Used e.g. in the Sycamore processor.
The most basic type of transmon is in Ciro's ASCII art circuit diagram notation, an LC circuit e.g. as mentioned at youtu.be/cb_f9KpYipk?t=180 from Video "The transmon qubit by Leo Di Carlo (2018)":
+----------+
| Island 1 |
+----------+
| |
X C
| |
+----------+
| Island 2 |
+----------+youtu.be/eZJjQGu85Ps?t=2443 from Video "Superconducting Qubits I Part 1 by Zlatko Minev (2020)" describes a (possibly simplified) physical model of it, as two superconducting metal islands linked up by a Josephson junction marked as The circuit is then analogous to a LC circuit, with the islands being the capacitor. The Josephson junction functions as a non-linear inductor.
X in the diagram as per-Ciro's ASCII art circuit diagram notation:+-------+ +-------+
| | | |
| Q_1() |---X---| Q_2() |
| | | |
+-------+ +-------+Others define it with a SQUID device instead: youtu.be/cb_f9KpYipk?t=328 from Video "The transmon qubit by Leo Di Carlo (2018)". He mentions that this allows tuning the inductive element without creating a new device.
The superconducting transmon qubit as a microwave resonator by Daniel Sank (2021)
Source. Calibration of Transmon Superconducting Qubits by Stefan Titus (2021)
Source. Possibly this Keysight which would make sense.This is a good review article.
But seriously, this is a valuable little list.
The course is basically exclusively about transmons.
The transmon qubit by Leo Di Carlo (2018)
Source. Via QuTech Academy.Circuit QED by Leo Di Carlo (2018)
Source. Via QuTech Academy.Measurements on transmon qubits by Niels Bultink (2018)
Source. Via QuTech Academy. I wish someone would show some actual equipment running! But this is of interest.Single-qubit gate by Brian Taraskinki (2018)
Source. Good video! Basically you make a phase rotation by controlling the envelope of a pulse.Two qubit gates by Adriaan Rol (2018)
Source. Assembling a Quantum Processor by Leo Di Carlo (2018)
Source. Via QuTech Academy.