Release Release 0.26.0 · PennyLaneAI/pennylane

New features since last release

Classical shadows 👤

PennyLane now provides built-in support for implementing the classical-shadows measurement protocol. (#2820) (#2821) (#2871) (#2968) (#2959) (#2968)

The classical-shadow measurement protocol is described in detail in the paper Predicting Many Properties of a Quantum System from Very Few Measurements. As part of the support for classical shadows in this release, two new finite-shot and fully-differentiable measurements are available:
- QNodes returning the new measurement qml.classical_shadow() will return two entities; bits (0 or 1 if the 1 or -1 eigenvalue is sampled, respectively) and recipes (the randomized Pauli measurements that are performed for each qubit, labelled by integer):
```
dev = qml.device("default.qubit", wires=2, shots=3)

@qml.qnode(dev)
def circuit():
    qml.Hadamard(wires=0)
    qml.CNOT(wires=[0, 1])
    return qml.classical_shadow(wires=[0, 1])
```
```
>>> bits, recipes = circuit()
>>> bits
tensor([[0, 0],
        [1, 0],
        [0, 1]], dtype=uint8, requires_grad=True)
>>> recipes
tensor([[2, 2],
        [0, 2],
        [0, 2]], dtype=uint8, requires_grad=True)
```
- QNodes returning qml.shadow_expval() yield the expectation value estimation using classical shadows:
```
dev = qml.device("default.qubit", wires=range(2), shots=10000)

@qml.qnode(dev)
def circuit(x, H):
    qml.Hadamard(0)
    qml.CNOT((0,1))
    qml.RX(x, wires=0)
    return qml.shadow_expval(H)

x = np.array(0.5, requires_grad=True) 
H = qml.Hamiltonian(
        [1., 1.], 
        [qml.PauliZ(0) @ qml.PauliZ(1), qml.PauliX(0) @ qml.PauliX(1)]
    )  
```
```
>>> circuit(x, H)
tensor(1.8486, requires_grad=True) 
>>> qml.grad(circuit)(x, H)
-0.4797000000000001
```
Fully-differentiable QNode transforms for both new classical-shadows measurements are also available via qml.shadows.shadow_state and qml.shadows.shadow_expval, respectively.

For convenient post-processing, we've also added the ability to calculate general Renyi entropies by way of the ClassicalShadow class' entropy method, which requires the wires of the subsystem of interest and the Renyi entropy order:
```
>>> shadow = qml.ClassicalShadow(bits, recipes)
>>> vN_entropy = shadow.entropy(wires=[0, 1], alpha=1)
```

Qutrits: quantum circuits for tertiary degrees of freedom ☘️

An entirely new framework for quantum computing is now simulatable with the addition of qutrit functionalities. (#2699) (#2781) (#2782) (#2783) (#2784) (#2841) (#2843)

Qutrits are like qubits, but instead live in a three-dimensional Hilbert space; they are not binary degrees of freedom, they are tertiary. The advent of qutrits allows for all sorts of interesting theoretical, practical, and algorithmic capabilities that have yet to be discovered.

To facilitate qutrit circuits requires a new device: default.qutrit. The default.qutrit device is a Python-based simulator, akin to default.qubit, and is defined as per usual:
```
>>> dev = qml.device("default.qutrit", wires=1)
```
The following operations are supported on default.qutrit devices:
- The qutrit shift operator, qml.TShift, and the ternary clock operator, qml.TClock, as defined in this paper by Yeh et al. (2022),
  which are the qutrit analogs of the Pauli X and Pauli Z operations, respectively.
- The qml.TAdd and qml.TSWAP operations which are the qutrit analogs of the CNOT and SWAP operations, respectively.
- Custom unitary operations via qml.QutritUnitary.
- qml.state and qml.probs measurements.
- Measuring user-specified Hermitian matrix observables via qml.THermitian.
A comprehensive example of these features is given below:
```
dev = qml.device("default.qutrit", wires=1)

U = np.array([
        [1, 1, 1], 
        [1, 1, 1], 
        [1, 1, 1]
    ]
) / np.sqrt(3) 

obs = np.array([
        [1, 1, 0], 
        [1, -1, 0], 
        [0, 0, np.sqrt(2)]
    ]
) / np.sqrt(2)

@qml.qnode(dev)
def qutrit_state(U, obs):
    qml.TShift(0)
    qml.TClock(0)
    qml.QutritUnitary(U, wires=0)
    return qml.state()

@qml.qnode(dev)
def qutrit_expval(U, obs):
    qml.TShift(0)
    qml.TClock(0)
    qml.QutritUnitary(U, wires=0)
    return qml.expval(qml.THermitian(obs, wires=0))
```
```
>>> qutrit_state(U, obs)
tensor([-0.28867513+0.5j, -0.28867513+0.5j, -0.28867513+0.5j], requires_grad=True) 
>>> qutrit_expval(U, obs)
tensor(0.80473785, requires_grad=True)
```
We will continue to add more and more support for qutrits in future releases.

Simplifying just got... simpler 😌

The qml.simplify() function has several intuitive improvements with this release. (#2978) (#2982) (#2922) (#3012)

qml.simplify can now perform the following:
- simplify parametrized operations
- simplify the adjoint and power of specific operators
- group like terms in a sum
- resolve products of Pauli operators
- combine rotation angles of identical rotation gates
Here is an example of qml.simplify in action with parameterized rotation gates. In this case, the angles of rotation are simplified to be modulo $4\pi$.
```
>>> op1 = qml.RX(30.0, wires=0)
>>> qml.simplify(op1)
RX(4.867258771281655, wires=[0])
>>> op2 = qml.RX(4 * np.pi, wires=0)
>>> qml.simplify(op2)
Identity(wires=[0])
```
All of these simplification features can be applied directly to quantum functions, QNodes, and tapes via decorating with @qml.simplify, as well:
```
dev = qml.device("default.qubit", wires=2)
@qml.simplify
@qml.qnode(dev)
def circuit():
    qml.adjoint(qml.prod(qml.RX(1, 0) ** 1, qml.RY(1, 0), qml.RZ(1, 0)))
    return qml.probs(wires=0)
```
```
>>> circuit()
>>> list(circuit.tape)
[RZ(11.566370614359172, wires=[0]) @ RY(11.566370614359172, wires=[0]) @ RX(11.566370614359172, wires=[0]),
 probs(wires=[0])]
```

QNSPSA optimizer 💪

A new optimizer called qml.QNSPSAOptimizer is available that implements the quantum natural simultaneous perturbation stochastic approximation (QNSPSA) method based on Simultaneous Perturbation Stochastic Approximation of the Quantum Fisher Information. (#2818)

qml.QNSPSAOptimizer is a second-order SPSA algorithm, which combines the convergence power of the quantum-aware Quantum Natural Gradient (QNG) optimization method with the reduced quantum evaluations of SPSA methods.

While the QNSPSA optimizer requires additional circuit executions (10 executions per step) compared to standard SPSA optimization (3 executions per step), these additional evaluations are used to provide a stochastic estimation of a second-order metric tensor, which often helps the optimizer to achieve faster convergence.

Use qml.QNSPSAOptimizer like you would any other optimizer:
```
max_iterations = 50
opt = qml.QNSPSAOptimizer() 

for _ in range(max_iterations):
    params, cost = opt.step_and_cost(cost, params)
```
Check out our demo on the QNSPSA optimizer for more information.

Operator and parameter broadcasting supplements 📈

Operator methods for exponentiation and raising to a power have been added. (#2799) (#3029)

The qml.exp function can be used to create observables or generic rotation gates:

>>> x = 1.234
>>> t = qml.PauliX(0) @ qml.PauliX(1) + qml.PauliY(0) @ qml.PauliY(1)
>>> isingxy = qml.exp(t, 0.25j * x)
>>> isingxy.matrix()
array([[1.       +0.j        , 0.       +0.j        ,
    1.       +0.j        , 0.       +0.j        ],
   [0.       +0.j        , 0.8156179+0.j        ,
    1.       +0.57859091j, 0.       +0.j        ],
   [0.       +0.j        , 0.       +0.57859091j,
    0.8156179+0.j        , 0.       +0.j        ],
   [0.       +0.j        , 0.       +0.j        ,
    1.       +0.j        , 1.       +0.j        ]])

The qml.pow function raises a given operator to a power:

>>> op = qml.pow(qml.PauliX(0), 2)
>>> op.matrix()
array([[1, 0], [0, 1]])

An operator called qml.PSWAP is now available. (#2667)

The qml.PSWAP gate -- or phase-SWAP gate -- was previously available within the PennyLane-Braket plugin only. Enjoy it natively in PennyLane with v0.26.

Check whether or not an operator is hermitian or unitary with qml.is_hermitian and qml.is_unitary. (#2960)

>>> op1 = qml.PauliX(wires=0)
>>> qml.is_hermitian(op1)
True
>>> op2 = qml.PauliX(0) + qml.RX(np.pi/3, 0) 
>>> qml.is_unitary(op2)
False

Embedding templates now support parameter broadcasting. (#2810)

Embedding templates like AmplitudeEmbedding or IQPEmbedding now support parameter broadcasting with a leading broadcasting dimension in their variational parameters. AmplitudeEmbedding, for example, would usually use a one-dimensional input vector of features. With broadcasting, we can now compute
```
>>> features = np.array([
...     [0.5, 0.5, 0., 0., 0.5, 0., 0.5, 0.],
...     [1., 0., 0., 0., 0., 0., 0., 0.],
...     [0.5, 0.5, 0., 0., 0., 0., 0.5, 0.5],
... ])
>>> op = qml.AmplitudeEmbedding(features, wires=[1, 5, 2])
>>> op.batch_size
3
```
An exception is BasisEmbedding, which is not broadcastable.

Improvements

The qml.math.expand_matrix() method now allows the sparse matrix representation of an operator to be extended to a larger hilbert space. (#2998)

>>> from scipy import sparse
>>> mat = sparse.csr_matrix([[0, 1], [1, 0]])
>>> qml.math.expand_matrix(mat, wires=[1], wire_order=[0,1]).toarray()
array([[0., 1., 0., 0.],
       [1., 0., 0., 0.],
       [0., 0., 0., 1.],
       [0., 0., 1., 0.]])

qml.ctrl now uses Controlled instead of ControlledOperation. The new Controlled class wraps individual Operator's instead of a tape. It provides improved representations and integration. (#2990)
qml.matrix can now compute the matrix of tapes and QNodes that contain multiple broadcasted operations or non-broadcasted operations after broadcasted ones. (#3025)

A common scenario in which this becomes relevant is the decomposition of broadcasted operations: the decomposition in general will contain one or multiple broadcasted operations as well as operations with no or fixed parameters that are not broadcasted.
Lists of operators are now internally sorted by their respective wires while also taking into account their commutativity property.(#2995)
Some methods of the QuantumTape class have been simplified and reordered to improve both readability and performance. (#2963)
The qml.qchem.molecular_hamiltonian function is modified to support observable grouping. (#2997)
qml.ops.op_math.Controlled now has basic decomposition functionality. (#2938)
Automatic circuit cutting has been improved by making better partition imbalance derivations. Now it is more likely to generate optimal cuts for larger circuits. (#2517)

By default, qml.counts only returns the outcomes observed in sampling. Optionally, specifying qml.counts(all_outcomes=True) will return a dictionary containing all possible outcomes. (#2889)

>>> 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(all_outcomes=True)
>>> result = circuit()
>>> result
{'00': 495, '01': 0, '10': 0,  '11': 505}

Internal use of in-place inversion is eliminated in preparation for its deprecation. (#2965)
Controlled operators now work with qml.is_commuting. (#2994)

qml.prod and qml.op_sum now support the sparse_matrix() method. (#3006)

>>> xy = qml.prod(qml.PauliX(1), qml.PauliY(1))
>>> op = qml.op_sum(xy, qml.Identity(0))
>>>
>>> sparse_mat = op.sparse_matrix(wire_order=[0,1])
>>> type(sparse_mat)
<class 'scipy.sparse.csr.csr_matrix'>
>>> sparse_mat.toarray()
[[1.+1.j 0.+0.j 0.+0.j 0.+0.j]
[0.+0.j 1.-1.j 0.+0.j 0.+0.j]
[0.+0.j 0.+0.j 1.+1.j 0.+0.j]
[0.+0.j 0.+0.j 0.+0.j 1.-1.j]]

Provided sparse_matrix() support for single qubit observables. (#2964)
qml.Barrier with only_visual=True now simplifies via op.simplify() to the identity operator or a product of identity operators.(#3016)
More accurate and intuitive outputs for printing some operators have been added. (#3013)
Results for the matrix of the sum or product of operators are stored in a more efficient manner. (#3022)
The computation of the (sparse) matrix for the sum or product of operators is now more efficient. (#3030)
When the factors of qml.prod don't share any wires, the matrix and sparse matrix are computed using a kronecker product for improved efficiency. (#3040)
qml.grouping.is_pauli_word now returns False for operators that don't inherit from qml.Observable instead of raising an error. (#3039)

Added functionality to iterate over operators created from qml.op_sum and qml.prod. (#3028)

>>> op = qml.op_sum(qml.PauliX(0), qml.PauliY(1), qml.PauliZ(2))
>>> len(op)
3
>>> op[1]
PauliY(wires=[1])
>>> [o.name for o in op]
['PauliX', 'PauliY', 'PauliZ']

Deprecations

In-place inversion is now deprecated. This includes op.inv() and op.inverse=value. Please use qml.adjoint or qml.pow instead. Support for these methods will remain till v0.28. (#2988)

Don't use:
```
>>> v1 = qml.PauliX(0).inv()
>>> v2 = qml.PauliX(0)
>>> v2.inverse = True
```
Instead use:
```
>>> qml.adjoint(qml.PauliX(0))
Adjoint(PauliX(wires=[0]))
>>> qml.pow(qml.PauliX(0), -1)
PauliX(wires=[0])**-1
>>> qml.pow(qml.PauliX(0), -1, lazy=False)
PauliX(wires=[0])
>>> qml.PauliX(0) ** -1
PauliX(wires=[0])**-1
```
qml.adjoint takes the conjugate transpose of an operator, while qml.pow(op, -1) indicates matrix inversion. For unitary operators, adjoint will be more efficient than qml.pow(op, -1), even though they represent the same thing.
The supports_reversible_diff device capability is unused and has been removed. (#2993)

Breaking changes

Measuring an operator that might not be hermitian now raises a warning instead of an error. To definitively determine whether or not an operator is hermitian, use qml.is_hermitian. (#2960)
The ControlledOperation class has been removed. This was a developer-only class, so the change should not be evident to any users. It is replaced by Controlled. (#2990)
The default execute method for the QubitDevice base class now calls self.statistics with an additional keyword argument circuit, which represents the quantum tape being executed. Any device that overrides statistics should edit the signature of the method to include the new circuit keyword argument. (#2820)
The expand_matrix() has been moved from pennylane.operation to pennylane.math.matrix_manipulation. (#3008)
qml.grouping.utils.is_commuting has been removed, and its Pauli word logic is now part of qml.is_commuting. (#3033)
qml.is_commuting has been moved from pennylane.transforms.commutation_dag to pennylane.ops.functions. (#2991)

Documentation

Updated the Fourier transform docs to use circuit_spectrum instead of spectrum, which has been deprecated. (#3018)
Corrected the docstrings for diagonalizing gates for all relevant operations. The docstrings used to say that the diagonalizing gates implemented $U$, the unitary such that $O = U \Sigma U^{\dagger}$, where $O$ is the original observable and $\Sigma$ a diagonal matrix. However, the diagonalizing gates actually implement $U^{\dagger}$, since $\langle \psi | O | \psi \rangle = \langle \psi | U \Sigma U^{\dagger} | \psi \rangle$, making $U^{\dagger} | \psi \rangle$ the actual state being measured in the Z-basis. (#2981)

Bug fixes

Fixed a bug with qml.ops.Exp operators when the coefficient is autograd but the diagonalizing gates don't act on all wires. (#3057)
Fixed a bug where the tape transform single_qubit_fusion computed wrong rotation angles for specific combinations of rotations. (#3024)
Jax gradients now work with a QNode when the quantum function was transformed by qml.simplify. (#3017)
Operators that have num_wires = AnyWires or num_wires = AnyWires now raise an error, with certain exceptions, when instantiated with wires=[]. (#2979)
Fixed a bug where printing qml.Hamiltonian with complex coefficients raises TypeError in some cases. (#3005)
Added a more descriptive error message when measuring non-commuting observables at the end of a circuit with probs, samples, counts and allcounts. (#3065)

Contributors

This release contains contributions from (in alphabetical order):

Juan Miguel Arrazola, Utkarsh Azad, Tom Bromley, Olivia Di Matteo, Isaac De Vlugt, Yiheng Duan, Lillian Marie Austin Frederiksen, Josh Izaac, Soran Jahangiri, Edward Jiang, Ankit Khandelwal, Korbinian Kottmann, Meenu Kumari, Christina Lee, Albert Mitjans Coma, Romain Moyard, Rashid N H M, Zeyue Niu, Mudit Pandey, Matthew Silverman, Jay Soni, Antal Száva, Cody Wang, David Wierichs.

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Release 0.26.0