Feasibility of continuous observation of quantum gate parameters

Since direct observation of quantum bits collapses the quantum state and prevents the original state from being obtained, does constant monitoring of the parameters of the quantum gate collapse the quantum state? How is it possible to continuously monitor the parameters of a quantum gate without directly observing the quantum state?

Hi @R-X, welcome to the Forum!

When you run variational algorithms you are indeed creating a circuit with a specific set of parameters. Then you measure it and find the gradient of the circuit with respect to those parameters. This gives you information on how to update those parameters. In the next iteration you create your circuit again but now you use the new parameters. The cycle continues until the gradients go to zero (or close) which means that your circuit (or cost function) can no longer improve if you move the parameters a little.

Note that, depending on how you calculate your gradients, additional runs of your circuit may be needed. Let me know if you have questions about this.

You can learn more about variational circuits in this page.

I hope this helps you and let me know if you have any further questions!

I would also recommend our new PennyLane YouTube channel which has great videos on optimizing quantum circuits and more.

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