separate DensityMatrixMP into its own measurement process #4558
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## master #4558 +/- ##
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Coverage 99.65% 99.65%
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Files 376 376
Lines 33104 33108 +4
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albi3ro
reviewed
Aug 31, 2023
mudit2812
approved these changes
Sep 1, 2023
rmoyard
approved these changes
Sep 1, 2023
mudit2812
pushed a commit
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this pull request
Sep 8, 2023
**Context:** Today, `qml.state()` returns a `StateMP` with no wires set, while `qml.density_matrix()` returns a `StateMP` with the wires of the subsystem being traced out. The new device implementation would benefit greatly from being able to assign wires to a state measurement, and also it just makes sense for these to be separate things. **Description of the Change:** Adds a new `DensityMatrixMP` that inherits from `StateMP` (not the parent `StateMeasurement` so that `isinstance(mp, StateMP)` still returns True for density matrix measurements). I literally copy-pasted the `StateMP` class, then deleted any density-matrix-related things from the `StateMP` class, and deleted and state-related things from the `DensityMatrixMP` class. **Benefits:** - Density matrix is its own measurement process! - We can assign wires to a `StateMP` without it thinking it represents a density matrix. **Possible Drawbacks:** This can also be viewed as a benefit (see above, no code changes outside module needed), but `isinstance(my_density_mp, StateMP)` returns true. If you want to know that your measurement _is_ a state measurement, you can do `type(mp) is StateMP`. **UPDATE for reviewers**: Responding to a comment, I've changed `StateMP` to not accept `wires` in its init, so now `DensityMatrixMP` directly calls `StateMeasurement.__init__(self, wires=wires, id=id)` to skip its parent (StateMP) which would now fail if given wires. I call it directly because this is the most pythonic way to call a grandparent's method (the other option being `super(StateMP, self).__init__` but I think `StateMeasurement` is a more reliable and responsible choice than "whoever StateMP's parent is"). Plus, I'll be removing that change when StateMP supports wires.
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Context:
Today,
qml.state()returns aStateMPwith no wires set, whileqml.density_matrix()returns aStateMPwith the wires of the subsystem being traced out. The new device implementation would benefit greatly from being able to assign wires to a state measurement, and also it just makes sense for these to be separate things.Description of the Change:
Adds a new
DensityMatrixMPthat inherits fromStateMP(not the parentStateMeasurementso thatisinstance(mp, StateMP)still returns True for density matrix measurements).I literally copy-pasted the
StateMPclass, then deleted any density-matrix-related things from theStateMPclass, and deleted and state-related things from theDensityMatrixMPclass.Benefits:
StateMPwithout it thinking it represents a density matrix.Possible Drawbacks:
This can also be viewed as a benefit (see above, no code changes outside module needed), but
isinstance(my_density_mp, StateMP)returns true. If you want to know that your measurement is a state measurement, you can dotype(mp) is StateMP.UPDATE for reviewers:
Responding to a comment, I've changed
StateMPto not acceptwiresin its init, so nowDensityMatrixMPdirectly callsStateMeasurement.__init__(self, wires=wires, id=id)to skip its parent (StateMP) which would now fail if given wires. I call it directly because this is the most pythonic way to call a grandparent's method (the other option beingsuper(StateMP, self).__init__but I thinkStateMeasurementis a more reliable and responsible choice than "whoever StateMP's parent is"). Plus, I'll be removing that change when StateMP supports wires.