There is a lot of quantum gates in IBM Q Composer, however, only few are implemented physically while others can be composed of them.

When one looks at description of a quantum processor in IBM Q interface, there is a list of basis gates. For example those are id, u1, u2, u3, cx for Melbourne processor. However, gates $I$, $U1$ and $U2$ are special cases of $U3$. If one removes statement include "qelib1.inc"; from QASM code, only $CNOT$ and $U3$ gates are left. So, it seems that all single qubit gates are implemented with $U3$ and there is of course two qubits $CNOT$.

According to IBM Q manual, $U3$ gate is implemented by three frame changes and two $X_{\pi/2}$ pulses.

My questions are these:

  1. Which gates are implemented physically on IBM Q? I would expect $CNOT$ and some single qubit gates but which ones?
  2. What are frame changes and $X_{\pi/2}$ pulses used for gate implementation?

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