execute(tapes, device, gradient_fn, interface='autograd', mode='best', gradient_kwargs=None, cache=True, cachesize=10000, max_diff=1, override_shots=False, expand_fn='device', max_expansion=10)¶
Execute a batch of tapes on a device in an autodifferentiable-compatible manner.
tapes (Sequence[QuantumTape]) – batch of tapes to execute
device (Device) – Device to use to execute the batch of tapes. If the device does not provide a
batch_executemethod, by default the tapes will be executed in serial.
gradient_fn (None or callable) – The gradient transform function to use for backward passes. If “device”, the device will be queried directly for the gradient (if supported).
interface (str) – The interface that will be used for classical autodifferentiation. This affects the types of parameters that can exist on the input tapes. Available options include
mode (str) – Whether the gradients should be computed on the forward pass (
forward) or the backward pass (
backward). Only applies if the device is queried for the gradient; gradient transform functions available in
qml.gradientsare only supported on the backward pass.
gradient_kwargs (dict) – dictionary of keyword arguments to pass when determining the gradients of tapes
cache (bool) – Whether to cache evaluations. This can result in a significant reduction in quantum evaluations during gradient computations.
cachesize (int) – the size of the cache
max_diff (int) – If
gradient_fnis a gradient transform, this option specifies the maximum number of derivatives to support. Increasing this value allows for higher order derivatives to be extracted, at the cost of additional (classical) computational overhead during the backwards pass.
expand_fn (function) – Tape expansion function to be called prior to device execution. Must have signature of the form
expand_fn(tape, max_expansion), and return a single
QuantumTape. If not provided, by default
max_expansion (int) – The number of times the internal circuit should be expanded when executed on a device. Expansion occurs when an operation or measurement is not supported, and results in a gate decomposition. If any operations in the decomposition remain unsupported by the device, another expansion occurs.
A nested list of tape results. Each element in the returned list corresponds in order to the provided tapes.
- Return type
Consider the following cost function:
dev = qml.device("lightning.qubit", wires=2) def cost_fn(params, x): with qml.tape.JacobianTape() as tape1: qml.RX(params, wires=0) qml.RY(params, wires=0) qml.expval(qml.PauliZ(0)) with qml.tape.JacobianTape() as tape2: qml.RX(params, wires=0) qml.RY(x, wires=1) qml.CNOT(wires=[0, 1]) qml.probs(wires=0) tapes = [tape1, tape2] # execute both tapes in a batch on the given device res = execute(tapes, dev, qml.gradients.param_shift) return res + res[0, 0] - res[0, 1]
In this cost function, two independent quantum tapes are being constructed; one returning an expectation value, the other probabilities. We then batch execute the two tapes, and reduce the results to obtain a scalar.
Let’s execute this cost function while tracking the gradient:
>>> params = np.array([0.1, 0.2, 0.3], requires_grad=True) >>> x = np.array([0.5], requires_grad=True) >>> cost_fn(params, x) 1.9305068163274222
executefunction is differentiable, we can also compute the gradient:
>>> qml.grad(cost_fn)(params, x) (array([-0.0978434 , -0.19767681, -0.29552021]), array([5.37764278e-17]))
Finally, we can also compute any nth-order derivative. Let’s compute the Jacobian of the gradient (that is, the Hessian):
>>> x.requires_grad = False >>> qml.jacobian(qml.grad(cost_fn))(params, x) array([[-0.97517033, 0.01983384, 0. ], [ 0.01983384, -0.97517033, 0. ], [ 0. , 0. , -0.95533649]])