qml.counts returns all outcomes when requested if MCMs present

[andrijapau]

Context:

Previously, despite using all_outcomes , qml.counts incorrectly returned only the states that have counts present,

import pennylane as qml
dev = qml.device("default.qubit", shots=10)

@qml.qnode(device=dev, mcm_method="one-shot")
def circuit():
    m1 = qml.measure(0)
    return qml.counts(m1, all_outcomes=True)

>>> circuit()
{0.0: 10}

Description of the Change:

If all_outcomes is specified, the intermediate tmp variable (used to keep track of measurement values from MCMs) defaults to a Counter object that initializes all possible basis states as keys with their values set to zero. This counter is then updated based on the measurement results, where each mcm sample contributes to updating the counts, weighted by a validity factor.

This results in the correct output,

dev = qml.device("default.qubit", shots=10)

@qml.qnode(device=dev, mcm_method="one-shot")
def circuit():
    m1 = qml.measure(0)
    return qml.counts(m1, all_outcomes=True)

>>> circuit()
{0.0: 10, 1.0: 0}
# {0.0: 10} if all_outcomes=False

or,

dev = qml.device("default.qubit", shots=10)

@qml.qnode(device=dev, mcm_method="one-shot")
def circuit():
    m1 = qml.measure(0)
    m2 = qml.measure(1)
    return qml.counts([m1,m2], all_outcomes=True)

>>> circuit()
{'00': 10, '01': 0, '10': 0, '11': 0}
# {'00': 10} if all_outcomes=False

Benefits: all_outcomes kwarg in qml.counts now works as intended when mcm_method="one-shot" and MCM operations are present.

Possible Drawbacks: tmp object creation doesn't scale well with higher qubit counts.

Related GitHub Issues: Fixes #6700

[sc-80206]

[JerryChen97]

All good! Thanks a lot for fixing this. Just minor suggestions nonblocking

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.60%. Comparing base (6f26036) to head (ff03b5c).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6732   +/-   ##
=======================================
  Coverage   99.60%   99.60%           
=======================================
  Files         476      476           
  Lines       45224    45232    +8     
=======================================
+ Hits        45047    45055    +8     
  Misses        177      177           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[PietropaoloFrisoni]

Thanks @andrijapau!

[lillian542]

LGTM!