JointMeasurements/JointMeasurements.ipynb

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   "source": [
    "# Joint Measurements Kata\n",
    "\n",
    "**Joint Measurements** quantum kata is a series of exercises designed to get you familiar with programming in Q#. It covers the joint parity measurements and using them for distinguishing quantum states or for performing multi-qubit gates.\n",
    "\n",
    "* In Q# joint measurements are implemented as the [Measure](https://docs.microsoft.com/qsharp/api/qsharp/microsoft.quantum.intrinsic.measure) operation.\n",
    "* You can read more about measurements of multi-qubit Pauli operators in the [Q# documentation](https://docs.microsoft.com/azure/quantum/concepts-pauli-measurements).\n",
    "\n",
    "Each task is wrapped in one operation preceded by the description of the task.  Your goal is to fill in the blank (marked with `// ...` comments)\n",
    "with some Q# code that solves the task. To verify your answer, run the cell using Ctrl+Enter (⌘+Enter on macOS).\n",
    "\n",
    "The tasks are given in approximate order of increasing difficulty; harder ones are marked with asterisks."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Task 1. Single-qubit measurement\n",
    "\n",
    "**Input:** Two qubits (stored in an array) which are guaranteed to be either in superposition of states $|00\\rangle$ and $|11\\rangle$ or in superposition of states $|01\\rangle$ and $|10\\rangle$.\n",
    "\n",
    "**Output:**  0 if qubits were in the first superposition, 1 if they were in the second superposition.  \n",
    "*The state of the qubits at the end of the operation does not matter.*\n",
    "\n",
    "<br/>\n",
    "<details closed>\n",
    "  <summary><b>Need a hint? Click here</b></summary>\n",
    "    Use two single-qubit measurements. \n",
    "</details>"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "%kata T01_SingleQubitMeasurement \n",
    "\n",
    "operation SingleQubitMeasurement (qs : Qubit[]) : Int {\n",
    "    // ...\n",
    "    return -1;\n",
    "}"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "*Can't come up with a solution? See the explained solution in the [Joint Measurements Workbook](./Workbook_JointMeasurements.ipynb#Task-1.-Single-qubit-measurement).*"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Task 2. Parity measurement\n",
    "\n",
    "**Inputs**: Two qubits (stored in an array) which are guaranteed to be either in superposition of states $|00\\rangle$ and $|11\\rangle$ or in superposition of states $|01\\rangle$ and $|10\\rangle$.\n",
    "\n",
    "**Output**: 0 if qubits were in the first superposition, 1 if they were in the second superposition.  \n",
    "*The state of the qubits at the end of the operation should be the same as the starting state.*"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "%kata T02_ParityMeasurement \n",
    "\n",
    "operation ParityMeasurement (qs : Qubit[]) : Int {\n",
    "    // ...\n",
    "    return -1;\n",
    "}"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "*Can't come up with a solution? See the explained solution in the [Joint Measurements Workbook](./Workbook_JointMeasurements.ipynb#Task-2.-Parity-measurement).*"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Task 3. $|0000\\rangle + |1111\\rangle$ or $|0011\\rangle + |1100\\rangle$  ?\n",
    "**Inputs**: Four qubits (stored in an array) which are guaranteed to be either in superposition of states $|0000\\rangle$ and $|1111\\rangle$ or in superposition of states $|0011\\rangle$ and $|1100\\rangle$.\n",
    "\n",
    "**Output** : 0 if qubits were in the first superposition, 1 if they were in the second superposition.  \n",
    "*The state of the qubits at the end of the operation should be the same as the starting state.*"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "%kata T03_GHZOrGHZWithX \n",
    "\n",
    "operation GHZOrGHZWithX (qs : Qubit[]) : Int {\n",
    "    // ...\n",
    "    return -1;\n",
    "}"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "*Can't come up with a solution? See the explained solution in the [Joint Measurements Workbook](./Workbook_JointMeasurements.ipynb#Task-3.-$|0000\\rangle-+-|1111\\rangle$-or-$|0011\\rangle-+-|1100\\rangle$--?).*"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Task 4. GHZ state or W state ?\n",
    "\n",
    "**Inputs:** An **even** number of qubits (stored in an array) which are guaranteed to be either in a superposition of states $|0..0\\rangle$ and $|1..1\\rangle$ (the [GHZ state](https://en.wikipedia.org/wiki/Greenberger%E2%80%93Horne%E2%80%93Zeilinger_state)) or in the [W state](https://en.wikipedia.org/wiki/W_state).\n",
    "\n",
    "**Output:** 0 if qubits were in the first superposition, 1 if they were in the second superposition.  \n",
    "*The state of the qubits at the end of the operation should be the same as the starting state.*"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "%kata T04_GHZOrWState\n",
    "\n",
    "operation GHZOrWState (qs : Qubit[]) : Int {\n",
    "    // ...\n",
    "    return -1;\n",
    "}"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "*Can't come up with a solution? See the explained solution in the [Joint Measurements Workbook](./Workbook_JointMeasurements.ipynb#Task-4.-GHZ-state-or-W-state-?).*"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### <a name=\"parity-x\"></a> Task 5*. Parity measurement in different basis\n",
    "\n",
    "**Inputs:** Two qubits (stored in an array) which are guaranteed to be\n",
    "* either in superposition $\\alpha |00\\rangle + \\beta |01\\rangle + \\beta |10\\rangle + \\alpha |11\\rangle$\n",
    "* or in superposition $\\alpha |00\\rangle - \\beta |01\\rangle + \\beta |10\\rangle - \\alpha |11\\rangle$\n",
    "\n",
    "**Output:** 0 if qubits were in the first superposition, 1 if they were in the second superposition.  \n",
    "*The state of the qubits at the end of the operation should be the same as the starting state.*"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "%kata T05_DifferentBasis\n",
    "\n",
    "operation DifferentBasis (qs : Qubit[]) : Int {\n",
    "    // ...\n",
    "    return -1;\n",
    "}"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "*Can't come up with a solution? See the explained solution in the [Joint Measurements Workbook](./Workbook_JointMeasurements.ipynb#parity-x).*"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### <a name=\"controlled-x-0\"></a> Task 6*. Controlled X gate with $|0\\rangle$ target\n",
    "\n",
    "**Input:** Two unentangled qubits (stored in an array of length 2). The first qubit (`qs[0]`) will be in state $|\\psi\\rangle = \\alpha |0\\rangle + \\beta |1\\rangle$ , the second (`qs[1]`) - in state $|0\\rangle$ (this can be written as two-qubit state $\\big(\\alpha |0\\rangle + \\beta |1\\rangle\\big) \\otimes |0\\rangle$). \n",
    "\n",
    "**Output:** Change the two-qubit state to $\\alpha |00\\rangle + \\beta |11\\rangle$ using only single-qubit gates and joint measurements. Do not use two-qubit gates. You do not need to allocate extra qubits."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "%kata T06_ControlledX\n",
    "\n",
    "operation ControlledX (qs : Qubit[]) : Unit {\n",
    "    // ...\n",
    "}"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "*Can't come up with a solution? See the explained solution in the [Joint Measurements Workbook](./Workbook_JointMeasurements.ipynb#controlled-x-0).*"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### <a name=\"controlled-x\"></a> Task 7**. Controlled X gate with arbitrary target\n",
    "\n",
    "**Input:** Two qubits (stored in an array of length 2) in an arbitrary two-qubit state $\\alpha |00\\rangle + \\beta |01\\rangle + \\color{blue}\\gamma |10\\rangle  + \\color{blue}\\delta |11\\rangle$.\n",
    "\n",
    "**Goal:** Change the two-qubit state to $\\alpha |00\\rangle + \\beta |01\\rangle + \\color{red}\\delta |10\\rangle  + \\color{red}\\gamma |11\\rangle$ using only single-qubit gates and joint measurements.  Do not use two-qubit gates.\n",
    "\n",
    "> A general-case implementation of CNOT gate via joint measurements is described in [this paper](https://arxiv.org/pdf/1201.5734.pdf).\n",
    "\n",
    "<details>\n",
    "  <summary><b>Need a hint? Click here</b></summary>\n",
    "  You can use an extra qubit to perform this operation.\n",
    "</details>"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "%kata T07_ControlledX_General\n",
    "\n",
    "operation ControlledX_General (qs : Qubit[]) : Unit {\n",
    "    // ...\n",
    "}"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "*Can't come up with a solution? See the explained solution in the [Joint Measurements Workbook](./Workbook_JointMeasurements.ipynb#controlled-x).*"
   ]
  }
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