dm_simulator ported to Qiskit-terra 0.20.1 with additional features

[fasihere]

Information

• Qiskit Terra version: 0.20.1

I have ported the dm_simulator to Qiskit-terra 0.20.1

The ported version can be found in this link: dmsim0.20.1

After porting, I tested out all the qiskit-textbook algorithms in the dm_simulator.

Issues identified during testing

• Plot option does not return to default value when not stated in backend options.
• Single measurements do not output partial probability
• Once the backend noise is applied, it does not go back to default once the backend options are removed.(The issue was that rotation_error was a dictionary and was shallow copied from DEFAULT_OPTIONS and the DEFAULT_OPTIONS changed once we updated rotation_error. Hence it had to be deep copied.)
• Result is a dictionary. Many qiskit algorithms expect the result to be an object similar to that in qasm_simulator, and they check for attributes which throws an error

Changes made

• Plot and show_partition options returns to default option(False).
• Rotation_error returns to default values
• Single measurements now output partial probability
• Result is changed to an object instead of dictionary, as in qasm simulator. This also improves user convenience as the usage is similar to that of qasm_simulator.
• Made necessary changes to bell measurements to remove add_params requirement. This change is made in reference to issue #7.

   qc2 = QuantumCircuit(3,3)
   qc2.measure(1,2,basis='Bell')           #Measures bell_probabilities on the pair of qubits [1,2]
   options2 = {
       'plot': True
   }
   backend = BasicAer.get_backend('dm_simulator')
   run2 = execute(qc2,backend,**options2,shots=1)
   result2 = run2.result()
   result2.get_probability()

Features added

• Added plot option for partial_probability
• Added new methods to result:
  • get_densitymatrix() outputs the densitymatrix
  • get_probability() outputs the partial_probability, ensemble_probability, etc depending on the basis of measurement
  • get_expectation() outputs the expectation when the basis is ‘Expect’

I have also added the qiskit algorithms jupyter notebooks taken directly from qiskit-textbook, with changes made for the dm_simulator. These algorithms can be found in the algorithm folder inside dm_simulator_user_guide.

Observations

• In a quantum circuit, if you measure some qubits using ensemble basis, and others using ‘X’, ’Y’, or ‘Z’ (default) basis, the partial_probabilities dictionary will contain partial probabilities of latter qubits. The get_probability() function will prioritize and output partial probabilities. Since I have also added plot option for partial probabilities, both partial probabilities and ensemble probabilities will be plotted.
• The partial probabilities outputs the probabilities similar to the format of qasm_simulator(little endian). i.e. they output the last qubit as the most significant qubit. This is opposite to that of ensemble probabilities dictionary.
• If we are taking an ensemble measurement in a circuit, we only need to take one such measurement. If we take multiple such ensemble measurements, multiple plots will be plotted(given plot=True).
• For removing add_params for bell measurement, the first argument(qubit) and the second argument(cbit) are added together to form add_param. Since the function is still measure function, a classical bit should be present in the quantum circuit that is equal to the second argument. Otherwise, it throws a 'CircuitError'.

Final Note

Let me know any issues/suggestions/comments you have regarding the update. You can add the branch to the repository as you wish.