Ciro Santilli @cirosantilli 35

 Incoming links: Boltzmann constant

2019 redefinition of the SI base units  Updated 2025-02-22  +Created 1970-01-01

web.archive.org/web/20181119214326/https://www.bipm.org/utils/common/pdf/CGPM-2018/26th-CGPM-Resolutions.pdf gives it in raw:

the unperturbed ground state hyperfine transition frequency of the caesium-133 atom $Δ v_{C s}$ is 9 192 631 770 Hz
the speed of light in vacuum c is 299 792 458 m/s
the Planck constant h is 6.626 070 15 × $1 0^{- 34}$ J s
the elementary charge e is 1.602 176 634 × $1 0^{- 19}$ C
the Boltzmann constant k is 1.380 649 × $1 0^{- 23}$ J/K
the Avogadro constant NA is 6.022 140 76 × $1 0^{23}$ mol
the luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz, Kcd, is 683 lm/W,

The breakdown is:

actually use some physical constant:
- the unperturbed ground state hyperfine transition frequency of the caesium-133 atom $Δ v_{C s}$ is 9 192 631 770 Hz
  Defines the second in terms of caesium-133 experiments. The beauty of this definition is that we only have to count an integer number of discrete events, which is what allows us to make things precise.
- the speed of light in vacuum c is 299 792 458 m/s
  Defines the meter in terms of speed of light experiments. We already had the second from the previous definition.
- the Planck constant h is 6.626 070 15 × $1 0^{- 34}$ J s
  Defines the kilogram in terms of the Planck constant.
- the elementary charge e is 1.602 176 634 × $1 0^{- 19}$ C
  Defines the Coulomb in terms of the electron charge.
arbitrary definitions based on the above just to match historical values as well as possible:
- the Boltzmann constant k is 1.380 649 × $1 0^{- 23}$ J/K
  Arbitrarily defines temperature from previously defined energy (J) to match historical values.
- the Avogadro constant NA is 6.022 140 76 × $1 0^{23}$ mol
  Arbitrarily defines the mol to match historical values. In particular, the kilogram is not an exact multiple of the weight of an atom of hydrogen.
- the luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz, Kcd, is 683 lm/W
  Arbitrarily defines the Candela in terms of previous values to match historical records. The most useless unit comes last as you'd expect.

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Superconducting quantum computing  Updated 2025-02-22  +Created 1970-01-01

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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:

transmon

Input:

microwave radiation to excite circuit, or do nothing and wait for it to fall to 0 spontaneously
interaction: TODO
readout: TODO

Video 1.

Quantum Computing with Superconducting Qubits by Alexandre Blais (2012)

Source.

Video 2.

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.

Video 3.

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 circuit
```
+-----+
|     |
|   +-+-+
|   |   |
C   X   X
|   |   |
|   +-+-+
|     |
+-----+
```
using the newly created just now Ciro's ASCII art circuit diagram notation. Note that the block on the right is a SQUID device.
youtu.be/uPw9nkJAwDY?t=471 mentions that the frequency between states 0 and 1 is chosen to be 6 GHz:
- 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 about $6^{9} \times h = 6 \times 1 0^{9} \times 6.62 \times 1 0^{- 34} \approx 4 \times 1 0^{- 24} J$
  From the definition of the Boltzmann constant, the temperature which has that average energe of particles is of the order of:
  $T = E / k_{b} = 4 \times 1 0^{- 24} / 1.38 \times 1 0^{- 23} \approx 0.3 K$
  (1)
This explains why we need to go to much lower temperatures than simply the superconducting temperature of aluminum!

Video 4.

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

Video 5.

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

Video 6.

Superconducting Qubits I Part 2 by Zlatko Minev (2020)

Source.

Video 7.

How to Turn Superconductors Into A Quantum Computer by Lukas's Lab (2023)

Source. This video is just the introduction, too basic. But if he goes through with the followups he promisses, then something might actually come out of it.

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Temperature  Updated 2025-02-22  +Created 1970-01-01

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For scales from absolute 0 like Kelvin, is proportional to the total kinetic energy of the material.

The Boltzmann constant tells us how much energy that is, i.e. gives the slope.

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