Ciro Santilli - requires intense refrigeration to 15mK in dilution refrigerator. Note that this is much lower than the actual superconducting temperature of the metal, we have to go even lower to reduce noise enough, see e.g. youtu.be/uPw9nkJAwDY?t=471 from Video "Building a quantum computer with superconducting qubits by Daniel Sank (2019)"
- less connectivity, normally limited to 4 nearest neighbours, or maybe 6 for 3D approaches, e.g. compared to trapped ion quantum computers, where each trapped ion can be entangled with every other on the same chip
This is unlike atomic systems like trapped ion quantum computers, where each atom is necessarily exactly the same as the other.
Superconducting qubits are regarded as promising because superconductivity is a macroscopic quantum phenomena of Bose Einstein condensation, and so as a macroscopic phenomena, it is easier to control and observe.
This is mentioned e.g. in this relatively early: physicsworld.com/a/superconducting-quantum-bits/. While most quantum phenomena is observed at the atomic scale, superconducting qubits are micrometer scale, which is huge!
Physicists are comfortable with the use of quantum mechanics to describe atomic and subatomic particles. However, in recent years we have discovered that micron-sized objects that have been produced using standard semiconductor-fabrication techniques – objects that are small on everyday scales but large compared with atoms – can also behave as quantum particles.
Atom-based qubits like trapped ion quantum computers have parameters fixed by the laws of physics.
However superconducting qubits have a limit on how precise their parameters can be set based on how well we can fabricate devices. This may require per-device characterisation.
In Ciro's ASCII art circuit diagram notation, it is a loop with three Josephson junctions:
+----X-----+
| |
| |
| |
+--X----X--+
Offers an interesting interpretation of superposition in that type of device (TODO precise name, seems to be a flux qubit): current going clockwise or current going counter clockwise at the same time.
Their circuit is a loop with three Josephson junctions, in Ciro's ASCII art circuit diagram notation:
+----X-----+
| |
| |
| |
+--X----X--+They name the clockwise and counter clockwise states and (named for Left and Right).
When half the magnetic flux quantum is applied as microwaves, this produces the ground state:
where and cancel each other out. And the first excited state is:
Then he mentions that:
- to go from 0 to 1, they apply the difference in energy
- if the duration is reduced by half, it creates a superposition of .
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.
But seriously, this is a valuable little list.
The course is basically exclusively about transmons.