Hardware-friendly decomposition

[shreyapkumar]

Context: This PR attempts to remove needless complications in the code arising from the difference between how theorists define T matrices and the equivalent transformations that the chip implements.

Currently, the unitary is decomposed into Clements T matrices which are later converted to phases on the chip by rectangular_symmetric function. Decomposing the unitary directly into Mach Zehnder units would make the code simpler. It would also make it easier to modify the code and decompositions as planned in order to account for imperfect beamsplitters and also for power reduction, etc.

Description of the Change:

• Adds MZ matrices to eventually replace T matrices to now represent the units on the chip.
• Removes need for function rectangular_symmetric by modifying rectangular_phase_end_MZ to directly calculate internal and external phases on chip.
• Tests very similar to the ones already present are added.

Possible Drawbacks:

• Doesn't match the theoretical conventions in Clements and Xanadu modular decompositions.

[shreyapkumar]

Possible reviewers could be @lneuhaus , @nquesada and/or @ishdhand

[codecov[bot]]

Codecov Report

Merging #363 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #363   +/-   ##
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  Coverage   97.77%   97.77%           
=======================================
  Files          52       52           
  Lines        6435     6435           
=======================================
  Hits         6292     6292           
  Misses        143      143           

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[josh146]

@shreyapkumar, by the way, CodeFactor runs some static analysis on the PR, and picked up the following issues. Running the Black formatter locally should automatically fix most of them,

black -l 100 strawberryfields/decompositions.py

and the remainder can be fixed by hand.

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CodeFactor found multiple issues last seen at 2fa0b43f300d92bae13c9424d1add9346de1e570:

Redefining name 'T' from outer scope (line 233)

strawberryfields\decompositions.py:499

No space allowed before :

if U[n, m] == 0 :
                ^

strawberryfields\decompositions.py:541

Trailing whitespace

strawberryfields\decompositions.py:493 strawberryfields\decompositions.py:654 strawberryfields\decompositions.py:661 strawberryfields\decompositions.py:492 strawberryfields\decompositions.py:548 strawberryfields\decompositions.py:500

No space allowed before :

if U[m, n] == 0 :
                ^

strawberryfields\decompositions.py:520

[shreyapkumar]

Documentation and formatting changes are fixed now. Comments about this or anything else are welcome!

[shreyapkumar]

Thanks so much, @josh146 for the very helpful comments! I learnt a lot :)

[josh146]

@shreyapkumar, is this ready to be merged in?

[shreyapkumar]

Yes, nothing more to add from my side!