Noise channels do not validate parameter values, leading to errors

[nahumsa]

Issue description

Description of the issue - include code snippets and screenshots here
if relevant. You may use the following template below

• Expected behavior: Raise an error showing that the probability (or sum of probabilities) are not compatible with the expected channel behaviour.

• Actual behavior: Raises a RuntimeWarning for an invalid value encountered in sqrt.

• Reproduces how often: 100%

• System information:

• Name: PennyLane
  Version: 0.16.0.dev0
  Summary: PennyLane is a Python quantum machine learning library by Xanadu Inc.
  Home-page: https://github.com/XanaduAI/pennylane
  Author: None
  Author-email: None
  License: Apache License 2.0
  Location: /home/nahum/Documents/pennylane-training/pen-env/lib/python3.6/site-packages
  Requires: scipy, toml, appdirs, autoray, semantic-version, networkx, autograd, numpy
  Required-by: PennyLane-qiskit
  Platform info: Linux-5.4.0-73-generic-x86_64-with-debian-buster-sid
  Python version: 3.6.8
  Numpy version: 1.19.5
  Scipy version: 1.5.4
  Installed devices:

• default.gaussian (PennyLane-0.16.0.dev0)
• default.mixed (PennyLane-0.16.0.dev0)
• default.qubit (PennyLane-0.16.0.dev0)
• default.qubit.autograd (PennyLane-0.16.0.dev0)
• default.qubit.jax (PennyLane-0.16.0.dev0)
• default.qubit.tf (PennyLane-0.16.0.dev0)
• default.tensor (PennyLane-0.16.0.dev0)
• default.tensor.tf (PennyLane-0.16.0.dev0)
• qiskit.aer (PennyLane-qiskit-0.15.0)
• qiskit.basicaer (PennyLane-qiskit-0.15.0)
• qiskit.ibmq (PennyLane-qiskit-0.15.0)

Source code and tracebacks

from pennylane.ops.channel import AmplitudeDamping

print(AmplitudeDamping(2, wires=0).kraus_matrices)

pennylane/ops/channel.py:61: RuntimeWarning: invalid value encountered in sqrt
  K0 = np.diag([1, np.sqrt(1 - gamma)])
[array([[ 1.,  0.],
       [ 0., nan]]), array([[0.        , 1.41421356],
       [0.        , 0.        ]])]

[josh146]

Thanks for reporting this @nahumsa!

Could you post the code that created this error? It will help us track down the noisy channel that is causing this bug.

(and if you already have a solution, feel free to make a PR!)

[nahumsa]

Hi @josh146. Actually, all noise channels have this error, thus maybe the solution can be on the Channel base class. I have added the code sample to reproduce the error.

I plan on making a PR for this issue later this week. 😄

[josh146]

I plan on making a PR for this issue later this week. 😄

Nice, great to hear 🙂