4 K. Enough for to make "low temperature superconductors" like regular metals superconducting, e.g. the superconducting temperature of aluminum if 1.2 K.
Contrast with liquid nitrogen, which is much cheaper but only goes to 77K.
Based on the Josephson effect. Yet another application of that phenomenal phenomena!
Philosophically, superconducting qubits are good because superconductivity is macroscopic.
It is fun to see that the representation of information in the QC basically uses an LC circuit, which is a very classical resonator circuit.
As mentioned at en.wikipedia.org/wiki/Superconducting_quantum_computing#Qubit_archetypes there are actually a few different types of superconducting qubits:
- flux
- charge
- phase
and hybridizations of those such as:
Input:
- microwave radiation to excite circuit, or do nothing and wait for it to fall to 0 spontaneously
- interaction: TODO
- readout: TODO
Quantum Transport, Lecture 16: Superconducting qubits by Sergey Frolov (2013)
Source. youtu.be/Kz6mhh1A_mU?t=1171 describes several possible realizations: charge, flux, charge/flux and phase.Building a quantum computer with superconducting qubits by Daniel Sank (2019)
Source. Daniel wears a "Google SB" t-shirt, which either means shabi in Chinese, or Santa Barbara. Google Quantum AI is based in Santa Barbara, with links to UCSB.- youtu.be/uPw9nkJAwDY?t=293 superconducting qubits are good because superconductivity is macroscopic. Explains how in non superconducting metal, each electron moves separatelly, and can hit atoms and leak vibration/photos, which lead to observation and quantum error
- youtu.be/uPw9nkJAwDY?t=429 made of aluminium
- youtu.be/uPw9nkJAwDY?t=432 shows the circuit diagram, and notes that the thing is basically a LC circuitusing the newly created just now Ciro's ASCII art circuit diagram notation. Note that the block on the right is a SQUID device.
+-----+ | | | +-+-+ | | | C X X | | | | +-+-+ | | +-----+ - youtu.be/uPw9nkJAwDY?t=471 mentions that the frequency between states 0 and 1 is chosen to be 6 GHz:This explains why we need to go to much lower temperatures than simply the superconducting temperature of aluminum!
- higher frequencies would be harder/more expensive to generate
- lower frequencies would mean less energy according to the Planck relation. And less energy means that thermal energy would matter more, and introduce more noise.6 GHz is aboutFrom the definition of the Boltzmann constant, the temperature which has that average energe of particles is of the order of:
A Brief History of Superconducting quantum computing by Steven Girvin (2021)
Source. - youtu.be/xjlGL4Mvq7A?t=138 superconducting quantum computer need non-linear components (too brief if you don't know what he means in advance)
- youtu.be/xjlGL4Mvq7A?t=169 quantum computing is hard because we want long coherence but fast control
Superconducting Qubits I Part 1 by Zlatko Minev (2020)
Source. The Q&A in the middle of talking is a bit annoying.
- youtu.be/eZJjQGu85Ps?t=2443 the first actually useful part, shows a transmon diagram with some useful formulas on it