changelog-dev.md

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Release 0.40.0-dev (development release)

New features since last release

• Two new methods: setup_execution_config and preprocess_transforms are added to the Device class. Device developers are encouraged to override these two methods separately instead of the preprocess method. For now, to avoid ambiguity, a device is allowed to override either these two methods or preprocess, but not both. In the long term, we will slowly phase out the use of preprocess in favour of these two methods for better separation of concerns. (#6617)

• Developers of plugin devices now have the option of providing a TOML-formatted configuration file to declare the capabilities of the device. See Device Capabilities for details.

  • An internal module pennylane.devices.capabilities is added that defines a new DeviceCapabilites data class, as well as functions that load and parse the TOML-formatted configuration files. (#6407)

      >>> from pennylane.devices.capabilities import DeviceCapabilities
      >>> capabilities = DeviceCapabilities.from_toml_file("my_device.toml")
      >>> isinstance(capabilities, DeviceCapabilities)
      True

  • Devices that extends qml.devices.Device now has an optional class attribute capabilities that is an instance of the DeviceCapabilities data class, constructed from the configuration file if it exists. Otherwise, it is set to None. (#6433)

    from pennylane.devices import Device
    
    class MyDevice(Device):
    
        config_filepath = "path/to/config.toml"
    
        ...

    >>> isinstance(MyDevice.capabilities, DeviceCapabilities)
    True

New `labs` module `dla` for handling dynamical Lie algebras (DLAs)

• Added a dense implementation of computing the Lie closure in a new function lie_closure_dense in pennylane.labs.dla. (#6371)

• Added a dense implementation of computing the structure constants in a new function structure_constants_dense in pennylane.labs.dla. (#6376)

• Added utility functions for handling dense matrices in the Lie theory context. (#6563)

New API for Qubit Mixed

• Added qml.devices.qubit_mixed module for mixed-state qubit device support (#6379). This module introduces an apply_operation helper function that features:

  • Two density matrix contraction methods using einsum and tensordot

  • Optimized handling of special cases including: Diagonal operators, Identity operators, CX (controlled-X), Multi-controlled X gates, Grover operators

• Added submodule 'initialize_state' featuring a create_initial_state function for initializing a density matrix from qml.StatePrep operations or qml.QubitDensityMatrix operations. (#6503)

• Added support for constructing BoseWord and BoseSentence, similar to FermiWord and FermiSentence. (#6518)

• Added method preprocess to the QubitMixed device class to preprocess the quantum circuit before execution. Necessary non-intrusive interfaces changes to class init method were made along the way to the QubitMixed device class to support new API feature. (#6601)

• Added a second class DefaultMixedNewAPI to the qml.devices.qubit_mixed module, which is to be the replacement of legacy DefaultMixed which for now to hold the implementations of preprocess and execute methods. (#6607)

• Added christiansen_mapping() function to map BoseWord and BoseSentence to qubit operators, using christiansen mapping. (#6623)

• Added unary_mapping() function to map BoseWord and BoseSentence to qubit operators, using unary mapping. (#6576)

• Added binary_mapping() function to map BoseWord and BoseSentence to qubit operators, using standard-binary mapping. (#6564)

• The qml.qchem.factorize function now supports new methods for double factorization: Cholesky decomposition (cholesky=True) and compressed double factorization (compressed=True). (#6573) (#6611)

• Added qml.qchem.symmetry_shift function to perform the block-invariant symmetry shift on the electronic integrals. (#6574)

Improvements 🛠

• Raises a comprehensive error when using qml.fourier.qnode_spectrum with standard numpy arguments and interface="auto". (#6622)

• Added support for the wire_options dictionary to customize wire line formatting in qml.draw_mpl circuit visualizations, allowing global and per-wire customization with options like color, linestyle, and linewidth. (#6486)

• QNode and qml.execute now forbid certain keyword arguments from being passed positionally. (#6610)

• Shortened the string representation for the qml.S, qml.T, and qml.SX operators. (#6542)

• Added functions and dunder methods to add and multiply Resources objects in series and in parallel. (#6567)

Capturing and representing hybrid programs

• jax.vmap can be captured with qml.capture.make_plxpr and is compatible with quantum circuits. (#6349) (#6422)

• qml.capture.PlxprInterpreter base class has been added for easy transformation and execution of pennylane variant jaxpr. (#6141)

• A DefaultQubitInterpreter class has been added to provide plxpr execution using python based tools, and the DefaultQubit.eval_jaxpr method is now implemented. (#6594) (#6328)

• An optional method eval_jaxpr is added to the device API for native execution of plxpr programs. (#6580)

• qml.capture.qnode_call has been made private and moved to the workflow module. (#6620)

Other Improvements

• Added PyTree support for measurements in a circuit. (#6378)

  import pennylane as qml
  
  @qml.qnode(qml.device("default.qubit"))
  def circuit():
      qml.Hadamard(0)
      qml.CNOT([0,1])
      return {"Probabilities": qml.probs(), "State": qml.state()}

  >>> circuit() 
  {'Probabilities': array([0.5, 0. , 0. , 0.5]), 'State': array([0.70710678+0.j, 0.        +0.j, 0.        +0.j, 0.70710678+0.j])} 

• _cache_transform transform has been moved to its own file located at qml.workflow._cache_transform.py. (#6624)

• qml.BasisRotation template is now JIT compatible. (#6019)

• The Jaxpr primitives for for_loop, while_loop and cond now store slices instead of numbers of args. (#6521)

• Expand ExecutionConfig.gradient_method to store TransformDispatcher type. (#6455)

• Fix the string representation of Resources instances to match the attribute names. (#6581)

Labs 🧪

• Added base class Resources, CompressedResourceOp, ResourceOperator for advanced resource estimation. (#6428)

• Added ResourceOperator classes for QFT and all operators in QFT's decomposition. (#6447)

Breaking changes 💔

• qml.fourier.qnode_spectrum no longer automatically converts pure numpy parameters to the Autograd framework. As the function uses automatic differentiation for validation, parameters from an autodiff framework have to be used. (#6622)

• qml.math.jax_argnums_to_tape_trainable is moved and made private to avoid a qnode dependency in the math module. (#6609)

• Gradient transforms are now applied after the user's transform program. (#6590)

• Legacy operator arithmetic has been removed. This includes qml.ops.Hamiltonian, qml.operation.Tensor, qml.operation.enable_new_opmath, qml.operation.disable_new_opmath, and qml.operation.convert_to_legacy_H. Note that qml.Hamiltonian will continue to dispatch to qml.ops.LinearCombination. For more information, check out the updated operator troubleshooting page. (#6548) (#6602) (#6589)

• The developer-facing qml.utils module has been removed. Specifically, the following 4 sets of functions have been either moved or removed(#6588):

  • qml.utils._flatten, qml.utils.unflatten has been moved and renamed to qml.optimize.qng._flatten_np and qml.optimize.qng._unflatten_np respectively.

  • qml.utils._inv_dict and qml._get_default_args have been removed.

  • qml.utils.pauli_eigs has been moved to qml.pauli.utils.

  • qml.utils.expand_vector has been moved to qml.math.expand_vector.

• The qml.qinfo module has been removed. Please see the respective functions in the qml.math and qml.measurements modules instead. (#6584)

• Top level access to Device, QubitDevice, and QutritDevice have been removed. Instead, they are available as qml.devices.LegacyDevice, qml.devices.QubitDevice, and qml.devices.QutritDevice respectively. (#6537)

• The 'ancilla' argument for qml.iterative_qpe has been removed. Instead, use the 'aux_wire' argument. (#6532)

• The qml.BasisStatePreparation template has been removed. Instead, use qml.BasisState. (#6528)

• The qml.workflow.set_shots helper function has been removed. We no longer interact with the legacy device interface in our code. Instead, shots should be specified on the tape, and the device should use these shots. (#6534)

• QNode.gradient_fn has been removed. Please use QNode.diff_method instead. QNode.get_gradient_fn can also be used to process the diff method. (#6535)

• The qml.QubitStateVector template has been removed. Instead, use qml.StatePrep. (#6525)

• qml.broadcast has been removed. Users should use for loops instead. (#6527)

• The max_expansion argument for qml.transforms.clifford_t_decomposition has been removed. (#6531) (#6571)

• The expand_depth argument for qml.compile has been removed. (#6531)

• The qml.shadows.shadow_expval transform has been removed. Instead, please use the qml.shadow_expval measurement process. (#6530) (#6561)

Deprecations 👋

• The tape and qtape properties of QNode have been deprecated. Instead, use the qml.workflow.construct_tape function. (#6583) (#6650)

• The max_expansion argument in qml.devices.preprocess.decompose is deprecated and will be removed in v0.41. (#6400)

• The decomp_depth argument in qml.transforms.set_decomposition is deprecated and will be removed in v0.41. (#6400)

• The output_dim property of qml.tape.QuantumScript has been deprecated. Instead, use method shape of QuantumScript or MeasurementProcess to get the same information. (#6577)

• The QNode.get_best_method and QNode.best_method_str methods have been deprecated. Instead, use the qml.workflow.get_best_diff_method function. (#6418)

• The qml.execute gradient_fn keyword argument has been renamed diff_method, to better align with the termionology used by the QNode. gradient_fn will be removed in v0.41. (#6549)

Documentation 📝

• Add reporting of test warnings as failures. (#6217)

• Add a warning message to Gradients and training documentation about ComplexWarnings. (#6543)

Bug fixes 🐛

• The qml.HilbertSchmidt and qml.LocalHilbertSchmidt templates now apply the complex conjugate of the unitaries instead of the adjoint, providing the correct result. (#6604)

• QNode return behaviour is now consistent for lists and tuples. (#6568)

• qml.QNode now accepts arguments with types defined in libraries that are not necessarily in the list of supported interfaces, such as the Graph class defined in networkx. (#6600)

• qml.math.get_deep_interface now works properly for autograd arrays. (#6557)

• Fixed Identity.__repr__ to return correct wires list. (#6506)

Contributors ✍️

This release contains contributions from (in alphabetical order):

Shiwen An, Utkarsh Azad, Astral Cai, Yushao Chen, Diksha Dhawan, Pietropaolo Frisoni, Austin Huang, Korbinian Kottmann, Christina Lee, William Maxwell, Andrija Paurevic, Justin Pickering, Jay Soni, David Wierichs,