}`control-flow.ipynb`**"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "id": "0ef5d6cd-cb12-4485-97fb-eae6e4c1cd32",
   "metadata": {},
   "outputs": [],
   "source": [
    "from guppylang import guppy\n",
    "from guppylang.std.builtins import result\n",
    "from guppylang.std.quantum import measure, qubit, discard, h, tdg, cx, t, z"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "9ab7cb6e-b11b-4b0b-aab9-a113bb2618b0",
   "metadata": {},
   "source": [
    "Implement an $V_3 = R_z(-2\\arctan(2)) = (I + 2iZ)/\\sqrt{5}$ gate using the repeat-until-success scheme from https://arxiv.org/abs/1311.1074."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "id": "1be5c488-36df-4a8b-b0af-0a330a8293fd",
   "metadata": {},
   "outputs": [],
   "source": [
    "@guppy\n",
    "def repeat_until_success(q: qubit) -> None:\n",
    "    attempts = 0\n",
    "    while True:\n",
    "        attempts += 1\n",
    "\n",
    "        # Prepare ancillas\n",
    "        a, b = qubit(), qubit()\n",
    "        h(a)\n",
    "        h(b)\n",
    "\n",
    "        tdg(a)\n",
    "        cx(b, a)\n",
    "        t(a)\n",
    "        h(a)\n",
    "        if measure(a):\n",
    "            # First part failed, try again\n",
    "            discard(b)\n",
    "            continue\n",
    "\n",
    "        t(q)\n",
    "        z(q)\n",
    "        cx(q, b)\n",
    "        t(b)\n",
    "        h(b)\n",
    "        if measure(b):\n",
    "            # Second part failed, apply correction\n",
    "            # and try again\n",
    "            z(q)\n",
    "            continue\n",
    "\n",
    "        result(\"attempts\", attempts)\n",
    "        break"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "id": "fbd490b3-8cc4-43c2-87f8-e799d00fa2c2",
   "metadata": {},
   "outputs": [],
   "source": [
    "@guppy\n",
    "def main() -> None:\n",
    "    q = qubit()\n",
    "    repeat_until_success(q)\n",
    "    measure(q)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "id": "831c0673-a3f1-4159-bf39-aae7ec9e3323",
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "1.49"
      ]
     },
     "execution_count": 4,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "shots = main.emulator(n_qubits=3).with_seed(0).with_shots(100).run()\n",
    "sum(int(shot.as_dict()[\"attempts\"]) for shot in shots) / len(shots)"
   ]
  }
 ],
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