(feat) Added Bitflip Channel #990

RajdeepAher · 2025-03-04T08:38:52Z

Description

Related to #989 (partially)

Implemented the bitflip function to model quantum bit-flip . This function simulates a bit-flip operation occurring with a given probability.

Changes

Added bitflip function to simulate bit-flip.
Implemented unit tests covering edge cases (probabilities 0, 1, and intermediate values).
Added input validation to check probability values are within the range ([0,1]).

Checklist

Before marking your PR ready for review, make sure you checked the following locally. If this is your first PR, you might be notified of some workflow failures after a maintainer has approved the workflow jobs to be run on your PR.

Additional information is available in the documentation.

Use ruff for errors related to code style and formatting.
Verify all previous and newly added unit tests pass in pytest.
Check the documentation build does not lead to any failures. Sphinx build can be checked locally for any failures related to your PR.
Use linkcheck to check for broken links in the documentation.
Use doctest to verify the examples in the function docstrings work as expected.

vprusso

Thanks for your contribution, @RajdeepAher !

I've added a few comments here on the MR.

vprusso · 2025-03-04T12:08:56Z

toqito/channels/bitflip.py

+        return kraus_ops
+
+    # Apply the channel to the input state
+    # print(type(input_mat))


Looks like an extra print statement was left here.

Yes, will remove it. Thanks

vprusso · 2025-03-04T12:09:54Z

toqito/channels/bitflip.py

+    # X gate for the flip case
+    if dim == 2:
+        k1 = flip_prob * np.array([[0, 1], [1, 0]])
+    else:


If the bitflip channel is only defined when dim == 2, why do we make this a parameter in the function?

I felt that my understanding was somewhat lacking, and there might be a future extension of this to higher dimensions that I am not aware of at the moment and hence the parameter. I can remove it too!

vprusso · 2025-03-04T12:11:10Z

toqito/channels/bitflip.py

+             or the result of applying the channel to input_mat.
+
+    """
+    if prob < 0 or prob > 1:


You can write this more succinctly as if not (0 <= prob <= 1):

vprusso · 2025-03-04T12:12:04Z

toqito/channels/bitflip.py

+        raise ValueError("Probability must be between 0 and 1.")
+
+    # If input is a CVX variable, handle it appropriately
+    if isinstance(input_mat, Variable):


I'm not sure why we would need to cover the case here when the input_mat is a CVXPY-type object.

I saw rest of the implementations in toqito/channels and thought it would be better to include CVXPY object. I can remove it , if it is wrong

vprusso · 2025-03-04T12:12:45Z

toqito/channels/tests/test_bitflip.py

@@ -0,0 +1,80 @@
+"""Test cases for the bit-flip channel in Toqito."""


We can remove the "in Toqito" part of the this docstring.

vprusso · 2025-03-04T12:13:13Z

toqito/channels/tests/test_bitflip.py

@@ -0,0 +1,80 @@
+"""Test cases for the bit-flip channel in Toqito."""
+
+import unittest


We should use pytest (not unittest) to conform with how testing is done elsewhere in toqito for consistency.

vprusso · 2025-03-04T12:14:43Z

.pre-commit-config.yaml

-      args: [ --fix ]
-    # Run the formatter.
-    - id: ruff-format 
+repos:


Seems okay, but since these seem identical, I wonder why GitHub is flagging them as having changed. Can probably ignore this comment, but mentioning it as it also seems to be the case in channels/__init__.py as well.

yes, even I am not aware of why it is happening

codecov · 2025-03-04T14:41:03Z

Codecov Report

Attention: Patch coverage is 77.77778% with 6 lines in your changes missing coverage. Please review.

Project coverage is 97.7%. Comparing base (3cde097) to head (d1d9457).

Files with missing lines	Patch %	Lines
toqito/channels/bitflip.py	77.7%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #990     +/-   ##
========================================
- Coverage    97.9%   97.7%   -0.2%     
========================================
  Files         169     170      +1     
  Lines        3321    3348     +27     
  Branches      727     733      +6     
========================================
+ Hits         3253    3274     +21     
- Misses         44      49      +5     
- Partials       24      25      +1

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

- Removed littered comments - Forced - Fixed range - Removed compatibility for CVXPY object - Rewrote tests in pytest

purva-thakre · 2025-03-05T15:08:38Z

@RajdeepAher Please make sure your function is properly covered by unit tests.

RajdeepAher · 2025-03-05T15:35:08Z

@RajdeepAher Please make sure your function is properly covered by unit tests.

Hello,
The error in coverage was due to inclusion of CVXPY-type object, but now I have removed it

purva-thakre · 2025-03-05T15:54:40Z

? There are a bunch of test failures though..

https://github.com/vprusso/toqito/actions/runs/13677568697/job/38245804389?pr=990

https://github.com/vprusso/toqito/actions/runs/13677568689/job/38245803706?pr=990

purva-thakre · 2025-03-05T15:55:12Z

toqito/channels/bitflip.py

+    The *bitflip channel* is a quantum channel that flips a qubit from :math:`|0\rangle` to :math:`|1\rangle`
+    and from :math:`|1\rangle` to :math:`|0\rangle` with probability :math:`p`. It is defined by
+    the following operation:
+    .. math::


Add a blankline before math mode.

https://toqito--990.org.readthedocs.build/en/990/autoapi/channels/bitflip/index.html

purva-thakre · 2025-03-05T15:56:24Z

toqito/channels/bitflip.py

+        [1] https://arxiv.org/abs/1106.1445
+        [2] https://www.quantumchannelzoo.org/person/bit-flip


Add these to refs.bib instead of directly listing links to references.

https://toqito.readthedocs.io/en/latest/contributing.html#references-in-docstrings

purva-thakre · 2025-03-05T16:01:57Z

toqito/channels/tests/test_bitflip.py

+def test_invalid_probability_negative():
+    """Test that a negative probability raises an error."""
+    with pytest.raises(ValueError, match="Probability must be between 0 and 1."):
+        bitflip(prob=-0.1)
+
+
+def test_invalid_probability_greater_than_1():
+    """Test that a probability greater than 1 raises an error."""
+    with pytest.raises(ValueError, match="Probability must be between 0 and 1."):
+        bitflip(prob=1.1)
+
+
+def test_invalid_dimension():
+    """Test that invalid dimensions raise an error."""
+    rho = np.eye(3)  # 3x3 matrix
+    with pytest.raises(ValueError, match="Bitflip channel is only defined for qubits"):
+        bitflip(rho, prob=0.3)


These can be condensed into one unit tests using parametrize.

https://docs.pytest.org/en/stable/how-to/parametrize.html

purva-thakre · 2025-03-05T16:03:12Z

toqito/channels/tests/test_bitflip.py

+    expected_k0 = np.sqrt(1 - prob) * np.eye(2)
+    expected_k1 = np.sqrt(prob) * np.array([[0, 1], [1, 0]])


You can create a list/array of these two and directly compare the calculated output to the expected output.

purva-thakre · 2025-03-05T16:04:33Z

toqito/channels/tests/test_bitflip.py

+def test_apply_to_state_0():
+    """Test bitflip application to |0><0| state."""
+    prob = 0.3
+    rho = np.array([[1, 0], [0, 0]])  # |0><0|
+    expected_output = (1 - prob) * rho + prob * np.array([[0, 0], [0, 1]])
+    result = bitflip(rho, prob=prob)
+
+    np.testing.assert_almost_equal(result, expected_output)
+
+
+def test_apply_to_state_1():
+    """Test bitflip application to |1><1| state."""
+    prob = 0.3
+    rho = np.array([[0, 0], [0, 1]])  # |1><1|
+    expected_output = (1 - prob) * rho + prob * np.array([[1, 0], [0, 0]])
+    result = bitflip(rho, prob=prob)
+
+    np.testing.assert_almost_equal(result, expected_output)
+
+
+def test_probability_0():
+    """Test bitflip with probability 0 (no change)."""
+    rho = np.array([[0.5, 0.5], [0.5, 0.5]])
+    result = bitflip(rho, prob=0)
+    np.testing.assert_almost_equal(result, rho)
+
+
+def test_probability_1():
+    """Test bitflip with probability 1 (always flips)."""
+    rho = np.array([[0.5, 0.5], [0.5, 0.5]])
+    expected_output = np.array([[0.5, 0.5], [0.5, 0.5]])  # Same for maximally mixed state
+    result = bitflip(rho, prob=1)
+    np.testing.assert_almost_equal(result, expected_output)


Same as my other comment, most of these unit tests can be condensed into 1 by using using parameterize.

purva-thakre · 2025-03-05T16:06:31Z

toqito/channels/bitflip.py

+    :return: Either the Kraus operators of the bitflip channel if input_mat is None,
+             or the result of applying the channel to input_mat.


If you are referring to a parameter, sandwich it between `

purva-thakre · 2025-03-05T16:10:04Z

toqito/channels/bitflip.py

+    if input_mat is not None:
+        dim = input_mat.shape[0]
+    else:
+        dim = 2
+
+    # Enforce 2-dimensional (qubit) requirement
+    if dim != 2:
+        raise ValueError("Bitflip channel is only defined for qubits (dim=2).")


Suggested change

if input_mat is not None:

dim = input_mat.shape[0]

else:

dim = 2

# Enforce 2-dimensional (qubit) requirement

if dim != 2:

raise ValueError("Bitflip channel is only defined for qubits (dim=2).")

if input_mat is not None:

if input_mat.shape[0] != 2:

raise ValueError("Bitflip channel is only defined for qubits (dim=2).")

dim = 2

(feat) added bitflip channel

d1d9457

vprusso reviewed Mar 4, 2025

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root added 3 commits March 5, 2025 19:06

Fix bitflip issues

1c6b2a3

- Removed littered comments - Forced - Fixed range - Removed compatibility for CVXPY object - Rewrote tests in pytest

minor bug

8c88cf3

removed unnecessary imports

d50e1f7

purva-thakre reviewed Mar 5, 2025

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(feat) Added Bitflip Channel #990

(feat) Added Bitflip Channel #990

RajdeepAher commented Mar 4, 2025 •

edited by purva-thakre

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		@@ -0,0 +1,80 @@
		"""Test cases for the bit-flip channel in Toqito."""

		@@ -0,0 +1,80 @@
		"""Test cases for the bit-flip channel in Toqito."""

		import unittest

		[1] https://arxiv.org/abs/1106.1445
		[2] https://www.quantumchannelzoo.org/person/bit-flip

		expected_k0 = np.sqrt(1 - prob) * np.eye(2)
		expected_k1 = np.sqrt(prob) * np.array([[0, 1], [1, 0]])

		:return: Either the Kraus operators of the bitflip channel if input_mat is None,
		or the result of applying the channel to input_mat.

(feat) Added Bitflip Channel #990

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(feat) Added Bitflip Channel #990

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RajdeepAher commented Mar 4, 2025 • edited by purva-thakre Loading

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codecov bot commented Mar 4, 2025

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RajdeepAher commented Mar 4, 2025 •

edited by purva-thakre

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