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Release 0.25.0

New features since last release

Estimate computational resource requirements 🧠

  • Functionality for estimating molecular simulation computations has been added with qml.resource. (#2646) (#2653) (#2665) (#2694) (#2720) (#2723) (#2746) (#2796) (#2797) (#2874) (#2944) (#2644)

    The new resource module allows you to estimate the number of non-Clifford gates and logical qubits needed to implement quantum phase estimation algorithms for simulating materials and molecules. This includes support for quantum algorithms using first and second quantization with specific bases:

    • First quantization using a plane-wave basis via the FirstQuantization class:

      >>> n = 100000        # number of plane waves
>>> eta = 156         # number of electrons
>>> omega = 1145.166  # unit cell volume in atomic units
>>> algo = FirstQuantization(n, eta, omega)
>>> print(algo.gates, algo.qubits)
1.10e+13, 4416

    • Second quantization with a double-factorized Hamiltonian via the DoubleFactorization class:

      symbols = ["O", "H", "H"]
geometry = np.array(
    [
        [0.00000000, 0.00000000, 0.28377432],
        [0.00000000, 1.45278171, -1.00662237],
        [0.00000000, -1.45278171, -1.00662237],
    ],
    requires_grad=False,
)

mol = qml.qchem.Molecule(symbols, geometry, basis_name="sto-3g")
core, one, two = qml.qchem.electron_integrals(mol)()

algo = DoubleFactorization(one, two)

      >>> print(algo.gates, algo.qubits)
103969925, 290

    The methods of the FirstQuantization and the DoubleFactorization classes, such as qubit_cost (number of logical qubits) and gate_cost (number of non-Clifford gates), can be also accessed as static methods:

    >>> qml.resource.FirstQuantization.qubit_cost(100000, 156, 169.69608, 0.01)
4377
>>> qml.resource.FirstQuantization.gate_cost(100000, 156, 169.69608, 0.01)
3676557345574

Differentiable error mitigation ⚙️

  • Differentiable zero-noise-extrapolation (ZNE) error mitigation is now available. (#2757)

    Elevate any variational quantum algorithm to a mitigated algorithm with improved results on noisy hardware while maintaining differentiability throughout.

    In order to do so, use the qml.transforms.mitigate_with_zne transform on your QNode and provide the PennyLane proprietary qml.transforms.fold_global folding function and qml.transforms.poly_extrapolate extrapolation function. Here is an example for a noisy simulation device where we mitigate a QNode and are still able to compute the gradient:

    # Describe noise
noise_gate = qml.DepolarizingChannel
noise_strength = 0.1

# Load devices
dev_ideal = qml.device("default.mixed", wires=1)
dev_noisy = qml.transforms.insert(noise_gate, noise_strength)(dev_ideal)

scale_factors = [1, 2, 3]
@mitigate_with_zne(
  scale_factors,
  qml.transforms.fold_global,
  qml.transforms.poly_extrapolate,
  extrapolate_kwargs={'order': 2}
)
@qml.qnode(dev_noisy)
def qnode_mitigated(theta):
    qml.RY(theta, wires=0)
    return qml.expval(qml.PauliX(0))

    >>> theta = np.array(0.5, requires_grad=True)
>>> qml.grad(qnode_mitigated)(theta)
0.5712737447327619

More native support for parameter broadcasting 📡

  • default.qubit now natively supports parameter broadcasting, providing increased performance when executing the same circuit at various parameter positions compared to manually looping over parameters, or directly using the qml.transforms.broadcast_expand transform. (#2627)

    dev = qml.device("default.qubit", wires=1)

@qml.qnode(dev)
def circuit(x):
    qml.RX(x, wires=0)
    return qml.expval(qml.PauliZ(0))

    >>> circuit(np.array([0.1, 0.3, 0.2]))
tensor([0.99500417, 0.95533649, 0.98006658], requires_grad=True)

    Currently, not all templates have been updated to support broadcasting.

  • Parameter-shift gradients now allow for parameter broadcasting internally, which can result in a significant speedup when computing gradients of circuits with many parameters. (#2749)

    The gradient transform qml.gradients.param_shift now accepts the keyword argument broadcast. If set to True, broadcasting is used to compute the derivative:

    dev = qml.device("default.qubit", wires=2)

@qml.qnode(dev)
def circuit(x, y):
    qml.RX(x, wires=0)
    qml.RY(y, wires=1)
    return qml.expval(qml.PauliZ(0) @ qml.PauliZ(1))

    >>> x = np.array([np.pi/3, np.pi/2], requires_grad=True)
>>> y = np.array([np.pi/6, np.pi/5], requires_grad=True)
>>> qml.gradients.param_shift(circuit, broadcast=True)(x, y)
(tensor([[-0.7795085,  0.       ],
         [ 0.       , -0.7795085]], requires_grad=True),
tensor([[-0.125, 0.  ],
        [0.  , -0.125]], requires_grad=True))

    The following example highlights how to make use of broadcasting gradients at the QNode level. Internally, broadcasting is used to compute the parameter-shift rule when required, which may result in performance improvements.

    @qml.qnode(dev, diff_method="parameter-shift", broadcast=True)
def circuit(x, y):
    qml.RX(x, wires=0)
    qml.RY(y, wires=1)
    return qml.expval(qml.PauliZ(0) @ qml.PauliZ(1))

    >>> x = np.array(0.1, requires_grad=True)
>>> y = np.array(0.4, requires_grad=True)
>>> qml.grad(circuit)(x, y)
(array(-0.09195267), array(-0.38747287))

    Here, only 2 circuits are created internally, rather than 4 with broadcast=False.

    To illustrate the speedup, for a constant-depth circuit with Pauli rotations and controlled Pauli rotations, the time required to compute qml.gradients.param_shift(circuit, broadcast=False)(params) (“No broadcasting”) and qml.gradients.param_shift(circuit, broadcast=True)(params) (“Broadcasting”) as a function of the number of qubits is given here.

  • Operations for quantum chemistry now support parameter broadcasting. (#2726)

    >>> op = qml.SingleExcitation(np.array([0.3, 1.2, -0.7]), wires=[0, 1])
>>> op.matrix().shape
(3, 4, 4)

Intuitive operator arithmetic 🧮

  • New functionality for representing the sum, product, and scalar-product of operators is available. (#2475) (#2625) (#2622) (#2721)

    The following functionalities have been added to facilitate creating new operators whose matrix, terms, and eigenvalues can be accessed as per usual, while maintaining differentiability. Operators created from these new features can be used within QNodes as operations or as observables (where physically applicable).

    • Summing any number of operators via qml.op_sum results in a “summed” operator:

      >>> ops_to_sum = [qml.PauliX(0), qml.PauliY(1), qml.PauliZ(0)]
>>> summed_ops = qml.op_sum(*ops_to_sum)
>>> summed_ops
PauliX(wires=[0]) + PauliY(wires=[1]) + PauliZ(wires=[0])
>>> qml.matrix(summed_ops)
array([[ 1.+0.j,  0.-1.j,  1.+0.j,  0.+0.j],
       [ 0.+1.j,  1.+0.j,  0.+0.j,  1.+0.j],
       [ 1.+0.j,  0.+0.j, -1.+0.j,  0.-1.j],
       [ 0.+0.j,  1.+0.j,  0.+1.j, -1.+0.j]])
>>> summed_ops.terms()
([1.0, 1.0, 1.0], (PauliX(wires=[0]), PauliY(wires=[1]), PauliZ(wires=[0])))

    • Multiplying any number of operators via qml.prod results in a “product” operator, where the matrix product or tensor product is used correspondingly:

      >>> theta = 1.23
>>> prod_op = qml.prod(qml.PauliZ(0), qml.RX(theta, 1))
>>> prod_op
PauliZ(wires=[0]) @ RX(1.23, wires=[1])
>>> qml.eigvals(prod_op)
[-1.39373197 -0.23981492  0.23981492  1.39373197]

    • Taking the product of a coefficient and an operator via qml.s_prod produces a “scalar-product” operator:

      >>> sprod_op = qml.s_prod(2.0, qml.PauliX(0))
>>> sprod_op
2.0*(PauliX(wires=[0]))
>>> sprod_op.matrix()
array([[ 0., 2.],
       [ 2., 0.]])
>>> sprod_op.terms()
([2.0], [PauliX(wires=[0])])

    Each of these new functionalities can be used within QNodes as operators or observables, where applicable, while also maintaining differentiability. For example:

    dev = qml.device("default.qubit", wires=2)

@qml.qnode(dev)
def circuit(angles):
    qml.prod(qml.PauliZ(0), qml.RY(angles[0], 1))
    qml.op_sum(qml.PauliX(1), qml.RY(angles[1], 0))

    return qml.expval(qml.op_sum(qml.PauliX(0), qml.PauliZ(1)))

    >>> angles = np.array([1.23, 4.56], requires_grad=True)
>>> circuit(angles)
tensor(0.33423773, requires_grad=True)
>>> qml.grad(circuit)(angles)
array([-0.9424888,  0.       ])

  • All PennyLane operators can now be added, subtracted, multiplied, scaled, and raised to powers using +, -, @, *, **, respectively. (#2849) (#2825) (#2891)

    • You can now add scalars to operators, where the interpretation is that the scalar is a properly-sized identity matrix;

      >>> sum_op = 5 + qml.PauliX(0)
>>> sum_op.matrix()
array([[5., 1.],
       [1., 5.]])

    • The + and - operators can be used to combine all Pennylane operators:

      >>> sum_op = qml.RX(phi=1.23, wires=0) + qml.RZ(phi=3.14, wires=0) - qml.RY(phi=0.12, wires=0)
>>> sum_op
RX(1.23, wires=[0]) + RZ(3.14, wires=[0]) + -1*(RY(0.12, wires=[0]))
>>> qml.matrix(sum_op)
array([[-0.18063077-0.99999968j,  0.05996401-0.57695852j],
       [-0.05996401-0.57695852j, -0.18063077+0.99999968j]])

      Note that the behavior of + and - with observables is different; it still creates a Hamiltonian.

    • The * and @ operators can be used to scale and compose all PennyLane operators.

      >>> prod_op = 2*qml.RX(1, wires=0) @ qml.RY(2, wires=0)
>>> prod_op
2*(RX(1, wires=[0])) @ RY(2, wires=[0])
>>> qml.matrix(prod_op)
array([[ 0.94831976-0.80684536j, -1.47692053-0.51806945j],
       [ 1.47692053-0.51806945j,  0.94831976+0.80684536j]])

    • The ** operator can be used to raise PennyLane operators to a power.

      >>> exp_op = qml.RZ(1.0, wires=0) ** 2
>>> exp_op
RZ**2(1.0, wires=[0])
>>> qml.matrix(exp_op)
array([[0.54030231-0.84147098j, 0.        +0.j        ],
       [0.        +0.j        , 0.54030231+0.84147098j]])

  • A new class called Controlled is available in qml.ops.op_math to represent a controlled version of any operator. This will eventually be integrated into qml.ctrl to provide a performance increase and more feature coverage. (#2634)

  • Arithmetic operations can now be simplified using qml.simplify. (#2835) (#2854)

    >>> op = qml.adjoint(qml.adjoint(qml.RX(x, wires=0)))
>>> op
Adjoint(Adjoint(RX))(tensor([1.04719755, 1.57079633], requires_grad=True), wires=[0])
>>> qml.simplify(op)
RX(tensor([1.04719755, 1.57079633], requires_grad=True), wires=[0])

  • A new function called qml.equal can be used to compare the equality of parametric operators. (#2651)

    >>> qml.equal(qml.RX(1.23, 0), qml.RX(1.23, 0))
True
>>> qml.equal(qml.RY(4.56, 0), qml.RY(7.89, 0))
False

Marvelous mixed state features 🙌

  • The default.mixed device now supports backpropagation with the "jax" interface, which can result in significant speedups. (#2754) (#2776)

    dev = qml.device("default.mixed", wires=2)

@qml.qnode(dev, diff_method="backprop", interface="jax")
def circuit(angles):
    qml.RX(angles[0], wires=0)
    qml.RY(angles[1], wires=1)
    return qml.expval(qml.PauliZ(0) + qml.PauliZ(1))

    >>> angles = np.array([np.pi/6, np.pi/5], requires_grad=True)
>>> qml.grad(circuit)(angles)
array([-0.8660254 , -0.25881905])

    Additionally, quantum channels now support Jax and TensorFlow tensors. This allows quantum channels to be used inside QNodes decorated by tf.function, jax.jit, or jax.vmap.

  • The default.mixed device now supports readout error. (#2786)

    A new keyword argument called readout_prob can be specified when creating a default.mixed device. Any circuits running on a default.mixed device with a finite readout_prob (upper-bounded by 1) will alter the measurements performed at the end of the circuit similarly to how a qml.BitFlip channel would affect circuit measurements:

    >>> dev = qml.device("default.mixed", wires=2, readout_prob=0.1)
>>> @qml.qnode(dev)
... def circuit():
...     return qml.expval(qml.PauliZ(0))
>>> circuit()
array(0.8)

Relative entropy is now available in qml.qinfo 💥

  • The quantum information module now supports computation of relative entropy. (#2772)

    We’ve enabled two cases for calculating the relative entropy:

    • A QNode transform via qml.qinfo.relative_entropy:

      dev = qml.device('default.qubit', wires=2)

@qml.qnode(dev)
def circuit(param):
    qml.RY(param, wires=0)
    qml.CNOT(wires=[0, 1])
    return qml.state()

      >>> relative_entropy_circuit = qml.qinfo.relative_entropy(circuit, circuit, wires0=[0], wires1=[0])
>>> x, y = np.array(0.4), np.array(0.6)
>>> relative_entropy_circuit((x,), (y,))
0.017750012490703237

    • Support in qml.math for flexible post-processing:

      >>> rho = np.array([[0.3, 0], [0, 0.7]])
>>> sigma = np.array([[0.5, 0], [0, 0.5]])
>>> qml.math.relative_entropy(rho, sigma)
tensor(0.08228288, requires_grad=True)

New measurements, operators, and more! ✨

  • A new measurement called qml.counts is available. (#2686) (#2839) (#2876)

    QNodes with shots != None that return qml.counts will yield a dictionary whose keys are bitstrings representing computational basis states that were measured, and whose values are the corresponding counts (i.e., how many times that computational basis state was measured):

    dev = qml.device("default.qubit", wires=2, shots=1000)

@qml.qnode(dev)
def circuit():
    qml.Hadamard(wires=0)
    qml.CNOT(wires=[0, 1])
    return qml.counts()

    >>> circuit()
{'00': 495, '11': 505}

    qml.counts can also accept observables, where the resulting dictionary is ordered by the eigenvalues of the observable.

    dev = qml.device("default.qubit", wires=2, shots=1000)

@qml.qnode(dev)
def circuit():
    qml.Hadamard(wires=0)
    qml.CNOT(wires=[0, 1])
    return qml.counts(qml.PauliZ(0)), qml.counts(qml.PauliZ(1))

    >>> circuit()
({-1: 470, 1: 530}, {-1: 470, 1: 530})

  • A new experimental return type for QNodes with multiple measurements has been added. (#2814) (#2815) (#2860)

    QNodes returning a list or tuple of different measurements return an intuitive data structure via qml.enable_return(), where the individual measurements are separated into their own tensors:

    qml.enable_return()
dev = qml.device("default.qubit", wires=2)

@qml.qnode(dev)
def circuit(x):
    qml.Hadamard(wires=[0])
    qml.CRX(x, wires=[0, 1])
    return (qml.probs(wires=[0]), qml.vn_entropy(wires=[0]), qml.probs(wires=0), qml.expval(wires=1))

    >>> circuit(0.5)
(tensor([0.5, 0.5], requires_grad=True), tensor(0.08014815, requires_grad=True), tensor([0.5, 0.5], requires_grad=True), tensor(0.93879128, requires_grad=True))

    In addition, QNodes that utilize this new return type support backpropagation. This new return type can be disabled thereafter via qml.disable_return().

  • An operator called qml.FlipSign is now available. (#2780)

    Mathematically, qml.FlipSign functions as follows: \(\text{FlipSign}(n) \vert m \rangle = (-1)^\delta_{n,m} \vert m \rangle\), where \(\vert m \rangle\) is an arbitrary qubit state and $n$ is a qubit configuration:

    basis_state = [0, 1]

dev = qml.device("default.qubit", wires=2)

@qml.qnode(dev)
def circuit():
  for wire in list(range(2)):
        qml.Hadamard(wires = wire)
  qml.FlipSign(basis_state, wires = list(range(2)))
  return qml.state()

    >>> circuit()
tensor([ 0.5+0.j  -0.5+0.j 0.5+0.j  0.5+0.j], requires_grad=True)

  • The simultaneous perturbation stochastic approximation (SPSA) optimizer is available via qml.SPSAOptimizer. (#2661)

    The SPSA optimizer is suitable for cost functions whose evaluation may involve noise. Use the SPSA optimizer like you would any other optimizer:

    max_iterations = 50
opt = qml.SPSAOptimizer(maxiter=max_iterations)

for _ in range(max_iterations):
    params, cost = opt.step_and_cost(cost, params)

More drawing styles 🎨

  • New PennyLane-inspired sketch and sketch_dark styles are now available for drawing circuit diagram graphics. (#2709)

Improvements 📈

  • default.qubit now natively executes any operation that defines a matrix except for trainable Pow operations. (#2836)

  • Added expm to the qml.math module for matrix exponentiation. (#2890)

  • When adjoint differentiation is requested, circuits are now decomposed so that all trainable operations have a generator. (#2836)

  • A warning is now emitted for qml.state, qml.density_matrix, qml.vn_entropy, and qml.mutual_info when using a device with finite shots or a shot list since these measurements are always analytic. (#2918)

  • The efficiency of the Hartree-Fock workflow has been improved by removing repetitive steps. (#2850)

  • The coefficients of the non-differentiable molecular Hamiltonians generated with openfermion now have requires_grad = False by default. (#2865)

  • Upgraded performance of the compute_matrix method of broadcastable parametric operations. (#2759)

  • Jacobians are now cached with the Autograd interface when using the parameter-shift rule. (#2645)

  • The qml.state and qml.density_matrix measurements now support custom wire labels. (#2779)

  • Add trivial behaviour logic to qml.operation.expand_matrix. (#2785)

  • Added an are_pauli_words_qwc function which checks if certain Pauli words are pairwise qubit-wise commuting. This new function improves performance when measuring hamiltonians with many commuting terms. (#2789)

  • Adjoint differentiation now uses the adjoint symbolic wrapper instead of in-place inversion. (#2855)

Breaking changes 💔

  • The deprecated qml.hf module is removed. Users with code that calls qml.hf can simply replace qml.hf with qml.qchem in most cases, or refer to the qchem documentation and demos for more information. (#2795)

  • default.qubit now uses stopping_condition to specify support for anything with a matrix. To override this behavior in inheriting devices and to support only a specific subset of operations, developers need to override stopping_condition. (#2836)

  • Custom devices inheriting from DefaultQubit or QubitDevice can break due to the introduction of parameter broadcasting. (#2627)

    A custom device should only break if all three following statements hold simultaneously:

    1. The custom device inherits from DefaultQubit, not QubitDevice.

    2. The device implements custom methods in the simulation pipeline that are incompatible with broadcasting (for example expval, apply_operation or analytic_probability).

    3. The custom device maintains the flag "supports_broadcasting": True in its capabilities dictionary or it overwrites Device.batch_transform without applying broadcast_expand (or both).

    The capabilities["supports_broadcasting"] is set to True for DefaultQubit. Typically, the easiest fix will be to change the capabilities["supports_broadcasting"] flag to False for the child device and/or to include a call to broadcast_expand in CustomDevice.batch_transform, similar to how Device.batch_transform calls it.

    Separately from the above, custom devices that inherit from QubitDevice and implement a custom _gather method need to allow for the kwarg axis to be passed to this _gather method.

  • The argument argnum of the function qml.batch_input has been redefined: now it indicates the indices of the batched parameters, which need to be non-trainable, in the quantum tape. Consequently, its default value (set to 0) has been removed. (#2873)

    Before this breaking change, one could call qml.batch_input without any arguments when using batched inputs as the first argument of the quantum circuit.

    dev = qml.device("default.qubit", wires=2, shots=None)

@qml.batch_input()  # argnum = 0
@qml.qnode(dev, diff_method="parameter-shift", interface="tf")
def circuit(inputs, weights):  # argument `inputs` is batched
    qml.RY(weights[0], wires=0)
    qml.AngleEmbedding(inputs, wires=range(2), rotation="Y")
    qml.RY(weights[1], wires=1)
    return qml.expval(qml.PauliZ(1))

    With this breaking change, users must set a value to argnum specifying the index of the batched inputs with respect to all quantum tape parameters. In this example the quantum tape parameters are [ weights[0], inputs, weights[1] ], thus argnum should be set to 1, specifying that inputs is batched:

    dev = qml.device("default.qubit", wires=2, shots=None)

@qml.batch_input(argnum=1)
@qml.qnode(dev, diff_method="parameter-shift", interface="tf")
def circuit(inputs, weights):
    qml.RY(weights[0], wires=0)
    qml.AngleEmbedding(inputs, wires=range(2), rotation="Y")
    qml.RY(weights[1], wires=1)
    return qml.expval(qml.PauliZ(1))

  • PennyLane now depends on newer versions (>=2.7) of the semantic_version package, which provides an updated API that is incompatible which versions of the package prior to 2.7. If you run into issues relating to this package, please reinstall PennyLane. (#2744) (#2767)

Documentation 📕

  • Added a dedicated docstring for the QubitDevice.sample method. (#2812)

  • Optimization examples of using JAXopt and Optax with the JAX interface have been added. (#2769)

  • Updated IsingXY gate docstring. (#2858)

Bug fixes 🐞

  • Fixes qml.equal so that operators with different inverse properties are not equal. (#2947)

  • Cleans up interactions between operator arithmetic and batching by testing supported cases and adding errors when batching is not supported. (#2900)

  • Fixed a bug where the parameter-shift rule wasn’t defined for qml.kUpCCGSD. (#2913)

  • Reworked the Hermiticity check in qml.Hermitian by using qml.math calls because calling .conj() on an EagerTensor from TensorFlow raised an error. (#2895)

  • Fixed a bug where the parameter-shift gradient breaks when using both custom grad_recipes that contain unshifted terms and recipes that do not contain any unshifted terms. (#2834)

  • Fixed mixed CPU-GPU data-locality issues for the Torch interface. (#2830)

  • Fixed a bug where the parameter-shift Hessian of circuits with untrainable parameters might be computed with respect to the wrong parameters or might raise an error. (#2822)

  • Fixed a bug where the custom implementation of the states_to_binary device method was not used. (#2809)

  • qml.grouping.group_observables now works when individual wire labels are iterable. (#2752)

  • The adjoint of an adjoint now has a correct expand result. (#2766)

  • Fixed the ability to return custom objects as the expectation value of a QNode with the Autograd interface. (#2808)

  • The WireCut operator now raises an error when instantiating it with an empty list. (#2826)

  • Hamiltonians with grouped observables are now allowed to be measured on devices which were transformed using qml.transform.insert(). (#2857)

  • Fixed a bug where qml.batch_input raised an error when using a batched operator that was not located at the beginning of the circuit. In addition, now qml.batch_input raises an error when using trainable batched inputs, which avoids an unwanted behaviour with duplicated parameters. (#2873)

  • Calling qml.equal with nested operators now raises a NotImplementedError. (#2877)

  • Fixed a bug where a non-sensible error message was raised when using qml.counts with shots=False. (#2928)

  • Fixed a bug where no error was raised and a wrong value was returned when using qml.counts with another non-commuting observable. (#2928)

  • Operator Arithmetic now allows Hamiltonian objects to be used and produces correct matrices. (#2957)

Contributors

This release contains contributions from (in alphabetical order):

Juan Miguel Arrazola, Utkarsh Azad, Samuel Banning, Prajwal Borkar, Isaac De Vlugt, Olivia Di Matteo, Kristiyan Dilov, David Ittah, Josh Izaac, Soran Jahangiri, Edward Jiang, Ankit Khandelwal, Korbinian Kottmann, Meenu Kumari, Christina Lee, Sergio Martínez-Losa, Albert Mitjans Coma, Ixchel Meza Chavez, Romain Moyard, Lee James O’Riordan, Mudit Pandey, Bogdan Reznychenko, Shuli Shu, Jay Soni, Modjtaba Shokrian-Zini, Antal Száva, David Wierichs, Moritz Willmann

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