Before embarking on simulating the dynamics of quantum systems, we will first look at the data structure used for returning the simulation results. This object is a ~qutip.solver.result.Result class that stores all the crucial data needed for analyzing and plotting the results of a simulation. A generic Result object result contains the following properties for storing simulation data:
table-striped
| Property | Description |
|---|---|
result.solver |
String indicating which solver was used to generate the data. |
result.times |
List/array of times at which simulation data is calculated. |
result.expect |
List/array of expectation values, if requested. |
result.e_data |
Dictionary of expectation values, if requested. |
result.states |
List/array of state vectors/density matrices calculated at times, if requested. |
result.final_state |
State vector or density matrix at the last time of the evolution. |
result.stats |
Various statistics about the evolution. |
To understand how to access the data in a Result object we will use an example as a guide, although we do not worry about the simulation details at this stage. Like all solvers, the Master Equation solver used in this example returns an Result object, here called simply result. To see what is contained inside result we can use the print function:
>>> print(result) <Result Solver: mesolve Solver stats: method: 'scipy zvode adams' init time: 0.0001876354217529297 preparation time: 0.007544517517089844 run time: 0.001268625259399414 solver: 'Master Equation Evolution' num_collapse: 1 Time interval: [0, 1.0] (2 steps) Number of e_ops: 1 State not saved. >
The first line tells us that this data object was generated from the Master Equation solver .mesolve. Next we have the statistics including the ODE solver used, setup time, number of collpases. Then the integration interval is described, followed with the number of expectation value computed. Finally, it says whether the states are stored.
Now we have all the information needed to analyze the simulation results. To access the data for the two expectation values one can do:
expt0 = result.expect[0] expt1 = result.expect[1]
Recall that Python uses C-style indexing that begins with zero (i.e., [0] => 1st collapse operator data). Alternatively, expectation values can be obtained as a dictionary:
e_ops = {"sx": sigmax(), "sy": sigmay(), "sz": sigmaz()} ... expt_sx = result.e_data["sx"]
When e_ops is a list, e_data ca be used with the list index. Together with the array of times at which these expectation values are calculated:
times = result.times
we can plot the resulting expectation values:
plot(times, expt0) plot(times, expt1) show()
State vectors, or density matrices, are accessed in a similar manner, although typically one does not need an index (i.e [0]) since there is only one list for each of these components. Some other solver can have other output, .heomsolve's results can have ado_states output if the options store_ados is set, similarly, .fmmesolve can return floquet_states.
Solver which compute multiple trajectories such as the Monte Carlo Equations Solvers or the Stochastics Solvers result will differ depending on whether the trajectories are flags to be saved. For example:
>>> mcsolve(H, psi, np.linspace(0, 1, 11), c_ops, e_ops=[num(N)], ntraj=25, options={"keep_runs_results": False}) >>> np.shape(result.expect) (1, 11)
>>> mcsolve(H, psi, np.linspace(0, 1, 11), c_ops, e_ops=[num(N)], ntraj=25, options={"keep_runs_results": True}) >>> np.shape(result.expect) (1, 25, 11)
When the runs are not saved, the expectation values and states are averaged over all trajectories, while a list over the runs are given when they are stored. For a fix output format, average_expect return the average, while runs_states return the list over trajectories. The runs_ output will return None when the trajectories are not saved. Standard derivation of the expectation values is also available:
| Reduced result | Trajectories results | Description |
|---|---|---|
average_states |
runs_states |
State vectors or density matrices calculated at each times of tlist |
average_final_state |
runs_final_state |
State vectors or density matrices calculated at the last time of tlist |
average_expect |
runs_expect |
List/array of expectation values, if requested. |
std_expect |
List/array of standard derivation of the expectation values. | |
average_e_data |
runs_e_data |
Dictionary of expectation values, if requested. |
std_e_data |
Dictionary of standard derivation of the expectation values. |
Multiple trajectories results also keep the trajectories seeds to allows recomputing the results.
seeds = result.seeds
One last feature specific to multi-trajectories results is the addition operation that can be used to merge sets of trajectories.
>>> run1 = smesolve(H, psi, np.linspace(0, 1, 11), c_ops, e_ops=[num(N)], ntraj=25)
>>> print(run1.num_trajectories)
25
>>> run2 = smesolve(H, psi, np.linspace(0, 1, 11), c_ops, e_ops=[num(N)], ntraj=25)
>>> print(run2.num_trajectories)
25
>>> merged = run1 + run2
>>> print(merged.num_trajectories)
50This allows one to improve statistics while keeping previous computations.