# The Wavefunction device¶

The rigetti.wavefunction device provides an interface between PennyLane and the Forest SDK wavefunction simulator. Because the wavefunction simulator allows access and manipulation of the underlying quantum state vector, rigetti.wavefunction is able to support the full suite of PennyLane and Quil quantum operations and observables.

In addition, it is generally faster than running equivalent simulations on the QVM, as the final state can be inspected and the expectation value calculated analytically, rather than by sampling measurements.

Note

By default, rigetti.wavefunction is initialized with shots=0, indicating that the exact analytic expectation value is to be returned.

If the number of trials or shots provided to the rigetti.wavefunction is instead non-zero, a spectral decomposition is performed and a Bernoulli distribution is constructed and sampled. This allows the rigetti.wavefunction device to ‘approximate’ the effect of sampling the expectation value.

## Usage¶

You can instantiate the device in PennyLane as follows:

import pennylane as qml

dev_wfun = qml.device('rigetti.wavefunction', wires=2)


This device can then be used just like other devices for the definition and evaluation of QNodes within PennyLane.

A simple quantum function that returns the expectation value and variance of a measurement and depends on three classical input parameters would look like:

@qml.qnode(dev_wfun)
def circuit(x, y, z):
qml.RZ(z, wires=[0])
qml.RY(y, wires=[0])
qml.RX(x, wires=[0])
qml.CNOT(wires=[0, 1])
return qml.expval(qml.PauliZ(0)), var(qml.PauliZ(1))


You can then execute the circuit like any other function to get the quantum mechanical expectation value and variance:

>>> circuit(0.2, 0.1, 0.3)
array([0.97517033, 0.04904283])


## Supported operations¶

All devices support all PennyLane operations and observables, with the exception of the PennyLane QubitStateVector state preparation operation.

## QVM and quilc server configuration¶

Note

If using the downloadable Rigetti SDK with the default server configurations for the QVM and the Quil compiler (i.e., you launch them with the commands qvm -S and quilc -R), then no special configuration is needed. If using a non-default port or host for either of the servers, see the pyQuil configuration documentation for details on how to override the default values.