# The GBS device¶

This device gives access to a near-term model of photonic quantum computing called Gaussian boson sampling (GBS). It is run on Strawberry Field’s Gaussian simulator backend. The GBS model can encode graphs using their adjacency matrix $$A$$, with a number of known applications. The encoding also fixes a mean number of photons generated by the device. Its output is a collection of samples listing the number of photons counted in each mode, resulting in a probability distribution that captures information about the encoded graph.

The GBS device can then be trained by varying the initial adjacency matrix according to some trainable parameters. When doing this, we can access the output GBS probability distribution as well as its derivative.

Note

It is also possible to perform GBS using other devices in Pennylane-SF. However, gradient evaluation is calculated using the finite-difference rule. The purpose of this device is to provide a setting where the gradient can be evaluated analytically using a fixed method of varying an encoded adjacency matrix.

## Usage¶

You can instantiate the GBS device in PennyLane as follows:

import pennylane as qml

dev = qml.device('strawberryfields.gbs', wires=4, cutoff_dim=4)


Note that a cutoff_dim argument is used to truncate the number of photons in the output GBS probability distribution.

The GBS device represents a fixed circuit where we can:

• Vary an input adjacency matrix according to some trainable parameters and embed the result into GBS,

• Measure the output probability distribution.

Hence, QNodes bound to the strawberryfields.gbs device must consist solely of the ParamGraphEmbed operation, and must return Fock probabilities. For example:

from pennylane_sf.ops import ParamGraphEmbed
from pennylane import numpy as np

A = np.array([
[0.0, 1.0, 1.0, 1.0],
[1.0, 0.0, 1.0, 0.0],
[1.0, 1.0, 0.0, 0.0],
n_mean = 2.5

@qml.qnode(dev)
def quantum_function(x):
ParamGraphEmbed(x, A, n_mean, wires=range(4))
return qml.probs(wires=range(4))


Here,

• A is a symmetric adjacency matrix of a graph, with the size of the adjacency matrix matching the number of wires on the device;

• n_mean is the initial mean number of photons for the output samples;

• x are a set of real, non-negative trainable parameters; and

• ParamGraphEmbed embeds a parametrized graph $$WAW$$ onto the GBS device, where $$W$$ is a diagonal matrix dependent on parameters x (see below for further details).

The initial values of the trainable parameters x must be non-negative and are typically chosen with an initial value close to one (which results in an unchanged A):

>>> x = 0.9 * np.ones(4)


The GBS probability distribution can then be evaluated using:

>>> quantum_function(x)


The derivative of the probability distribution can also be calculated using standard methods in PennyLane. For example,

>>> d_quantum_function = qml.jacobian(quantum_function)
>>> d_quantum_function(x)


The GBS probability distribution can also be post-processed and used as the input to other QNodes or classical nodes.

## Background¶

The GBS device can be trained by varying the adjacency matrix $$A$$ of a graph according to some trainable parameters $$\mathbf{w}$$. One way to include trainable parameters is to transform $$A$$ according to

$A \rightarrow WAW,$

where $$W$$ is a diagonal matrix with values given by $$\sqrt{\mathbf{w}}$$. Using this approach, a recent paper has shown how to calculate the derivative of the output GBS probability distribution $$P(\mathbf{n}, \mathbf{w})$$:

$\partial_{\mathbf{w}} P(\mathbf{n}, \mathbf{w}) = \frac{\mathbf{n} - \langle\mathbf{n}\rangle}{\mathbf{w}}P(\mathbf{n}, \mathbf{w}),$

where $$\mathbf{n}$$ is a sample given by counting the number of photons observed in each mode.

## Caching¶

The probability $$P(\mathbf{n}, \mathbf{w})$$ of a sample $$\mathbf{n}$$ according to trainable parameters $$\mathbf{w}$$ can be calculated as:

$P(\mathbf{n}, \mathbf{w}) = \frac{Z(A)}{Z(WAW)} \prod_{i=1}^{n} w_{i}^{n_{i}},$

where $$Z$$ is a normalization coefficient. This means that the probability distribution and its derivative can be calculated directly from the probability distribution of $$A$$. This observation is particularly useful for devices in non-analytic mode, allowing us to generate a reference set of samples from $$A$$ and rescale the resulting probability distribution to give $$P(\mathbf{n}, \mathbf{w})$$ for any choice of $$\mathbf{w}$$.

This behaviour can be realized in the GBS device by setting the use_cache=True argument (in non-analytic mode):

>>> dev = qml.device('strawberryfields.gbs', wires=4, cutoff_dim=4, shots=10, use_cache=True)


When the probability distribution using this device and the above quantum_function() is first evaluated, samples will instead be generated from $$A$$ and cached (stored). Subsequent evaluations of the probability distribution will then make use of this internal cache rather than generating new samples, resulting in a faster evaluation.

It is also possible to input a NumPy array of pre-generated samples from $$A$$ when instantiating the GBS device using the samples argument:

>>> dev = qml.device('strawberryfields.gbs', wires=4, cutoff_dim=4, shots=10,
...                  samples=my_array, use_cache=True)


This allows the initial generation of samples during the first evaluation of the probability distribution to be skipped.

## Device options¶

The GBS device accepts additional arguments beyond the PennyLane default device arguments.

cutoff_dim

the Fock basis truncation to be applied when computing probabilities in the Fock basis.

shots=None

The number of circuit evaluations/random samples used to estimate probabilities. The default value of None means that exact probabilities are returned.

use_cache

Indicates whether to use samples from previous evaluations to speed up the calculation of the probability distribution. Only used when shots is not None.

samples

Allows pre-generated samples of the input adjacency matrix to be provided in non-analytic mode. When use_cache=True, these samples will be used to infer the probability distribution for any choice of trainable parameter.

## Supported operations¶

The GBS device supports is a restricted model of quantum computing and supports only the following operations and return types:

Supported operations:

 ParamGraphEmbed A parametrized embedding of a graph into GBS.

Supported return types:

 probs Probability of each computational basis state.