# What does U mean in pulse schedules for superconducting quantum computers?

What does U channel mean in pulse schedules for superconducting devices? In the image you can see the schedule for a CX gate.

I assume D stands for the drive channels. What does U mean, is it perhaps the control channel? Why the letter U, what it stands for?

• $U$ is generally used to denote a unitary operation, but for your specific case, please link the resource you are referring to, so people can exactly answer your question. From the diagram, I believe $U$ is a signal for performing controlled rotation, where the first pulse is the unitary operation $CR(\pi/4)$ and the second pulse is the unitary operation $CR(-\pi /4)$. Nov 14 at 8:52
• This would be a good answer to my question Nov 14 at 8:53
• $U$ is typically only used for unitaries in a mathematical context -- here we're looking at the pulse schedule of the device and U denotes a channel to drive the qubit at a specific frequency, not a unitary. Nov 14 at 8:55
• Just to make my question more precise, I updated my question and replaced original image with an image for a CX gate schedule. Nov 14 at 8:58

You're already answering your own question: U0 stands for the control channel on qubit 0, as you can read in Table 1 of https://iopscience.iop.org/article/10.1088/2058-9565/aba404/pdf.
In your schedule the CX is implemented as echoed cross resonance gate (ECR), where we drive the control qubit 0 at the frequency of the target qubit 1. Therefore you can see a cross resonance (CR) pulse on the control channel of qubit 0, U0. Since we echo the pulse, you can also see a secondary CR pulse with inverted angle, interleaved with X gates.
You can also see something happening on the drive channel of qubit 1, D1, during the execution of the CR pulse on the control qubit. This is a cancellation tone to improve the CX fidelity. Note that even though the channels are drawn separately, they can be implemented on the same wire on the device.