Qutip

Improvements
------------------
- **MAJOR FEATURE**: Added methods and techniques to the stochastic solvers (by **Eric Giguère**) which allows to use a much broader set of solvers and much more efficiently.

- **MAJOR FEATURE**: Optimization of the montecarlo solver (by **Eric Giguère**). Computation are faster in many cases. Collapse information available to time dependant information.

- Added the QObjEvo class and methods (by **Eric Giguère**), which is used behind the scenes by the dynamical solvers, making the code more efficient and tidier. More built-in function available to string coefficients. The coefficients can be made from interpolated array with variable timesteps and can obtain state information more easily. Time-dependant collapse operator can have multiple terms.

- New wigner_transform and plot_wigner_sphere function. (by **Nithin Ramu**).

- ptrace is faster and work on bigger systems, from 15 Qbits to 30 Qbits.

- QIP module: added the possibility for user-defined gates, added the possibility to remove or add gates in any point of an already built circuit, added the molmer_sorensen gate, and fixed some bugs (by **Boxi Li**).

- Added the quantum Hellinger distance to qutip.metrics (by **Wojciech Rzadkowski**).

- Implemented possibility of choosing a random seed (by **Marek marekyggdrasil**).

- Added a code of conduct to Github.

Bug Fixes
-------------

- Fixed bug that made QuTiP incompatible with SciPy 1.3.

- quite.cite() is improved.

- Added matplotlib as optional dependency and updated numpy and scipy requirements.

- Fixed matrix generating coherent spin states in qutip.piqs to include complex coefficients.

Improvements
------------
- **MAJOR FEATURE**: Added the Permutational Invariant Quantum Solver (PIQS) module (**by Nathan Shammah** and **Shahnawaz Ahmed**) which allows the simluation of large TLSs ensembles including collective and local Lindblad dissipation. Applications range from superradiance to spin squeezing.

- **MAJOR FEATURE**: Added a photon scattering module (**by Ben Bartlett**) which can be used to study scattering in arbitrary driven systems coupled to some configuration of output waveguides.

- Cubic_Spline functions as time-dependent arguments for the collapse operators in mesolve are now allowed.

- Added a faster version of bloch_redfield_tensor, using components from the time-dependent version. About 3x+ faster for secular tensors, and 10x+ faster for non-secular tensors.

- Computing Q.overlap() [inner product] is now ~30x faster.

- Added projector method to Qobj class.

- Added fast projector method, ``Q.proj()``.

- Computing matrix elements, ``Q.matrix_element`` is now ~10x faster.

- Computing expectation values for ket vectors using ``expect`` is now ~10x faster.

- ``Q.tr()`` is now faster for small Hilbert space dimensions.

- Unitary operator evolution added to sesolve

- Use OPENMP for tidyup if installed.


Bug Fixes
---------

- Fixed bug that stopped simdiag working for python 3.

- Fixed semidefinite cvxpy Variable and Parameter.

- Fixed iterative lu solve atol keyword issue.

- Fixed unitary op evolution rhs matrix in ssesolve.

- Fixed interpolating function to return zero outside range.

- Fixed dnorm complex casting bug.

- Fixed control.io path checking issue.

- Fixed ENR fock dimension.

- Fixed hard coded options in propagator 'batch' mode

- Fixed bug in trace-norm for non-Hermitian operators.

- Fixed bug related to args not being passed to coherence_function_g2

- Fixed MKL error checking dict key error

Release of QuTiP 4.2.

Improvements
--------------
- *MAJOR FEATURE*: Initial implementation of time-dependent Bloch-Redfield Solver.
- Qobj tidyup is now an order of magnitude faster.
- Time-dependent codegen now generates output NumPy arrays faster.
- Improved calculation for analytic coefficients in coherent states (Sebastian Kramer).
- Input array to correlation FFT method now checked for validity.
- Function-based time-dependent mesolve and sesolve routines now faster.
- Codegen now makes sure that division is done in C, as opposed to Python.
- Can now set different controls for a each timeslot in quantum optimization.
- This allows time-varying controls to be used in pulse optimisation.

Bug Fixes
---------
- rcsolve importing old Odeoptions Class rather than Options.
- Non-int issue in spin Q and Wigner functions.
- Qobj’s should tidyup before determining isherm.
- Fixed time-dependent RHS function loading on Win.
- Fixed several issues with compiling with Cython 0.26.
- Liouvillian superoperators were hard setting isherm=True by default.
- Fixed an issue with the solver safety checks when inputing a list with Python functions as time-dependence.
- Fixed non-int issue in Wigner_cmap.
- MKL solver error handling not working properly.