**This is a pre-release.**
QuTiP 5 is a redesign of many of the core components of QuTiP (``Qobj``, ``QobjEvo``, solvers) to make them more consistent and more flexible.
``Qobj`` may now be stored in either sparse or dense representations, and the two may be mixed sensibly as needed. ``QobjEvo`` is now used consistently throughout QuTiP, and the implementation has been substantially cleaned up. A new ``Coefficient`` class is used to represent the time-dependent factors inside ``QobjEvo``.
The solvers have been rewritten to work well with the new data layer and the concept of ``Integrators`` which solve ODEs has been introduced. In future, new data layers may provide their own ``Integrators`` specialized to their representation of the underlying data.
Much of the user-facing API of QuTiP remains familiar, but there have had to be many small breaking changes. If we can make changes to easy migrating code from QuTiP 4 to QuTiP 5, please let us know.
Any extensive list of changes follows.
Contributors
------------
QuTiP 5 has been a large effort by many people over the last three years.
In particular:
- Jake Lishman led the implementation of the new data layer and coefficients.
- Eric Giguère led the implementation of the new QobjEvo interface and solvers.
- Boxi Li led the updating of QuTiP's QIP support and the creation of ``qutip_qip``.
Other members of the QuTiP Admin team have been heavily involved in reviewing, testing and designing QuTiP 5:
- Alexander Pitchford
- Asier Galicia
- Nathan Shammah
- Shahnawaz Ahmed
- Neill Lambert
- Simon Cross
Two Google Summer of Code contributors updated the tutorials and benchmarks to QuTiP 5:
- Christian Staufenbiel updated many of the tutorials (https://github.com/qutip/qutip-tutorials/).
- Xavier Sproken update the benchmarks (https://github.com/qutip/qutip-benchmark/).
Four experimental data layers backends were written either as part of Google Summer of Code or as separate projects. While these are still alpha quality, the helped significantly to test the data layer API:
- ``qutip-tensorflow``: a TensorFlow backend by Asier Galicia (https://github.com/qutip/qutip-tensorflow)
- ``qutip-cupy``: a CuPy GPU backend by Felipe Bivort Haiek (https://github.com/qutip/qutip-cupy)
- ``qutip-tensornetwork``: a TensorNetwork backend by Asier Galicia (https://github.com/qutip/qutip-tensornetwork)
- ``qutip-jax``: a JAX backend by Eric Giguère (https://github.com/qutip/qutip-jax)
We have also had many other contributors, whose specific contributions are detailed below:
- Pieter Eendebak (updated the required SciPy to 1.4+, [1982](https://github.com/qutip/qutip/pull/1982)).
- Pieter Eendebak (reduced import times by setting logger names, [1981](https://github.com/qutip/qutip/pull/1981))
- Xavier Sproken (included C header files in the source distribution, [1971](https://github.com/qutip/qutip/pull/1971))
- Christian Staufenbiel (added support for multiple collapse operators to the Floquet solver, [1962](https://github.com/qutip/qutip/pull/1962))
- Christian Staufenbiel (fixed the basis used in the Floquet Master Equation solver, [1952](https://github.com/qutip/qutip/pull/1952))
- Christian Staufenbiel (allowed the ``bloch_redfield_tensor`` function to accept strings and callables for `a_ops`, [1951](https://github.com/qutip/qutip/pull/1951))
- Henrique Silvéro (allowed ``qutip_qip`` to be imported as ``qutip.qip``, [1920](https://github.com/qutip/qutip/pull/1920))
- Florian Hopfmueller (added a vastly improved implementations of ``process_fidelity`` and ``average_gate_fidelity``, [1712](https://github.com/qutip/qutip/pull/1712), [#1748](https://github.com/qutip/qutip/pull/1748), [#1788](https://github.com/qutip/qutip/pull/1788))
- Felipe Bivort Haiek (fixed inaccuracy in docstring of the dense implementation of negation, [1608](https://github.com/qutip/qutip/pull/1608/))
- Rajath Shetty (added support for specifying colors for individual points, vectors and states display by `qutip.Bloch`, [1335](https://github.com/qutip/qutip/pull/1335))
Qobj changes
------------
Previously ``Qobj`` data was stored in a SciPy-like sparse matrix. Now the representation is flexible. Implementations for dense and sparse formats are included in QuTiP and custom implementations are possible. QuTiP's performance on dense states and operators is significantly improved as a result.
Some highlights:
- The data is still acessible via the ``.data`` attribute, but is now an instance of the underlying data type instead of a SciPy-like sparse matrix. The operations available in ``qutip.core.data`` may be used on ``.data``, regardless of the data type.
- ``Qobj`` with different data types may be mixed in arithmetic and other operations. A sensible output type will be automatically determined.
- The new ``.to(...)`` method may be used to convert a ``Qobj`` from one data type to another. E.g. ``.to("dense")`` will convert to the dense representation and ``.to("csr")`` will convert to the sparse type.
- Many ``Qobj`` methods and methods that create ``Qobj`` now accepted a ``dtype`` parameter that allows the data type of the returned ``Qobj`` to specified.
- The new ``&`` operator may be used to obtain the tensor product.
- The new ` operator may be used to obtain the matrix / operator product. ``bar ket`` returns a scalar.
- The new ``.contract()`` method will collapse 1D subspaces of the dimensions of the ``Qobj``.
- The new ``.logm()`` method returns the matrix logarithm of an operator.
- The methods ``.set_data``, ``.get_data``, ``.extract_state``, ``.eliminate_states``, ``.evaluate`` and ``.check_isunitary`` have been removed.
QobjEvo changes
---------------
The ``QobjEvo`` type for storing time-dependent quantum objects has been significantly expanded, standardized and extended. The time-dependent coefficients are now represented using a new ``Coefficient`` type that may be independently created and manipulated if required.
Some highlights:
- The ``.compile()`` method has been removed. Coefficients specified as strings are automatically compiled if possible and the compilation is cached across different Python runs and instances.
- Mixing coefficient types within a single ``Qobj`` is now supported.
- Many new attributes were added to ``QobjEvo`` for convenience. Examples include ``.dims``, ``.shape``, ``.superrep`` and ``.isconstant``.
- Many old attributes such as ``.cte``, ``.use_cython``, ``.type``, ``.const``, and ``.coeff_file`` were removed.
- A new ``Spline`` coefficient supports spline interpolations of different orders. The old ``Cubic_Spline`` coefficient has been removed.
- The new ``.arguments(...)`` method allows additional arguments to the underlying coefficient functions to be updated.
- The ``_step_func_coeff`` argument has been replaced by the ``order`` parameter. ``_step_func_coeff=False`` is equivalent to ``order=3``. ``_step_func_coeff=True`` is equivalent to ``order=0``. Higher values of ``order`` gives spline interpolations of higher orders.
Solver changes
--------------
The solvers in QuTiP have been heavily reworked and standardized. Under the hood solvers now make use of swappable ODE ``Integrators``. Many ``Integrators`` are included (see the list below) and custom implementations are possible. Solvers now consistently accept a ``QobjEvo`` instance at the Hamiltonian or Liouvillian, or any object which can be passed to the ``QobjEvo`` constructor.
A breakdown of highlights follows.
All solvers:
- Solver options are now supplied in an ordinary Python dict. ``qutip.Options`` is deprecated and returns a dict for backwards compatibility.
- A specific ODE integrator may be selected by supplying a ``method`` option.
- Each solver provides a class interface. Creating an instance of the class allows a solver to be run multiple times for the same system without having to repeatedly reconstruct the right-hand side of the ODE to be integrated.
- A ``QobjEvo`` instance is accepted for most operators, e.g., ``H``, ``c_ops``, ``e_ops``, ``a_ops``.
- The progress bar is now selected using the ``progress_bar`` option. A new progess bar using the ``tqdm`` Python library is provided.
- Dynamic arguments, where the value of an operator depends on the current state of the evolution, have been removed. They may be re-implemented later if there is demand for them.
Integrators:
- The SciPy zvode integrator is available with the BDF and Adams methods as ``bdf`` and ``adams``.
- The SciPy dop853 integrator (an eighth order Runge-Kutta method by Dormand & Prince) is available as ``dop853``.
- The SciPy lsoda integrator is available as ``lsoda``.
- QuTiP's own implementation of Verner's "most efficient" Runge-Kutta methods of order 7 and 9 are available as ``vern7`` and ``vern9``. See http://people.math.sfu.ca/~jverner/ for a description of the methods.
- QuTiP's own implementation of a solver that directly diagonalizes the the system to be integrated is available as ``diag``. It only works on time-independent systems and is slow to setup, but once the diagonalization is complete, it generates solutions very quickly.
- QuTiP's own implementatoin of an approximate Krylov subspace integrator is available as ``krylov``. This integrator is only usable with ``sesolve``.
Result class:
- A new ``.e_data`` attribute provides expectation values as a dictionary. Unlike ``.expect``, the values are provided in a Python list rather than a numpy array, which better supports non-numeric types.
- The contents of the ``.stats`` attribute changed significantly and is now more consistent across solvers.
Monte-Carlo Solver (mcsolve):
- The system, H, may now be a super-operator.
- The ``seed`` parameter now supports supplying numpy ``SeedSequence`` or ``Generator`` types.
- The new ``timeout`` and ``target_tol`` parameters allow the solver to exit early if a timeout or target tolerance is reached.
- The ntraj option no longer supports a list of numbers of trajectories. Instead, just run the solver multiple times and use the class ``MCSolver`` if setting up the solver uses a significant amount of time.
- The ``map_func`` parameter has been replaced by the ``map`` option. In addition to the existing ``serial`` and ``parallel`` values, the value ``loky`` may be supplied to use the loky package to parallelize trajectories.
- The result returned by ``mcsolve`` now supports calculating photocurrents and calculating the steady state over N trajectories.
- The old ``parfor`` parallel execution function has been removed from ``qutip.parallel``. Use ``parallel_map`` or ``loky_map`` instead.
Bloch-Redfield Master Equation Solver (brmesolve):
- The ``a_ops`` and ``spectra`` support implementaitons been heavily reworked to reuse the techniques from the new Coefficient and QobjEvo classes.
- The ``use_secular`` parameter has been removed. Use ``sec_cutoff=-1`` instead.
- The required tolerance is now read from ``qutip.settings``.
Krylov Subspace Solver (krylovsolve):
- The Krylov solver is now implemented using ``SESolver`` and the ``krylov`` ODE integrator. The function ``krylovsolve`` is maintained for convenience and now supports many more options.
- The ``sparse`` parameter has been removed. Supply a sparse ``Qobj`` for the Hamiltonian instead.
Floquet Solver (fsesolve and fmmesolve):
- The Floquet solver has been rewritten to use a new ``FloquetBasis`` class which manages the transformations from lab to Floquet basis and back.
- Many of the internal methods used by the old Floquet solvers have been removed. The Floquet tensor may still be retried using the function ``floquet_tensor``.
- The Floquet Markov Master Equation solver has had many changes and new options added. The environment temperature may be specified using ``w_th``, and the result states are stored in the lab basis and optionally in the Floquet basis using ``store_floquet_state``.
- The spectra functions supplied to ``fmmesolve`` must now be vectorized (i.e. accept and return numpy arrays for frequencies and densities) and must accept negative frequence (i.e. usually include a ``w > 0`` factor so that the returned densities are zero for negative frequencies).
- The number of sidebands to keep, ``kmax`` may only be supplied when using the ``FMESolver``
- The ``Tsteps`` parameter has been removed from both ``fsesolve`` and ``fmmesolve``. The ``precompute`` option to ``FloquetBasis`` may be used instead.
Evolution of State Solver (essovle):
- The function ``essolve`` has been removed. Use the ``diag`` integration method with ``sesolve`` or ``mesolve`` instead.
Steady-state solvers (steadystate module):
- The ``method`` parameter and ``solver`` parameters have been separated. Previously they were mixed together in the ``method`` parameter.
- The previous options are now passed as parameters to the steady state solver and mostly passed through to the underlying SciPy functions.
- The logging and statistics have been removed.
Correlation functions (correlation module):
- A new ``correlation_3op`` function has been added. It supports ``MESolver`` or ``BRMESolver``.
- The ``correlation``, ``correlation_4op``, and ``correlation_ss`` functions have been removed.
- Support for calculating correlation with ``mcsolve`` has been removed.
Propagators (propagator module):
- A class interface, ``qutip.Propagator``, has been added for propagators.
- Propagation of time-dependent systems is now supported using ``QobjEvo``.
- The ``unitary_mode`` and ``parallel`` options have been removed.
Correlation spectra (spectrum module):
- The functions ``spectrum_ss`` and ``spectrum_pi`` have been removed and are now internal functions.
- The ``use_pinv`` parameter for ``spectrum`` has been removed and the functionality merged into the ``solver`` parameter. Use ``solver="pi"`` instead.
QuTiP core
----------
There have been numerous other small changes to core QuTiP features:
- ``qft(...)`` the function that returns the quantum Fourier transform operator was moved from ``qutip.qip.algorithm`` into ``qutip``.
- The Bloch-Redfield solver tensor, ``brtensor``, has been moved into ``qutip.core``. See the section above on the Bloch-Redfield solver for details.
- The functions ``mat2vec`` and ``vec2mat`` for transforming states to and from super-operator states have been renamed to ``stack_columns`` and ``unstack_columns``.
- The function ``liouvillian_ref`` has been removed. Used ``liouvillian`` instead.
- The superoperator transforms ``super_to_choi``, ``choi_to_super``, ``choi_to_kraus``, ``choi_to_chi`` and ``chi_to_choi`` have been removed. Used ``to_choi``, ``to_super``, ``to_kraus`` and ``to_chi`` instead.
- All of the random object creation functions now accepted a numpy ``Generator`` as a seed.
- The ``dims`` parameter of all random object creation functions has been removed. Supply the dimensions as the first parameter if explicit dimensions are required.
- The function ``rand_unitary_haar`` has been removed. Use ``rand_unitary(distribution="haar")`` instead.
- The functions ``rand_dm_hs`` and ``rand_dm_ginibre`` have been removed. Use ``rand_dm(distribution="hs")`` and ``rand_dm(distribution="ginibre")`` instead.
- The function ``rand_ket_haar`` has been removed. Use ``rand_ket(distribution="haar")`` instead.
- The measurement functions have had the ``target`` parameter for expanding the measurement operator removed. Used ``expand_operator`` to expand the operator instead.
- ``qutip.Bloch`` now supports applying colours per-point, state or vector in
``add_point``, ``add_states``, and ``add_vectors``.
QuTiP settings
--------------
Previously ``qutip.settings`` was an ordinary module. Now ``qutip.settings`` is an instance of a settings class. All the runtime modifiable settings for core operations are in ``qutip.settings.core``. The other settings are not modifiable at runtime.
- Removed ``load``. ``reset`` and ``save`` functions.
- Removed ``.debug``, ``.fortran``, ``.openmp_thresh``.
- New ``.compile`` stores the compilation options for compiled coefficients.
- New ``.core["rtol"]`` core option gives the default relative tolerance used by QuTiP.
- The absolute tolerance setting ``.atol`` has been moved to ``.core["atol"]``.
Package reorganization
----------------------
- ``qutip.qip`` has been moved into its own package, qutip-qip. Once installed, qutip-qip is available as either ``qutip.qip`` or ``qutip_qip``. Some widely useful gates have been retained in ``qutip.gates``.
- ``qutip.lattice`` has been moved into its own package, qutip-lattice. It is available from `<https://github.com/qutip/qutip-lattice>`.
- ``qutip.sparse`` has been removed. It contained the old sparse matrix representation and is replaced by the new implementation in ``qutip.data``.
- ``qutip.piqs`` functions are no longer available from the ``qutip`` namespace. They are accessible from ``qutip.piqs`` instead.
Miscellaneous
-------------
- Support has been added for 64-bit integer sparse matrix indices, allowing sparse matrices with up to 2**63 rows and columns. This support needs to be enabled at compilation time by calling ``setup.py`` and passing ``--with-idxint-64``.
Feature removals
----------------
- Support for OpenMP has been removed. If there is enough demand and a good plan for how to organize it, OpenMP support may return in a future QuTiP release.
- The ``qutip.parfor`` function has been removed. Use ``qutip.parallel_map`` instead.
- ``qutip.graph`` has been removed and replaced by SciPy's graph functions.
- ``qutip.topology`` has been removed. It contained only one function ``berry_curvature``.
- The ``~/.qutip/qutiprc`` config file is no longer supported. It contained settings for the OpenMP support.