Fix #45: Parameterized measurement patterns

[thierry-martinez]

This commit is yet another tentative to implement parameterized measurement patterns, to fulfill issue #45 (previous tentative: #68).

This commit adds two methods to the class Pattern:

• is_parameterized() returns True if there is at least one measurement angle that is not just an instance of numbers.Number: indeed, a parameterized pattern is a pattern where at least one measurement angle is an expression that is not a number, typically an instance of sympy.Expr (but we don't force to choose sympy here).

• subs(variable, substitute) returns a copy of the pattern where occurrences of the given variable in measurement angles are substituted by the given value. Substitution is performed by calling the method subs on measurement angles, if the method exists, which is the case in particular for sympy.Expr. If the substitution returns a number, this number is coerced to float, to get numbers that implement the full number protocol (in particular, sympy numbers don't implement cos).

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 81.87050% with 126 lines in your changes missing coverage. Please review.

Project coverage is 72.95%. Comparing base (ec4c582) to head (5feac05). Report is 5 commits behind head on master.

Files	Patch %	Lines
graphix/parameter.py	80.54%	122 Missing :warning:
graphix/transpiler.py	80.95%	4 Missing :warning:

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```diff @@ Coverage Diff @@ ## master #158 +/- ## ========================================== + Coverage 71.82% 72.95% +1.13% ========================================== Files 30 31 +1 Lines 5359 6038 +679 ========================================== + Hits 3849 4405 +556 - Misses 1510 1633 +123 ```

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[shinich1]

@thierry-martinez great, is the intention to merge @pafloxy's #68-equivalent code first (when it's ready), before merging this?

[thierry-martinez]

@thierry-martinez great, is the intention to merge @pafloxy's #68-equivalent code first (when it's ready), before merging this?

I adapted #68 code for Parameter class in the commit 81bbeb7 I just pushed. #68 was incorrect because arithmetic operators changed parameters in place (computing alpha + 1 did change the value of alpha to alpha + 1!). In this PR, we distinguish between Parameter (alpha) and ParameterExpression (alpha + 1), and a Parameter is a particular case of ParameterExpression.

The test test_parameter_simulation is illustrative: we can either substitute a parameter by a value in a pattern then simulate, or simulate symbolically (given that the pattern is deterministic!) then substitute in the (symbolic) state vector or the density matrix, and we get the same result.

[shinich1]

@thierry-martinez @pafloxy great! give us a few days to look through. A few quick comments:

1. is TN backend working with parameters, or should we throw error in TN backend if pattern is parametrised? 2. could you add sympy to requirements.txt?
2. will circuit.simulate_statevector, pattern.perform_pauli_measurements, as well as visualization tool, work with parameters?

[EarlMilktea]

Let me review it later.

Anyway, is it absolutely necessary to use sympy? I'm concerned with...

• performance: Personally I feel sympy is far from performant.
• type stability: sympy has no type annotations, and it seems that not even planned.
• developer experience: Sometimes sympy makes linter (ex. flake8, Ruff) extremely slow.

Additionally, let me point out that @masa10-f is planning to completely eliminate sympy from the source.

[EarlMilktea]

If sympy is absolutely necessary, it's OK, but we may need to verify that we're using it wisely.

[pafloxy]

@thierry-martinez @pafloxy great! give us a few days to look through. A few quick comments:

1. is TN backend working with parameters, or should we throw error in TN backend if pattern is parametrised? ~2. could you add sympy to requirements.txt?~
2. will circuit.simulate_statevector, pattern.perform_pauli_measurements, as well as visualization tool, work with parameters?

@shinich1 @thierry-martinez Maybe you guys figured it already but I think we need to make some small modification to the visualization.py module as well. There are some checks to determine if a measurement angle is Pauli in some of the class methods for GraphVisualizer of the form

elif (
                show_pauli_measurement
                and self.meas_angles is not None
                and (
                    2 * self.meas_angles[node] == int(2 * self.meas_angles[node])
                )  # measurement angle is integer or half-integer
            ):

where we might need to add another line or so to exclude the case where the measurement angles is an instance of Parameter/ParameterExpression.

[shinich1]

Hi @pafloxy @thierry-martinez

I think the best way is to move parameter implementation with sympy out of main graphix repository, for example a separate repo (wrapper/module) dedicated to parameterized patterns and their executions (it seems possible, given the relatively small changes required to implement?). I am happy to initiate a creation of such repository in teamgraphix organization, if that makes sense to you? for example, that can be part of extra pip installation.

The reason mostly follows @EarlMilktea 's. Sustaining maintainability is going to help a lot in the long run for everyone contributing to this repo and will help a lot those maintaining.

1. sympy has quite a performance issue. Looking at their github repo, it seems like a recurring issue in qiskit (which has sympy incorporated) and quite a bit of effort can be saved by deciding to move it out at this point (nb as @EarlMilktea mentioned sympy will be removed from requirements.txt when gflow is refactored for performance soon.)
2. typing, linter compatibility, as in @EarlMilktea's comment

[thierry-martinez]

I propose in my last commits a version which does not require mypy.

• The module parameter.py implements symbolic expressions so as to support parameters, but without all the symbolic. machinery of mypy (computations with known values are done numerically, not symbolically).

• The rest of the code does not depend on parameter.py: in particular, the code for simulators is generic. If we still need the sympy version of parameters, we can implement it separately, and simulators should work without any specific modification in the code of graphix.

• Tests for parameters are done in a specific test_parameter.py module, and the tests include circuit simulation.

[shinich1]

I propose in my last commits a version which does not require mypy.

• The module parameter.py implements symbolic expressions so as to support parameters, but without all the symbolic. machinery of mypy (computations with known values are done numerically, not symbolically).
• The rest of the code does not depend on parameter.py: in particular, the code for simulators is generic. If we still need the sympy version of parameters, we can implement it separately, and simulators should work without any specific modification in the code of graphix.
• Tests for parameters are done in a specific test_parameter.py module, and the tests include circuit simulation.

Thank you! I believe you mean sympy instead of mypy above? Let me take a detailed look at the code soon.

[shinich1]

@thierry-martinez the implementation looks good to me! we need to:

• [ ] fix visualization which seems to throw error in the presence of parameter in pattern
• [ ] test more situations, so we can catch more errors (currently rz only with no arithmetic of param covered?)
• [ ] do we intend to cover simulations without subs? if so we should throw error if parameterized and being simulated?
• [ ] update QAOA, VQE and QNN examples.
• [ ] add appropriate page to docs (just autodoc, not much to write - see existing pages)
• [ ] device interface - may need care. easy solution is to ask subs before PatternRunner initialization.

[EarlMilktea]

I'm a little bit concerned about...

• complexity: Is it possible to maintain parameter.py ? It's already really huge.
• performance: I feel that current impl. cannot be faster than sympy as it's doing almost the same things.

[shinich1]

We suggest the following:

• implement 'placeholder' parameter implementation in graphix
• move symbolic stuff outside, in https://github.com/TeamGraphix/graphix-symbolic/tree/master.