[BUG] Confusing counts labels for list of MCMs including a postselected MCM #5513
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dwierichs
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**Context:** [sc-61314] Issue #5513 was being caused by samples that were converted to decimals to be cast to `int8`. This caused overflow with 8 or more wires, resulting in negative values being present in the counts dictionary. **Description of the Change:** * Update `qml.counts.process_samples` and `QubitDevice._samples_to_counts` to cast to `int64` instead of `int8`. **Benefits:** No more overflow issues with `qml.counts`. **Possible Drawbacks:** **Related GitHub Issues:** #5513
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Just noting here that this issue is still not resolved with #5544 because circuits with more than 64 qubits will have this behaviour still |
mudit2812
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**Context:**
`qml.counts` was converting samples to integers and using f-strings to
format as binary strings, however, it caused wires > 64 to have
overflow.
Without the changes in this PR:
```python
import pennylane as qml
import numpy as np
np.random.seed(582)
num_wires = 70
wires = list(range(num_wires))
dev = qml.device("default.clifford", wires=wires, shots=100)
@qml.qnode(dev)
# @qml.defer_measurements
def circuit():
[qml.PauliX(w) for w in wires]
return qml.counts()
circuit()
```
```pycon
{'-000000000000000000000000000000000000000000000000000000000000000000001': tensor(100, requires_grad=True)}
```
With the changes:
```python
import pennylane as qml
import numpy as np
np.random.seed(582)
num_wires = 70
wires = list(range(num_wires))
dev = qml.device("default.clifford", wires=wires, shots=100)
@qml.qnode(dev)
# @qml.defer_measurements
def circuit():
[qml.PauliX(w) for w in wires]
return qml.counts()
circuit()
```
```pycon
{'1111111111111111111111111111111111111111111111111111111111111111111111': tensor(100, requires_grad=True)}
```
**Description of the Change:**
* Change `qml.counts` to convert directly from list of samples to binary
string without first converting to integers.
* Changed `QubitDevice._samples_to_counts` to cast to `int8` instead of
`int64`. This is only for casting samples, not converting to ints, as
`int8` is more memory efficient.
**Benefits:**
* `qml.counts` now works with arbitrary wires. This is useful for
devices like `default.clifford`, which can simulate hundreds of qubits.
**Possible Drawbacks:**
**Related GitHub Issues:**
#5537 , #5513
---------
Co-authored-by: David Wierichs <[email protected]>
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Expected behavior
Meaningful strings are created to reference mid-circuit measurement values.
Actual behavior
For a very particular setup, strings are produced that contain a
"-"character, which is unexpected and not meaningful.Note that the output strings should consist of 4 varying integers (
first_mcms), 1 integer that is fixed to"1"(postselected_mcm) and 3 more varying integers (second_mcms).Additional information
No response
Source code
Tracebacks
{'-0000001': 1, '-0010010': 1, '-0100011': 1, '-0110100': 3, '-1000101': 1, '-1010110': 2, '-1100111': 3, '-1111000': 5}System information
Existing GitHub issues
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