Deep-copy circuit with Program._linked_copy

[thisac]

Before posting a bug report

• I have searched exisisting GitHub issues to make sure the issue does not already exist.

Expected behavior

When creating a linked copy of a program (e.g., when calling the compiler), the register references are retained. Gate parameters may contains MeasuredParameter objects, which are used as placeholders for mid-circuit measurements. They contain register references to the registers that are measured, and thus have to be kept in the linked copy.

Currently, the circuits are referenced in the linked copy, rather than copied. Expected behaviour would be that only the register references are retained, while the rest of the circuit (parameters, etc.) should be copied.

>>> cprog = prog._linked_copy()
>>> cprog.circuit is prog.circuit
False

Above: the program circuit should be equal but not the same.

Actual behavior

>>> cprog = prog._linked_copy()
>>> cprog.circuit is prog.circuit
True

Reproduces how often

Every time a program is copied.

System information

Strawberry Fields: a Python library for continuous-variable quantum circuits.
Copyright 2018-2020 Xanadu Quantum Technologies Inc.

Python version:            3.9.6
Platform info:             macOS-12.2.1-arm64-arm-64bit
Installation path:         /Users/theodor/Repos/strawberryfields/strawberryfields
Strawberry Fields version: 0.22.0-dev
Numpy version:             1.21.3
Scipy version:             1.7.0
SymPy version:             1.8
NetworkX version:          2.6.1
The Walrus version:        0.19.0-dev
Blackbird version:         0.5.0-dev
XCC version:               0.2.0-dev
TensorFlow version:        2.4.1

Source code

No response

Tracebacks

No response

Additional information

No response