Refactor simulator (#213)

* Define SimulatorBase class, which can be used to define target-specific classes.
* Simulator is now a factory function and not a class
* We provide some built-in backends, and support user-defined backends. This can also support quantum hardware as backends.

examples/linq/1.the_basics.ipynb

@@ -329,9 +329,10 @@
     }
    ],
    "source": [
-    "from tangelo.linq import SUPPORTED_GATES\n",
+    "from tangelo.linq import get_supported_gates\n",
     "\n",
-    "for backend, gates in SUPPORTED_GATES.items():\n",
+    "supported_gates = get_supported_gates()\n",
+    "for backend, gates in supported_gates.items():\n",
     "    print(f'{backend} : {gates}')"
    ]
   },
@@ -844,7 +845,7 @@
     "# Option2: Assume quantum circuit simulation was performed separately (by this package or different services)\n",
     "freqs, _ = sim.simulate(c)  # circuit c must be consistent with operator, regarding measurement along non-z axes\n",
     "term, coef = tuple(op.terms.items())[0]  # This yields ((0, 'Z'),), 1.0\n",
-    "expval2 = coef * Simulator.get_expectation_value_from_frequencies_oneterm(term, freqs)\n",
+    "expval2 = coef * sim.get_expectation_value_from_frequencies_oneterm(term, freqs)\n",
     "print(expval2)"
    ]
   },

examples/linq/2.qpu_connection.ipynb

@@ -767,24 +767,24 @@
     }
    ],
    "source": [
-    "from tangelo.linq import Simulator\n",
+    "from tangelo.linq import get_expectation_value_from_frequencies_oneterm\n",
     "from tangelo.linq.helpers.circuits.measurement_basis import pauli_string_to_of\n",
     "\n",
     "# Rescale counts to obtain frequency histogram\n",
     "freqs = {k:v/100 for k,v in counts.items()}\n",
     "print(f\"Frequencies: {freqs}\")\n",
     "\n",
     "# Compute expectation value for ZZ operator\n",
-    "exp_ZZ = Simulator.get_expectation_value_from_frequencies_oneterm(pauli_string_to_of(\"ZZ\"), freqs)\n",
+    "exp_ZZ = get_expectation_value_from_frequencies_oneterm(pauli_string_to_of(\"ZZ\"), freqs)\n",
     "print(f\"Expectation value for ZZ operator {exp_ZZ}\")"
    ]
   }
  ],
  "metadata": {
   "kernelspec": {
-   "display_name": "main_release0.3.1",
+   "display_name": "Python 3.9.7 64-bit ('qsdkmain': venv)",
    "language": "python",
-   "name": "main_release0.3.1"
+   "name": "python3"
   },
   "language_info": {
    "codemirror_mode": {
@@ -796,7 +796,12 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.8.10"
+   "version": "3.9.10"
+  },
+  "vscode": {
+   "interpreter": {
+    "hash": "95050af2697fca56ed7491a4fb0b04c1282c0de0a7e0a7cacd318a8297b0b1d8"
+   }
   }
  },
  "nbformat": 4,

tangelo/algorithms/variational/tests/test_vqe_solver.py

@@ -16,6 +16,8 @@
 import numpy as np
 
 from tangelo.linq import Simulator
+from tangelo.helpers.utils import installed_backends
+from tangelo.linq.target import QiskitSimulator
 from tangelo.algorithms import BuiltInAnsatze, VQESolver
 from tangelo.molecule_library import mol_H2_sto3g, mol_H4_sto3g, mol_H4_cation_sto3g, mol_NaH_sto3g, mol_H4_sto3g_symm
 from tangelo.toolboxes.ansatz_generator.uccsd import UCCSD
@@ -197,9 +199,10 @@ def test_simulate_vsqs_h2(self):
         energy = vqe_solver.simulate()
         self.assertAlmostEqual(energy, -1.137270, delta=1e-4)
 
+    @unittest.skipIf("qiskit" not in installed_backends, "Test Skipped: Backend not available \n")
     def test_simulate_h2_qiskit(self):
         """Run VQE on H2 molecule, with UCCSD ansatz, JW qubit mapping, initial
-        parameters, exact qiskit simulator.
+        parameters, exact qiskit simulator. Both string and class input.
         """
 
         backend_options = {"target": "qiskit", "n_shots": None, "noise_model": None}
@@ -212,6 +215,16 @@ def test_simulate_h2_qiskit(self):
 
         self.assertAlmostEqual(energy, -1.13727042117, delta=1e-6)
 
+        backend_options = {"target": QiskitSimulator, "n_shots": None, "noise_model": None}
+        vqe_options = {"molecule": mol_H2_sto3g, "ansatz": BuiltInAnsatze.UCCSD, "qubit_mapping": "jw",
+                        "initial_var_params": [6.28531447e-06, 5.65431626e-02], "verbose": True,
+                        "backend_options": backend_options}
+        vqe_solver = VQESolver(vqe_options)
+        vqe_solver.build()
+        energy = vqe_solver.simulate()
+
+        self.assertAlmostEqual(energy, -1.13727042117, delta=1e-6)
+
     def test_simulate_h4(self):
         """Run VQE on H4 molecule, with UCCSD ansatz, JW qubit mapping, initial
         parameters, exact simulator.

tangelo/helpers/utils.py

@@ -19,6 +19,7 @@
 
 import os
 import sys
+from importlib import util
 
 
 class HiddenPrints:
@@ -34,13 +35,9 @@ def __exit__(self, exc_type, exc_val, exc_tb):
 
 
 def is_package_installed(package_name):
-    try:
-        exec(f"import {package_name}")
-        # DEBUG print(f'{package_name:16s} :: found')
-        return True
-    except ModuleNotFoundError:
-        # DEBUG print(f'{package_name:16s} :: not found')
-        return False
+    """Check if module is installed without importing."""
+    spam_spec = util.find_spec(package_name)
+    return spam_spec is not None
 
 
 # List all backends and statevector ones supported by Simulator class

tangelo/linq/__init__.py

@@ -15,4 +15,5 @@
 from .gate import *
 from .circuit import Circuit, stack, remove_small_rotations, remove_redundant_gates
 from .translator import *
+from .simulator_base import get_expectation_value_from_frequencies_oneterm
 from .simulator import Simulator, backend_info