I'm going through tutorials on Qiskit Pulse. From this documentation, I can see that it is possible to give a pulse, such as a Gaussian a few different parameters. One of the parameters that I can specify is the amplitude.

But the amplitude isn't limited between 0 and 1. It can be negative as well, and shows up as such on the pulse schedule. What is the meaning of a negative amplitude?

For example, a simple implementation of a CNOT on IBMQ systems uses a single cross-resonance pulse. From there, I understand that it is possible to cancel out some error terms by using an echo. But when using an echo, why is it not enough to use an X pulse? Why does the amplitude of the cross-resonance pulse become negative?

Below is a sample of a negative pulse, taken from the Qiskit documentation on pulses.

Simple negative pulse example from Qiskit documentation



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