Summary

• Modify CircuitConverter to convert circuit components with Perceval parameter expressions, Expression.

Context

• Perceval components may contain parameter expressions, Expression. These are objects which represent arithmetic manipulations of the Parameter class.
• Currently, when converting circuits with Expression, MerLin ignores the Expression and does not raise any error - leading the user to think that their circuit may have been successfully converted into a tensor.

Proposed Changes

• Add hidden method, _parse_expression to CircuitConverter, which evaluates a Perceval Expression by parsing its algebraic representation using the ast library and substituting in the values of its parent parameters.

from perceval import Circuit, PS, P
from merlin import CircuitConverter
import torch
import numpy as np

# Consider a single phase shifter
circuit = Circuit(2) // PS(2 * P("A"))

# Let's print the unitary for phase of pi and a phase of 2pi
phases = np.pi * torch.tensor([[0.5], [1]])

circuit_graph = CircuitConverter(circuit, ["A"])
print(circuit_graph.to_tensor(phases).real)

tensor([[[-1.,  0.],
         [ 0.,  1.]],

        [[ 1.,  0.],
         [ 0.,  1.]]])