Assume PhasedXPowGate is different from XPowGate and YPowGate

[codrut3]

A similar change was done for PhasedISwapPowGate. This addresses issue #6528.

[daxfohl]

Can this be removed completely? I assume by the description in #4585, removing it entirely would have the same effect. But I could be wrong.

[pavoljuhas]

+1 - the _value_equality_values_cls_ can be indeed removed provided manual_cls in the decorator is changed to the False default. The _value_equality_approximate_values_ defaults to _value_equality_values_ so it can be removed as well.

diff --git a/cirq-core/cirq/ops/phased_x_gate.py b/cirq-core/cirq/ops/phased_x_gate.py
index d769b863..51bc5f39 100644
--- a/cirq-core/cirq/ops/phased_x_gate.py
+++ b/cirq-core/cirq/ops/phased_x_gate.py
@@ -30,3 +30,3 @@ from cirq.ops import raw_types
 
-@value.value_equality(manual_cls=True, approximate=True)
+@value.value_equality(approximate=True)
 class PhasedXPowGate(raw_types.Gate):
@@ -244,5 +244,2 @@ class PhasedXPowGate(raw_types.Gate):
 
-    def _value_equality_values_cls_(self):
-        return PhasedXPowGate
-
     def _value_equality_values_(self):
@@ -250,5 +247,2 @@ class PhasedXPowGate(raw_types.Gate):
 
-    def _value_equality_approximate_values_(self):
-        return self.phase_exponent, self._canonical_exponent, self._global_shift
-
     def _json_dict_(self) -> Dict[str, Any]:

[codrut3]

Hi Dax and Pavol,

Thank you a lot for your feedback! I updated the code following your suggestion.

However, in the meantime I noticed that several tests were failing. I updated the tests so that you can see the effect, but I think this change is overall problematic.
The problem is that:

• circuit optimization,
• merge gate operations,
• and conversion to proto,

all transform gates X and Y to PhasedXPowGate. This worked well until now because the gate was correctly interpreted as X or Y, for example when printing the circuit. However, with this change this is no longer the case. In particular, users will be surprised to optimize a circuit and instead of X, end up with a PhasedXPowGate(phase_exponent=0). It's harder to read, and it's a poor user experience.

I am inclined to drop this pull request. Let me know what you think and if there is a way around this problem.

[daxfohl]

Should an optimizer have a final stage where it puts all the gates in canonical form perhaps?

[pavoljuhas]

Should an optimizer have a final stage where it puts all the gates in canonical form perhaps?

Indeed, that would be nice. This change actually shows the affected transformations could work better.

@tanujkhattar - would you have some tips for a transformer that does such conversion,
e.g., cirq.PhasedXPowGate(phase_exponent=0) to cirq.X?

Here is an example code adapted from a unit test -

import cirq
q = cirq.q(0)
c0 = cirq.Circuit([cirq.Moment([cirq.Z(q) ** 0.5]), cirq.Moment([cirq.Y(q) ** 0.25])])
c1 = cirq.eject_z(c0)
print(repr(c1))

output:

cirq.Circuit([
    cirq.Moment(
        cirq.PhasedXPowGate(phase_exponent=0.0, exponent=0.25).on(cirq.LineQubit(0)),
    ),
    cirq.Moment(
        cirq.S(cirq.LineQubit(0)),
    ),
])