From 2ca9773c59b9771b29c1870ef717cbfd032e0b84 Mon Sep 17 00:00:00 2001
From: Jim Garrison <garrison@ibm.com>
Date: Wed, 15 May 2024 12:46:21 -0400
Subject: [PATCH] Remove use of qasm simulator (#589)

* Remove use of qasm simulator

Closes #502.

* Replace default configuration change with a warning

* Add `stacklevel` to warning

(cherry picked from commit e3db27bb8a921a1ccd4c1a881abc35eda1636a45)
---
 .../cutting/cutqc/wire_cutting_evaluation.py  |  9 +++
 .../tutorial_1_automatic_cut_finding.ipynb    |  6 +-
 .../tutorials/tutorial_2_manual_cutting.ipynb | 65 +++----------------
 3 files changed, 22 insertions(+), 58 deletions(-)

diff --git a/circuit_knitting/cutting/cutqc/wire_cutting_evaluation.py b/circuit_knitting/cutting/cutqc/wire_cutting_evaluation.py
index 5a35c3f03..dcc27a14d 100644
--- a/circuit_knitting/cutting/cutqc/wire_cutting_evaluation.py
+++ b/circuit_knitting/cutting/cutqc/wire_cutting_evaluation.py
@@ -17,6 +17,7 @@
 import copy
 from typing import Sequence, Any
 from multiprocessing.pool import ThreadPool
+from warnings import warn
 
 import numpy as np
 
@@ -81,6 +82,14 @@ def run_subcircuit_instances(
                     options[i % len(options)] for i, _ in enumerate(subcircuits)
                 ]
         else:
+            warn(
+                "Please provide a list of `backend_names` alongside the `service`. "
+                "With no backend specified, CutQC defaults to using "
+                "ibmq_qasm_simulator, but cloud simulators are not expected to be "
+                "operational past May 15, 2024. For more details, see: "
+                "https://docs.quantum.ibm.com/announcements/product-updates/2024-03-22-announcement-cloud-simulators-and-lab",
+                stacklevel=2,
+            )
             backend_names_repeated = ["ibmq_qasm_simulator"] * len(subcircuits)
             if options:
                 options_repeated = [options[0]] * len(subcircuits)
diff --git a/docs/circuit_cutting/cutqc/tutorials/tutorial_1_automatic_cut_finding.ipynb b/docs/circuit_cutting/cutqc/tutorials/tutorial_1_automatic_cut_finding.ipynb
index 9177e30fc..82672cf65 100644
--- a/docs/circuit_cutting/cutqc/tutorials/tutorial_1_automatic_cut_finding.ipynb
+++ b/docs/circuit_cutting/cutqc/tutorials/tutorial_1_automatic_cut_finding.ipynb
@@ -246,8 +246,10 @@
     "# Set the Sampler and runtime options\n",
     "options = Options(execution={\"shots\": 4000})\n",
     "\n",
-    "# Run 2 parallel qasm simulator threads\n",
-    "backend_names = [\"ibmq_qasm_simulator\"] * 2"
+    "# Use the least busy backend for both threads\n",
+    "backend_names = None\n",
+    "if service is not None:\n",
+    "    backend_names = [service.least_busy().name] * 2"
    ]
   },
   {
diff --git a/docs/circuit_cutting/cutqc/tutorials/tutorial_2_manual_cutting.ipynb b/docs/circuit_cutting/cutqc/tutorials/tutorial_2_manual_cutting.ipynb
index 4b3037e35..18ba96bcc 100644
--- a/docs/circuit_cutting/cutqc/tutorials/tutorial_2_manual_cutting.ipynb
+++ b/docs/circuit_cutting/cutqc/tutorials/tutorial_2_manual_cutting.ipynb
@@ -75,60 +75,6 @@
     "circuit.draw(\"mpl\", fold=-1, scale=0.75)"
    ]
   },
-  {
-   "cell_type": "markdown",
-   "id": "461e57e3",
-   "metadata": {},
-   "source": [
-    "## Set up the Qiskit Runtime Service\n",
-    "\n",
-    "The Qiskit Runtime Service provides access to IBM Runtime Primitives and quantum backends.\n",
-    "Alternatively, a local statevector simulator can be used with the Qiskit primitives."
-   ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": 5,
-   "id": "5d1fb2ca",
-   "metadata": {},
-   "outputs": [],
-   "source": [
-    "from qiskit_ibm_runtime import (\n",
-    "    QiskitRuntimeService,  # noqa: F401\n",
-    "    Options,\n",
-    ")\n",
-    "\n",
-    "# Use local versions of the primitives by default.\n",
-    "service = None\n",
-    "\n",
-    "# Uncomment the following line to instead use Qiskit Runtime.\n",
-    "# service = QiskitRuntimeService()"
-   ]
-  },
-  {
-   "cell_type": "markdown",
-   "id": "5fb383d2",
-   "metadata": {},
-   "source": [
-    "The wire cutter tool uses a `Sampler` primitive to evaluate the probabilities of each subcircuit. Here, we configure the options for the Runtime Sampler and specify the backend(s) to be used to evaluate the subcircuits.\n",
-    "\n",
-    "If no service was set up, the `backend_names` argument will be ignored, and Qiskit primitives will be used with statevector simulator."
-   ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": 6,
-   "id": "d409553d",
-   "metadata": {},
-   "outputs": [],
-   "source": [
-    "# Set the Sampler and runtime options\n",
-    "options = Options(execution={\"shots\": 4000})\n",
-    "\n",
-    "# Run 2 parallel qasm simulator threads\n",
-    "backend_names = [\"ibmq_qasm_simulator\"] * 2"
-   ]
-  },
   {
    "attachments": {
     "how-to-manual-cut.png": {
@@ -244,6 +190,11 @@
    "metadata": {},
    "outputs": [],
    "source": [
+    "from qiskit_ibm_runtime import (\n",
+    "    QiskitRuntimeService,  # noqa: F401\n",
+    "    Options,\n",
+    ")\n",
+    "\n",
     "# Use local versions of the primitives by default.\n",
     "service = None\n",
     "\n",
@@ -273,8 +224,10 @@
     "# Set the Sampler and runtime options\n",
     "options = Options(execution={\"shots\": 4000})\n",
     "\n",
-    "# Run 2 parallel qasm simulator threads\n",
-    "backend_names = [\"ibmq_qasm_simulator\"] * 2"
+    "# Use the least busy backend for both threads\n",
+    "backend_names = None\n",
+    "if service is not None:\n",
+    "    backend_names = [service.least_busy().name] * 2"
    ]
   },
   {