Release 0.41.0

New features since last release

Resource-efficient Decompositions 🔎

A new, experimental graph-based decomposition system is now available in PennyLane, offering more resource-efficiency and versatility. (#6950) (#6951) (#6952) (#7028) (#7045) (#7058) (#7064) (#7149) (#7184) (#7223) (#7263)

PennyLane's new experimental graph decomposition system offers a resource-efficient and versatile alternative to the current system. This is done by traversing an internal graph structure that is weighted by the resources (e.g., gate counts) required to decompose down to a given set of gates.

The graph-based system is experimental and is disabled by default, but it can be enabled by adding qml.decomposition.enable_graph() to the top of your program. Conversely, qml.decomposition.disable_graph() disables the new system from being active.

With qml.decomposition.enable_graph(), the following new features are available:

• Operators in PennyLane can now accommodate multiple decompositions, which can be queried with the new qml.list_decomps function:

  >>> import pennylane as qml
  >>> qml.decomposition.enable_graph()
  >>> qml.list_decomps(qml.CRX)
  [<pennylane.decomposition.decomposition_rule.DecompositionRule at 0x136da9de0>,
    <pennylane.decomposition.decomposition_rule.DecompositionRule at 0x136da9db0>,
    <pennylane.decomposition.decomposition_rule.DecompositionRule at 0x136da9f00>]
  >>> decomp_rule = qml.list_decomps(qml.CRX)[0]
  >>> print(decomp_rule, "\n")
  @register_resources(_crx_to_rx_cz_resources)
  def _crx_to_rx_cz(phi: TensorLike, wires: WiresLike, **__):
      qml.RX(phi / 2, wires=wires[1])
      qml.CZ(wires=wires)
      qml.RX(-phi / 2, wires=wires[1])
      qml.CZ(wires=wires) 
  
  >>> print(qml.draw(decomp_rule)(0.5, wires=[0, 1]))
  0: ───────────╭●────────────╭●─┤
  1: ──RX(0.25)─╰Z──RX(-0.25)─╰Z─┤

  When an operator within a circuit needs to be decomposed (e.g., when qml.transforms.decompose is present), the chosen decomposition rule is that which is most resource efficient (results in the smallest gate count).

• New decomposition rules can be globally added to operators in PennyLane with the new qml.add_decomps function. Creating a valid decomposition rule requires:

  • Defining a quantum function that represents the decomposition.
  • Adding resource requirements (gate counts) to the above quantum function by decorating it with the new qml.register_resources function, which requires a dictionary mapping operator types present in the quantum function to their number of occurrences.

  qml.decomposition.enable_graph()
  
  @qml.register_resources({qml.H: 2, qml.CZ: 1})
  def my_cnot(wires):
      qml.H(wires=wires[1])
      qml.CZ(wires=wires)
      qml.H(wires=wires[1])
  
  qml.add_decomps(qml.CNOT, my_cnot)

  This newly added rule for qml.CNOT can be verified as being available to use:

  >>> my_new_rule = qml.list_decomps(qml.CNOT)[-1]
  >>> print(my_new_rule)
  @qml.register_resources({qml.H: 2, qml.CZ: 1})
  def my_cnot(wires):
      qml.H(wires=wires[1])
      qml.CZ(wires=wires)
      qml.H(wires=wires[1])

  Operators with dynamic resource requirements must be declared in a resource estimate using the new qml.resource_rep function. For each operator class, the set of parameters that affects the type of gates and their number of occurrences in its decompositions is given by the resource_keys attribute.

  >>> qml.MultiRZ.resource_keys
  {'num_wires'}

  The output of resource_keys indicates that custom decompositions for the operator should be registered to a resource function (as opposed to a static dictionary) that accepts those exact arguments and returns a dictionary. Consider this dummy example of a fictitious decomposition rule comprising three qml.MultiRZ gates:

  qml.decomposition.enable_graph()
  
  def resource_fn(num_wires):
      return {
          qml.resource_rep(qml.MultiRZ, num_wires=num_wires - 1): 1,
          qml.resource_rep(qml.MultiRZ, num_wires=3): 2
      }
  
  @qml.register_resources(resource_fn)
  def my_decomp(theta, wires):
      qml.MultiRZ(theta, wires=wires[:3])
      qml.MultiRZ(theta, wires=wires[1:])
      qml.MultiRZ(theta, wires=wires[:3])

  More information for defining complex decomposition rules can be found in the documentation for qml.register_resources.

• The qml.transforms.decompose transform works when the new decompositions system is enabled and offers the ability to inject new decomposition rules for operators in QNodes. (#6966)

  With the graph-based system enabled, the qml.transforms.decompose transform offers the ability to inject new decomposition rules via two new keyword arguments:

  • fixed_decomps: decomposition rules provided to this keyword argument will be used by the new system, bypassing all other decomposition rules that may exist for the relevant operators.
  • alt_decomps: decomposition rules provided to this keyword argument are alternative decomposition rules that the new system may choose if they're the most resource efficient.

  Each keyword argument must be assigned a dictionary that maps operator types to decomposition rules. Here is an example of both keyword arguments in use:

  qml.decomposition.enable_graph()
  
  @qml.register_resources({qml.CNOT: 2, qml.RX: 1})
  def my_isingxx(phi, wires, **__):
      qml.CNOT(wires=wires)
      qml.RX(phi, wires=[wires[0]])
      qml.CNOT(wires=wires)
  
  @qml.register_resources({qml.H: 2, qml.CZ: 1})
  def my_cnot(wires, **__):
      qml.H(wires=wires[1])
      qml.CZ(wires=wires)
      qml.H(wires=wires[1])
  
  @partial(
      qml.transforms.decompose,
      gate_set={"RX", "RZ", "CZ", "GlobalPhase"},
      alt_decomps={qml.CNOT: my_cnot},
      fixed_decomps={qml.IsingXX: my_isingxx},
  )
  @qml.qnode(qml.device("default.qubit"))
  def circuit():
      qml.CNOT(wires=[0, 1])
      qml.IsingXX(0.5, wires=[0, 1])
      return qml.state()

  >>> circuit()
  array([ 9.68912422e-01+2.66934210e-16j, -1.57009246e-16+3.14018492e-16j,
        8.83177008e-17-2.94392336e-17j,  5.44955495e-18-2.47403959e-01j])

  More details about what fixed_decomps and alt_decomps do can be found in the usage details section in the qml.transforms.decompose documentation.

Capturing and Representing Hybrid Programs 📥

Quantum operations, classical processing, structured control flow, and dynamicism can be efficiently expressed with program capture (enabled with qml.capture.enable).

In the last several releases of PennyLane, we have been working on a new and experimental feature called
program capture, which allows for compactly expressing details about hybrid workflows while also providing a smoother integration with just-in-time compilation frameworks like Catalyst (via the ~.pennylane.qjit decorator) and JAX-jit.

Internally, program capture is supported by representing hybrid programs via a new intermediate representation (IR) called plxpr, rather than a quantum tape. The plxpr IR is an adaptation of JAX's jaxpr IR.

There are some quirks and restrictions to be aware of, which are outlined in the :doc:/news/program_capture_sharp_bits page. But with this release, many of the core features of PennyLane—and more!—are available with program capture enabled by adding qml.capture.enable() to the top of your program:

• QNodes can now contain mid-circuit measurements (MCMs) and classical processing on MCMs with mcm_method = "deferred" when program capture is enabled. (#6838) (#6937) (#6961)

  With mcm_method = "deferred", workflows with mid-circuit measurements can be executed with program capture enabled. Additionally, program capture unlocks the ability to classically process MCM values and use MCM values as gate parameters.

  import jax 
  import jax.numpy as jnp
  jax.config.update("jax_enable_x64", True)
  
  qml.capture.enable()
  
  @qml.qnode(qml.device("default.qubit", wires=3), mcm_method="deferred")
  def f(x):
      m0 = qml.measure(0)
      m1 = qml.measure(0)
  
      # classical processing on m0 and m1
      a = jnp.sin(0.5 * jnp.pi * m0)
      phi = a - (m1 + 1) ** 4
  
      qml.s_prod(x, qml.RX(phi, 0))
  
      return qml.expval(qml.Z(0))

  >>> f(0.1)
  Array(0.00540302, dtype=float64)

  Note that with capture enabled, automatic qubit management is not supported on devices; the number of wires given to the device must coincide with how many MCMs are present in the circuit, since deferred measurements add one auxiliary qubit per MCM.

• Quantum circuits can now be differentiated on default.qubit and lightning.qubit with diff_method="finite-diff", "adjoint", and "backprop" when program capture is enabled. (#6853) (#6875) (#7019)

  qml.capture.enable()
  
  @qml.qnode(qml.device("default.qubit", wires=3), diff_method="adjoint")
  def f(phi):
      qml.RX(phi, 0)
      return qml.expval(qml.Z(0))

  >>> qml.grad(f)(jnp.array(0.1))
  Array(-0.09983342, dtype=float64, weak_type=True)

• QNode arguments can now be assigned as static. In turn, PennyLane can now determine when plxpr needs to be reconstructed based on dynamic and static arguments, providing efficiency for repeated circuit executions. (#6923)

  Specifying static arguments can be done at the QNode level with the static_argnums keyword argument. Its values (integers) indicate which arguments are to be treated as static. By default, all QNode arguments are dynamic.

  Consider the following example, where the first argument is indicated to be static:

  qml.capture.enable()
  
  @qml.qnode(qml.device("default.qubit", wires=1), static_argnums=0)
  def f(x, y):
      print("I constructed plxpr")
      qml.RX(x, 0)
      qml.RY(y, 0)
      return qml.expval(qml.Z(0))

  When the value of x changes, PennyLane must (re)construct the plxpr representation of the program for the program to execute. In this example, the act of (re)constructing plxpr is indicated by the print statement executing. However, if the value of y changes (a dynamic argument), PennyLane does not need to reconstruct the plxpr representation of the program:

  >>> f(0.1, 0.2)
  I constructed plxpr
  0.97517043
  >>> f(0.1, 0.3)
  0.9505638
  >>> f(0.2, 0.3)
  I constructed plxpr
  0.93629336

• All PennyLane transforms that return one device execution are compatible with program capture, including those without a plxpr-native implementation. (#6916) (#6922) (#6925) (#6945) (#6946) (#6957) (#6977) (#7020) (#7199) (#7247)

  The following transforms now have native support for program capture (i.e., they directly transform plxpr) and can be used as you would normally use a transform in PennyLane:

  • merge_rotations
  • single_qubit_fusion
  • unitary_to_rot
  • merge_amplitude_embedding
  • commute_controlled
  • decompose
  • map_wires
  • cancel_inverses

  Other transforms without a plxpr-native implementation are also supported by internally converting the tape implementation. As an example, here is a custom tape transform that shifts every qml.RX gate to the end of the circuit:

  qml.capture.enable()
  
  @qml.transform
  def shift_rx_to_end(tape):
      """Transform that moves all RX gates to the end of the operations list."""
      new_ops, rxs = [], []
  
      for op in tape.operations:
          if isinstance(op, qml.RX):
              rxs.append(op)
          else:
                new_ops.append(op)
  
      operations = new_ops + rxs
      new_tape = tape.copy(operations=operations)
      return [new_tape], lambda res: res[0]
  
  @shift_rx_to_end
  @qml.qnode(qml.device("default.qubit", wires=1))
  def circuit1():
      qml.RX(0.1, wires=0)
      qml.H(wires=0)
      return qml.state()
  
  @qml.qnode(qml.device("default.qubit", wires=1))
  def circuit2():
      qml.H(wires=0)
      qml.RX(0.1, wires=0)
      return qml.state()

  >>> circuit1() == circuit2()
  Array([ True,  True], dtype=bool)

  There are some exceptions to getting transforms without a plxpr-native implementation to work with capture enabled:

  • Transforms that return multiple device executions cannot be converted.
  • Transforms that return non-trivial post-processing functions cannot be converted.
  • Transforms will fail to execute if the transformed quantum function or QNode contains:
    • qml.cond with dynamic parameters as predicates.
    • qml.for_loop with dynamic parameters for start, stop, or step.
    • qml.while_loop.

• Python control flow (if/else, for, while) is now supported when program capture is enabled. (#6837) (#6931)

  One of the strengths of program capture is preserving a program's structure, eliminating the need to unroll control flow operations. In previous releases, this could only be accomplished by using qml.for_loop, qml.cond, and qml.while_loop. With this release, standard Python control flow operations are now supported with program capture:

  qml.capture.enable()
  dev = qml.device("default.qubit", wires=[0, 1, 2])
  
  @qml.qnode(dev)
  def circuit(num_loops: int):
      for i in range(num_loops):
          if i % 2 == 0:
              qml.H(i)
          else:
              qml.RX(1, i)
      return qml.state()

  >>> print(qml.draw(circuit)(num_loops=3))
  0: ──H────────┤  State
  1: ──RX(1.00)─┤  State
  2: ──H────────┤  State
  >>> circuit(3)
  Array([0.43879125+0.j        , 0.43879125+0.j        ,
         0.        -0.23971277j, 0.        -0.23971277j,
         0.43879125+0.j        , 0.43879125+0.j        ,
         0.        -0.23971277j, 0.        -0.23971277j], dtype=complex64)

  This is enabled internally by diastatic-malt, which is our home-brewed implementation of Autograph.

  Support for this can be disabled by setting autograph=False at the QNode level (by default, autograph=True).

End-to-end Sparse Execution 🌌

• Sparse data structures in compressed-sparse-row (csr) format are now supported end-to-end in PennyLane, resulting in faster execution for large sparse objects. (#6883) (#7139) (#7191)

  Sparse-array input and execution are now supported on default.qubit and lightning.qubit with a variety of templates, preserving sparsity throughout the entire simulation.

  Specifically, the following templates now support sparse data structures:

  • qml.StatePrep (#6863)
  • qml.QubitUnitary (#6889) (#6986) (#7143)
  • qml.BlockEncode (#6963) (#7140)
  • qml.SWAP (#6965)
  • Controlled operations (#6994)

  import scipy
  import numpy as np
  
  sparse_state = scipy.sparse.csr_array([0, 1, 0, 0])
  mat = np.kron(np.identity(2**12), qml.X.compute_matrix())
  sparse_mat = scipy.sparse.csr_array(mat)
  sparse_x = scipy.sparse.csr_array(qml.X.compute_matrix())
  
  dev = qml.device("default.qubit")
  
  @qml.qnode(dev)
  def circuit():
      qml.StatePrep(sparse_state, wires=range(2))
  
      for i in range(10):
          qml.H(i)
          qml.CNOT(wires=[i, i + 1])
  
      qml.QubitUnitary(sparse_mat, wires=range(13))
      qml.ctrl(qml.QubitUnitary(sparse_x, wires=0), control=1)
      return qml.state()

  >>> circuit()
  array([ 0.     +0.j,  0.03125+0.j,  0.     +0.j, ..., -0.03125+0.j,
          0.     +0.j,  0.     +0.j])

• Operators that have a sparse_matrix method can now choose a sparse-matrix format and the order of their wires. Available scipy sparse-matrix formats include 'bsr', 'csr', 'csc', 'coo', 'dia', 'dok', and 'lil'. (#6995)

  >>> op = qml.CNOT([0,1])
  >>> type(op.sparse_matrix(format='dok'))
  scipy.sparse._dok.dok_matrix
  >>> type(op.sparse_matrix(format='csc'))
  scipy.sparse._csc.csc_matrix
  >>> print(op.sparse_matrix(wire_order=[1,0]))
  (0, 0)	1.0
  (1, 3)	1.0
  (2, 2)	1.0
  (3, 1)	1.0
  >>> print(op.sparse_matrix(wire_order=[0,1]))
  (0, 0)	1
  (1, 1)	1
  (2, 3)	1
  (3, 2)	1

• Sparse functionality is now available in qml.math:

  • qml.math.sqrt_matrix_sparse is available to compute the square root of a sparse Hermitian matrix. (#6976)

  • Most qml.math functions can now correctly handle sparse matrices as input by dispatching to scipy.sparse.linalg internally. (#6947)

QROM State Preparation 📖

• A new state-of-the-art state preparation technique based on QROM is now available with the qml.QROMStatePreparation template. (#6974)

  Using qml.QROMStatePreparation is analogous to using other state preparation techniques in PennyLane.

  state_vector = np.array([0.5, -0.5, 0.5, 0.5])
  
  dev = qml.device("default.qubit")
  wires = qml.registers({"work_wires": 1, "prec_wires": 3, "state_wires": 2})
  
  @qml.qnode(dev)
  def circuit():
      qml.QROMStatePreparation(
          state_vector, wires["state_wires"], wires["prec_wires"], wires["work_wires"]
      )
      return qml.state()

  >>> print(circuit()[:4].real)
  [ 0.5 -0.5  0.5  0.5]

Dynamical Lie Algebras 🕓

The new qml.liealg module provides a variety of Lie algebra functionality:

• Compute the dynamical Lie algebra from a set of generators with qml.lie_closure. This function accepts and outputs matrices when matrix=True.

  import pennylane as qml
  from pennylane import X, Y, Z, I
  n = 2
  gens = [qml.X(0), qml.X(0) @ qml.X(1), qml.Y(1)]
  dla = qml.lie_closure(gens)

  >>> dla
  [X(0), X(0) @ X(1), Y(1), X(0) @ Z(1)]

• Compute the structure constants that make up the adjoint representation of a Lie algebra using qml.structure_constants. This function accepts and outputs matrices when matrix=True.

  >>> adjoint_rep = qml.structure_constants(dla)
  >>> adjoint_rep.shape
  (4, 4, 4)

• The center of a Lie algebra, which is the collection of operators that commute with all other operators in the DLA, can be found with qml.center.

  >>> qml.center(dla)
  [X(0)]

• Cartan decompositions, g = k + m, can be performed with qml.liealg.cartan_decomp. These use involution functions that return a boolean value. A variety of typically encountered involution functions are included in the module, such as even_odd_involution, concurrence_involution, A, AI, AII, AIII, BD, BDI, DIII, C, CI, CII.

  from pennylane.liealg import concurrence_involution
  k, m = qml.liealg.cartan_decomp(dla, concurrence_involution)

  >>> k, m
  ([Y(1)], [X(0), X(0) @ X(1), X(0) @ Z(1)])

  The vertical subspace k and m fulfill the commutation relations [k, m] ⊆ m, [k, k] ⊆ k and [m, m] ⊆ k that make them a proper Cartan decomposition. These can be verified using the function qml.liealg.check_cartan_decomp.

  >>> qml.liealg.check_cartan_decomp(k, m)
  True

• The horizontal Cartan subalgebra a of m can be computed with qml.liealg.horizontal_cartan_subalgebra.

  from pennylane.liealg import horizontal_cartan_subalgebra
  newg, k, mtilde, a, new_adj = horizontal_cartan_subalgebra(k, m, return_adjvec=True)

  >>> newg.shape, k.shape, mtilde.shape, a.shape, new_adj.shape
  ((4, 4), (1, 4), (1, 4), (2, 4), (4, 4, 4))

  newg is ordered such that the elements are newg = k + mtilde + a, where mtilde is the remainder of m without a. A Cartan subalgebra is an Abelian subalgebra of m, and we can confirm that all elements in a are mutually commuting via qml.liealg.check_abelian.

  >>> qml.liealg.check_abelian(a)
  True

• The following functions have also been added:

  • qml.liealg.check_commutation_relation(A, B, C) checks if all commutators between A and B map to a subspace of C, i.e. [A, B] ⊆ C.
  • qml.liealg.adjvec_to_op and qml.liealg.op_to_adjvec allow transforming operators within a Lie algebra to and from their adjoint vector representations.
  • qml.liealg.change_basis_ad_rep allows the transformation of an adjoint representation tensor according to a basis transformation on the underlying Lie algebra, without re-computing the representation.
  • qml.pauli.trace_inner_product can handle batches of dense matrices.

(#6811) (#6861) (#6935) (#7026) (#7054) (#7129)

Qualtran Integration 🔗

• It's now possible to use Qualtran bloqs in PennyLane with the new qml.FromBloq class. (#7148)

  qml.FromBloq translates Qualtran bloqs into equivalent PennyLane operators. It requires two inputs:

  • bloq: an initialized Qualtran Bloq
  • wires: the wires the operator acts on

  The following example applies a PennyLane Operator and Qualtran Bloq in the same circuit:

  >>> from qualtran.bloqs.basic_gates import CNOT
  
  >>> dev = qml.device("default.qubit")
  >>> @qml.qnode(dev)
  ... def circuit():
  ...    qml.X(wires=0)
  ...    qml.FromBloq(CNOT(), wires=[0, 1])
  ...    return qml.state()
  >>> circuit()
  array([0.+0.j, 0.+0.j, 0.+0.j, 1.+0.j])

• A new function called qml.bloq_registers is available to help determine the required wires for complex Qualtran Bloqs with multiple registers. (#7148)

  Given a Qualtran Bloq, this function returns a dictionary of register names and wires.

  >>> from qualtran.bloqs.phase_estimation import RectangularWindowState, TextbookQPE
  >>> from qualtran.bloqs.basic_gates import ZPowGate
  >>> textbook_qpe = TextbookQPE(ZPowGate(exponent=2 * 0.234), RectangularWindowState(3))
  >>> registers = qml.bloq_registers(textbook_qpe)
  >>> registers
  {'q': Wires([0]), 'qpe_reg': Wires([1, 2, 3])}

  In the following example, we use these registers to measure the correct qubits in quantum phase estimation:

  dev = qml.device("default.qubit")
  @qml.qnode(dev)
  def circuit():
      qml.FromBloq(textbook_qpe, wires=range(textbook_qpe.signature.n_qubits()))
      return qml.probs(wires=registers['qpe_reg'])

  >>> print(qml.draw(circuit)())
  0: ─╭FromBloq─┤       
  1: ─├FromBloq─┤ ╭Probs
  2: ─├FromBloq─┤ ├Probs
  3: ─╰FromBloq─┤ ╰Probs

Hadamard Gradient Variants and Improvements 🌈

• Variants of the Hadamard gradient outlined in arXiv:2408.05406 have been added as new differentiation methods: diff_method="reversed-hadamard", "direct-hamadard", and "reversed-direct-hadamard". (#7046) (#7198)\

  The three variants of the Hadamard gradient added in this release offer tradeoffs that could be advantageous in certain cases:

  • "reversed-hadamard": the observable being measured and the generators of the unitary operations in the circuit are reversed; the generators are now the observables, and the Pauli decomposition of the observables are now gates in the circuit.
  • "direct-hadamard": the additional auxiliary qubit needed in the standard Hadamard gradient is exchanged for additional circuit executions.
  • "reversed-direct-hadamard": a combination of the "direct" and "reversed" modes, where the role of the observable and the generators of the unitary operations in the circuit swap, and the additional auxiliary qubit is exchanged for additional circuit executions.

  Using them in your code is just like any other differentiation method in PennyLane:

  import pennylane as qml
  
  dev = qml.device("default.qubit")
  
  @qml.qnode(dev, diff_method="reversed-hadamard")
  def circuit(x):
      qml.RX(x, 0)
      return qml.expval(qml.Z(0))

  >>> qml.grad(circuit)(qml.numpy.array(0.5))
  np.float64(-0.47942553860420284)

  More information on how these three new gradient methods work can be found in arXiv:2408.05406.

• The Hadamard gradient method and its variants can now differentiate any operator with a generator defined, and can accept circuits with non-commuting measurements. (#6928)

Improvements 🛠

QNode execution configuration

• QNodes now have an update method that allows for re-configuring settings like diff_method, mcm_method, and more. This allows for easier on-the-fly adjustments to workflows. (#6803)

  After constructing a QNode,

  import pennylane as qml
  
  @qml.qnode(device=qml.device("default.qubit"))
  def circuit():
      qml.H(0)
      qml.CNOT([0,1])
      return qml.probs()

  its settings can be modified with update, which returns a new QNode object (note: any arguments not specified in update will retain their original value). Here is an example of updating a QNode's diff_method:

  >>> print(circuit.diff_method)
  best
  >>> new_circuit = circuit.update(diff_method="parameter-shift")
  >>> print(new_circuit.diff_method)
  'parameter-shift'

• A new helper function called qml.workflow.construct_execution_config(qnode)(*args,**kwargs) is now available, which allows users to construct an execution configuration from a given QNode instance. (#6901)

  @qml.qnode(qml.device("default.qubit", wires=1))
  def circuit(x):
      qml.RX(x, 0)
      return qml.expval(qml.Z(0))

  >>> config = qml.workflow.construct_execution_config(circuit)(1)
  >>> print(config)
  ExecutionConfig(grad_on_execution=False,
                  use_device_gradient=True,
                  use_device_jacobian_product=False,
                  gradient_method='backprop',
                  gradient_keyword_arguments={},
                  device_options={'max_workers': None,
                                  'prng_key': None,
                                  'rng': Generator(PCG64) at 0x15F6BB680},
                  interface=<Interface.NUMPY: 'numpy'>,
                  derivative_order=1,
                  mcm_config=MCMConfig(mcm_method=None, postselect_mode=None),
                  convert_to_numpy=True)

• QNodes now store their ExecutionConfig instead of qnode_kwargs. (#6991)

Decompositions

• The qml.QROM template has been upgraded to decompose more efficiently when work_wires are not used. #6967)

• The decomposition of a single-qubit qml.QubitUnitary now includes the global phase. (#7143)

• The decompositions of qml.SX, qml.X and qml.Y use qml.GlobalPhase instead of qml.PhaseShift. (#7073)

• A new decomposition for multi-controlled global phases that uses one less controlled phase shift has been added. (#6936)

• qml.ops.sk_decomposition has been improved to produce less gates for certain edge cases. This greatly impacts the performance of qml.clifford_t_decomposition, which should now give less extraneous qml.T gates. (#6855)

• qml.MPSPrep now has a gate decomposition. This enables its use with any device. Additionally, the right_canonicalize_mps function has also been added to transform an MPS into its right-canonical form. (#6896)

• The qml.clifford_t_decomposition has been improved to use less gates when decomposing qml.PhaseShift. (#6842)

• An empty basis set in qml.compile is now recognized as valid, resulting in decomposition of all operators that can be decomposed. (#6821)

• The assert_valid method now validates that an operator's decomposition does not contain the operator itself, instead of checking that it does not contain any operators of the same class as the operator. (#7099)

Improved drawing

• qml.draw_mpl can now split deep circuits over multiple figures via a max_length keyword argument. (#7128)

• qml.draw now re-displays wire labels at the start of each partitioned chunk when using the max_length keyword argument. (#7250)

• qml.draw and qml.draw_mpl can now reuse lines for different classical wires, saving whitespace without changing the represented circuit. (#7163)

• qml.PrepSelPrep now has a concise representation when drawn with qml.draw or qml.draw_mpl. (#7164)

Device improvements

• Devices can now configure whether or not ML framework data is sent to them via an ExecutionConfig.convert_to_numpy parameter. End-to-end jitting on default.qubit is used if the user specified a jax.random.PRNGKey as a seed. (#6899) (#6788) (#6869)

• The reference.qubit device now enforces sum(probs)==1 in sample_state. (#7076)

• The default.mixed device now adheres to the newer device API introduced in v0.33. This means that default.mixed now supports not having to specify the number of wires, more predictable behaviour with interfaces, support for qml.Snapshot, and more. (#6684)

• null.qubit can now execute jaxpr. (#6924)

Experimental FTQC module

• A template class, qml.ftqc.GraphStatePrep, has been added for the Graph state construction. (#6985) (#7092)

• A new utility module qml.ftqc.utils is provided, with support for functionality such as dynamic qubit recycling. (#7075)

• A new class, qml.ftqc.QubitGraph, is now available for representing a qubit memory-addressing model for mappings between logical and physical qubits. This representation allows for nesting of lower-level qubits with arbitrary depth to allow easy insertion of arbitrarily many levels of abstractions between logical qubits and physical qubits. (#6962)

• A Lattice class and a generate_lattice method has been added to the qml.ftqc module. The generate_lattice method is to generate 1D, 2D, 3D grid graphs with the given geometric parameters. (#6958)

• Measurement functions measure_x, measure_y and measure_arbitrary_basis are added in the experimental ftqc module. These functions apply a mid-circuit measurement and return a MeasurementValue. They are analogous to qml.measure for the computational basis, but instead measure in the X-basis, Y-basis, or an arbitrary basis, respectively. Function qml.ftqc.measure_z is also added as an alias for qml.measure. (#6953)

• The function cond_measure has been added to the experimental ftqc module to apply a mid-circuit measurement with a measurement basis conditional on the function input. (#7037)

• A ParametrizedMidMeasure class has been added to represent a mid-circuit measurement in an arbitrary measurement basis in the XY, YZ or ZX plane. Subclasses XMidMeasureMP and YMidMeasureMP represent X-basis and Y-basis measurements. These classes are part of the experimental ftqc module. (#6938) (#6953)

• A diagonalize_mcms transform has been added that diagonalizes any ParametrizedMidMeasure, for devices that only natively support mid-circuit measurements in the computational basis. (#6938) (#7037)

Program capture and dynamic shapes

• Workflows that create dynamically shaped arrays via jnp.ones, jnp.zeros, jnp.arange, and jnp.full can now be captured. #6865)

• Workflows wherein the sizes of dynamically shaped arrays are updated in a qml.while_loop or qml.for_loop are now capturable. (#7084) (#7098)

• Workflows containing qml.cond instances that return arrays with dynamic shapes can now be captured. (#6888) (#7080)

• A qml.capture.pause() context manager has been added for pausing program capture in an error-safe way. (#6911)

• The requested diff_method is now validated when program capture is enabled. (#6852)

• A qml.capture.register_custom_staging_rule has been added for handling higher-order primitives that return new dynamically shaped arrays. (#7086)

• Support has been improved for when wires are specified as jax.numpy.ndarray if program capture is enabled. (#7108)

• qml.cond, qml.adjoint, qml.ctrl, and QNodes can now handle accepting dynamically shaped arrays with the abstract shape matching another argument. (#7059)

• A new qml.capture.eval_jaxpr function has been implemented. This is a variant of jax.core.eval_jaxpr that can handle the creation of arrays with dynamic shapes. (#7052)

• A new, experimental Operator method called compute_qfunc_decomposition has been added to represent decompositions with structure (e.g., control flow). This method is only used when capture is enabled with qml.capture.enable(). (#6859) (#6881) (#7022) (#6917) (#7081)

• The higher order primitives in program capture can now accept inputs with abstract shapes. (#6786)

• Execution interpreters and qml.capture.eval_jaxpr can now handle jax pjit primitives when dynamic shapes are being used. (#7078) (#7117)

• Device preprocessing is now being performed in the execution pipeline for program capture. (#7057) (#7089) (#7131) (#7135)

Other improvements

• The poly_to_angles function has been improved to correctly work with different interfaces and no longer manipulate the input angles tensor internally. (#6979)

• Dynamic one-shot workloads are now faster for null.qubit by removing a redundant functools.lru_cache call that was capturing all SampleMP objects in a workload. (#7077)

• The coefficients of observables now have improved differentiability. (#6598)

• An informative error is now raised when a QNode with diff_method=None is differentiated. (#6770)

• qml.gradients.finite_diff_jvp has been added to compute the jvp of an arbitrary numeric function. (#6853)

• The qchem functions that accept a string input have been updated to consistently work with both lower-case and upper-case inputs. (#7186)

• PSWAP.matrix() and PSWAP.eigvals() now support parameter broadcasting. (#7179) (#7228)

• Device.eval_jaxpr now accepts an execution_config keyword argument. (#6991)

• merge_rotations now correctly simplifies merged qml.Rot operators whose angles yield the identity operator. (#7011)

• The qml.measurements.NullMeasurement measurement process has been added to allow for profiling problems without the overheads associated with performing measurements. (#6989)

• The pauli_rep property is now accessible for Adjoint operators when there is a Pauli representation. (#6871)

• qml.pauli.PauliVSpace is now iterable. (#7054)

• qml.qchem.taper now handles wire ordering for the tapered observables more robustly. (#6954)

• A RuntimeWarning is now raised by qml.QNode and qml.execute if executing JAX workflows and the installed version of JAX is greater than 0.4.28. (#6864)

• The rng_salt version has been bumped to v0.40.0. (#6854)

Labs: a place for unified and rapid prototyping of research software 🧪

• pennylane.labs.dla.lie_closure_dense has been removed and integrated into qml.lie_closure using the new dense keyword. (#6811)

• pennylane.labs.dla.structure_constants_dense has been removed and integrated into qml.structure_constants using the new matrix keyword. (#6861)

• ResourceOperator.resource_params has been changed to a property. (#6973)

• ResourceOperator implementations for the ModExp, PhaseAdder, Multiplier, ControlledSequence, AmplitudeAmplification, QROM, SuperPosition, MottonenStatePreparation, StatePrep, BasisState templates have been added. (#6638)

• pennylane.labs.khaneja_glaser_involution has been removed, pennylane.labs.check_commutation has moved to qml.liealg.check_commutation_relation. pennylane.labs.check_cartan_decomp has moved to qml.liealg.check_cartan_decomp. All involution functions have been moved to qml.liealg. pennylane.labs.adjvec_to_op has moved to qml.liealg.adjvec_to_op. pennylane.labs.op_to_adjvec has moved to qml.liealg.op_to_adjvec. pennylane.labs.change_basis_ad_rep has moved to qml.liealg.change_basis_ad_rep. pennylane.labs.cartan_subalgebra has moved to qml.liealg.horizontal_cartan_subalgebra. (#7026) (#7054)

• New classes called HOState and VibronicHO have been added for representing harmonic oscillator states. (#7035)

• Base classes for Trotter error estimation on Realspace Hamiltonians have been added: RealspaceOperator, RealspaceSum, RealspaceCoeffs, and RealspaceMatrix (#7034)

• Functions for Trotter error estimation and Hamiltonian fragment generation have been added: trotter_error, perturbation_error, vibrational_fragments, vibronic_fragments, and generic_fragments. (#7036)

  As an example we compute the perturbation error of a vibrational Hamiltonian. First we generate random harmonic frequencies and Taylor coefficients to initialize the vibrational Hamiltonian.

  >>> from pennylane.labs.trotter_error import HOState, vibrational_fragments, perturbation_error
  >>> import numpy as np
  >>> n_modes = 2
  >>> r_state = np.random.RandomState(42)
  >>> freqs = r_state.random(n_modes)
  >>> taylor_coeffs = [
  >>>     np.array(0),
  >>>     r_state.random(size=(n_modes, )),
  >>>     r_state.random(size=(n_modes, n_modes)),
  >>>     r_state.random(size=(n_modes, n_modes, n_modes))
  >>> ]

  We call vibrational_fragments to get the harmonic and anharmonic fragments of the vibrational Hamiltonian.

  >>> frags = vibrational_fragments(n_modes, freqs, taylor_coeffs)

  We build state vectors in the harmonic oscillator basis with the HOState class.

  >>> gridpoints = 5
  >>> state1 = HOState(n_modes, gridpoints, {(0, 0): 1})
  >>> state2 = HOState(n_modes, gridpoints, {(1, 1): 1})

  Finally, we compute the error by calling perturbation_error.

  >>> perturbation_error(frags, [state1, state2])
  [(-0.9189251160920879+0j), (-4.797716682426851+0j)]

• Function qml.labs.trotter_error.vibronic_fragments now returns RealspaceMatrix objects with the correct number of electronic states. (#7251)

Breaking changes 💔

• Executing qml.specs is now much more efficient with the removal of accessing num_diagonalizing_gates. The calculation of this quantity is extremely expensive, and the definition is ambiguous for non-commuting observables. (#7047)

• qml.gradients.gradient_transform.choose_trainable_params has been renamed to choose_trainable_param_indices to better reflect what it actually does. (#6928)

• qml.MultiControlledX no longer accepts strings as control values. (#6835)

• The control_wires argument in qml.MultiControlledX has been removed. (#6832) (#6862)

• qml.execute now has a collection of keyword-only arguments. (#6598)

• The decomp_depth argument in qml.transforms.set_decomposition has been removed. (#6824)

• The max_expansion argument in qml.devices.preprocess.decompose has been removed. (#6824)

• The tape and qtape properties of QNode have been removed. Instead, use the qml.workflow.construct_tape function. (#6825)

• The gradient_fn keyword argument in qml.execute has been removed. Instead, it has been replaced with diff_method. (#6830)

• The QNode.get_best_method and QNode.best_method_str methods have been removed. Instead, use the qml.workflow.get_best_diff_method function. (#6823)

• The output_dim property of qml.tape.QuantumScript has been removed. Instead, use method shape of QuantumScript or MeasurementProcess to get the same information. (#6829)

• The qsvt_legacy method, along with its private helper _qsp_to_qsvt, has been removed. (#6827)

Deprecations 👋

• The qml.qnn.KerasLayer class has been deprecated because Keras 2 is no longer actively maintained. Please consider using a different machine learning framework instead of TensorFlow/Keras 2, like Pytorch or JAX. (#7097)

• Specifying pipeline=None with qml.compile is now deprecated. A sequence of transforms should now always be specified. (#7004)

• qml.ControlledQubitUnitary will stop accepting qml.QubitUnitary objects as arguments as its base. Instead, use qml.ctrl to construct a controlled qml.QubitUnitary. A follow-up PR fixed accidental double-queuing when using qml.ctrl with QubitUnitary. (#6840) (#6926)

• The control_wires argument in qml.ControlledQubitUnitary has been deprecated. Instead, use the wires argument as the second positional argument. (#6839)

• The mcm_method keyword in qml.execute has been deprecated. Instead, use the mcm_method and postselect_mode arguments. (#6807)

• Specifying gradient keyword arguments as any additional keyword argument to the qnode is deprecated and will be removed in v0.42. The gradient keyword arguments should be passed to the new keyword argument gradient_kwargs via an explicit dictionary. This change will improve QNode argument validation. (#6828)

• The qml.gradients.hamiltonian_grad function has been deprecated. This gradient recipe is not required with the new operator arithmetic system. (#6849)

• The inner_transform_program and config keyword arguments in qml.execute have been deprecated. If more detailed control over the execution is required, use qml.workflow.run with these arguments instead. (#6822) (#6879)

• The property MeasurementProcess.return_type has been deprecated. If observable type checking is needed, please use direct isinstance; if other text information is needed, please use class name, or another internal temporary private member _shortname. (#6841) (#6906) (#6910)

• Pauli module level imports of lie_closure, structure_constants, and center are deprecated, as functionality has moved to the new liealg module. (#6935)

Internal changes ⚙️

• An informative error message has been added if Autograph is employed on a function that has a lambda loop condition in qml.while_loop. (#7178)

• Logic in qml.drawer.tape_text has been cleaned. (#7133)

• An intermediate caching to null.qubit zero-value generation has been added to improve memory consumption for larger workloads. (#7155)

• All use of ABC for intermediate variables has been renamed to preserve the label for the Python abstract base class abc.ABC. (#7156)

• The error message when device wires are not specified when program capture is enabled is more clear. (#7130)

• Logic in _capture_qnode.py has been cleaned. (#7115)

• The test for qml.math.quantum._denman_beavers_iterations has been improved such that tested random matrices are guaranteed positive. (#7071)

• The matrix_power dispatch for the scipy interface has been replaced with an in-place implementation. (#7055)

• Support has been added to CollectOpsandMeas for handling QNode primitives. (#6922)

• Change some scipy imports from submodules to whole module to reduce memory footprint of importing pennylane. (#7040)

• NotImplementedErrors have been added for grad and jacobian in CollectOpsandMeas. (#7041)

• Quantum transform interpreters now perform argument validation and will no longer check if the equation in the jaxpr is a transform primitive. (#7023)

• qml.for_loop and qml.while_loop have been moved from the compiler module to a new control_flow module. (#7017)

• qml.capture.run_autograph is now idempotent. This means run_autograph(fn) = run_autograph(run_autograph(fn)). (#7001)

• Minor changes to DQInterpreter have been made for speedups with program capture execution. (#6984)

• no-member pylint issues from JAX are now globally silenced (#6987)

• pylint=3.3.4 errors in our source code have been fixed. (#6980) (#6988)

• QNode.get_gradient_fn has been removed from the source code. (#6898)

• The source code has been updated use black==25.1.0. (#6897)

• The InterfaceEnum object has been improved to prevent direct comparisons to str objects. (#6877)

• A QmlPrimitive class has been added that inherits jax.core.Primitive to a new qml.capture.custom_primitives module. This class contains a prim_type property so that we can differentiate between different sets of PennyLane primitives. Consequently, QmlPrimitive is now used to define all PennyLane primitives. (#6847)

• The RiemannianGradientOptimizer has been updated to take advantage of newer features. (#6882)

• The keep_intermediate=True flag is now used to keep Catalyst's IR when testing. (#6990)

Documentation 📝

• A page on sharp bits and debugging tips has been added for PennyLane program capture: :doc:/news/program_capture_sharp_bits. This page is recommended to consult any time errors occur when qml.capture.enable() is present. (#7062)

• The :doc:Compiling Circuits page <../introduction/compiling_circuits> has been updated to include information on using the new experimental decompositions system. (#7066)

• The docstring for qml.transforms.decompose now recommends the qml.clifford_t_decomposition transform when decomposing to the Clifford + T gate set. (#7177)

• Typos were fixed in the docstring for qml.QubitUnitary. (#7187)

• The docstring for qml.prod has been updated to explain that the order of the output may seem reversed, but it is correct. (#7083)

• The docstring for qml.labs.trotter_error has been updated. (#7190)

• The gates, qubits and lamb attributes of DoubleFactorization and FirstQuantization have dedicated documentation. (#7173)

• The code example in the docstring for qml.PauliSentence now properly copy-pastes. (#6949)

• The docstrings for qml.unary_mapping, qml.binary_mapping, qml.christiansen_mapping, qml.qchem.localize_normal_modes, and qml.qchem.VibrationalPES have been updated to include better code examples. (#6717)

• The docstrings for qml.qchem.localize_normal_modes and qml.qchem.VibrationalPES have been updated to include examples that can be copied. (#6834)

• A typo has been fixed in the code example for qml.labs.dla.lie_closure_dense. (#6858)

• The code example in the docstring for qml.BasisRotation was corrected by including wire_order in the call to qml.matrix. (#6891)

• The docstring of qml.noise.meas_eq has been updated to make its functionality clearer. (#6920)

• The docstring for qml.devices.default_tensor.DefaultTensor has been updated to clarify differentiation support. (#7150)

• The docstring for QuantumScripts has been updated to remove outdated references to set_parameters. (#7174)

• The documentation for qml.device has been updated to include null.qubit in the list of readily available devices. (#7233)

Bug fixes 🐛

• Using transforms with program capture enabled now works correctly with functions that accept pytree arguments. (#7233)

• qml.math.requires_grad no longer returns True for JAX inputs other than jax.interpreters.ad.JVPTracer instances. (#7233)

• PennyLane is now compatible with pyzx 0.9. (#7188)

• Fixed a bug when qml.matrix is applied on a sparse operator, which caused the output to have unnecessary epsilon inaccuracy. (#7147) (#7182)

• Reverted (#6933) to remove non-negligible performance impact due to wire flattening. (#7136)

• Fixed a bug that caused the output of qml.fourier.qnode_spectrum() to differ depending if equivalent gate generators are defined using different PennyLane operators. This was resolved by updating qml.operation.gen_is_multi_term_hamiltonian to work with more complicated generators. (#7121)

• Modulo operator calls on MCMs now correctly offload to the autoray-backed qml.math.mod dispatch. (#7085)

• qml.transforms.single_qubit_fusion and qml.transforms.cancel_inverses now correctly handle mid-circuit measurements when program capture is enabled. (#7020)

• qml.math.get_interface now correctly extracts the "scipy" interface if provided a list/array of sparse matrices. (#7015)

• qml.ops.Controlled.has_sparse_matrix now provides the correct information by checking if the target operator has a sparse or dense matrix defined. (#7025)

• qml.capture.PlxprInterpreter now flattens pytree arguments before evaluation. (#6975)

• qml.GlobalPhase.sparse_matrix now correctly returns a sparse matrix of the same shape as matrix. (#6940)

• qml.expval no longer silently casts to a real number when observable coefficients are imaginary. (#6939)

• Fixed qml.wires.Wires initialization to disallow Wires objects as wires labels. Now, Wires is idempotent, e.g. Wires([Wires([0]), Wires([1])])==Wires([0, 1]). (#6933)

• qml.capture.PlxprInterpreter now correctly handles propagation of constants when interpreting higher-order primitives. (#6913)

• qml.capture.PlxprInterpreter now uses Primitive.get_bind_params to resolve primitive calling signatures before binding primitives. (#6913)

• The interface is now detected from the data in the circuit, not the arguments to the QNode. This allows interface data to be strictly passed as closure variables and still be detected. (#6892)

• qml.BasisState now casts its input to integers. (#6844)

• The workflow.contstruct_batch and workflow.construct_tape functions now correctly reflect the mcm_method passed to the QNode, instead of assuming the method is always deferred. (#6903)

• Applying mid-circuit measurements inside qml.cond is not supported, and previously resulted in unclear error messages or incorrect results. It is now explicitly not allowed, and raises an error when calling the function returned by qml.cond. (#7027) (#7051)

• qml.qchem.givens_decomposition no longer raises a RuntimeWarning when the input is a zero matrix. #7053)

• Comparing an adjoint of an Observable with another Operation using qml.equal no longer incorrectly skips the check ensuring that the operator types match. (#7107)

• Downloading specific attributes of datasets in the 'other' category via qml.data.load no longer fails. (#7144)

Contributors ✍️

This release contains contributions from (in alphabetical order):

Guillermo Alonso, Daniela Angulo, Ali Asadi, Utkarsh Azad, Astral Cai, Joey Carter, Henry Chang, Yushao Chen, Isaac De Vlugt, Diksha Dhawan, Lillian M.A. Frederiksen, Pietropaolo Frisoni, Marcus Gisslén, Diego Guala, Austin Huang, Soran Jahangiri, Korbinian Kottmann, Christina Lee, Joseph Lee, Dantong Li, William Maxwell, Anton Naim Ibrahim, Lee J. O'Riordan, Mudit Pandey, Vyom Patel, Andrija Paurevic, Justin Pickering, Alex Preciado, Shuli Shu, David Wierichs