Ciro Santilli @cirosantilli  40

) as the first power of 2 that is outside of the human hearing range, and then how a frequency divider is used to reduce the frequency to get the second counter.

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Atomic clock Updated 2025-07-16

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How atomic clock works? answer by Ciro Santilli on Physics Stack Exchange.

Video 1.

How an atomic clock works, and its use in the global positioning system (GPS) by EngineerGuy (2012)

Source. Shows how conceptually an atomic clock is based on a feedback loop of two hyperfine structure states of caesium atoms (non-radioactive caesium-133 as clarified by the Wikipedia page). Like a quartz clock, it also relies on the piezoelectricity of quartz, but unlike the quartz clock, the quartz is not shaped like a tuning fork, and has a much larger resonating frequency of about 7 MHz. The feedback is completed by producing photons that resonate at the right frequency to excite the caesium.

Video 2.

Inside the HP 5061A Cesium Clock by CuriousMarc (2020)

Source.

A similar model was used in the Hafele-Keating experiment to test special relativity on two planes flying in opposite directions. Miniaturization was key.

Contains a disposable tube with 6g of Caesium. You boil it, so when it runs out, you change the tube, 40k USD. Their tube is made by Agilent Technologies, so a replacement since that opened in 1999, and the original machine is from the 60s.

Detection is done with an electron multiplier.

youtu.be/eOti3kKWX-c?t=1166 They compare it with their 100 dollar GPS disciplined oscillator, since GPS satellites have atomic clocks in them.

Video 3.

Quick presentation of the atomic clock at the National Physical Laboratory (2010)

Source. Their super accurate setup first does laser cooling on the caesium atoms.

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Clifford plus T Updated 2025-07-16

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Set of quantum logic gate composed of the Clifford gates plus the Toffoli gate. It forms a set of universal quantum gates.

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Gottesman-Knill theorem Updated 2025-07-16

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Making the Cisco connection Updated 2025-07-16

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archive.org/details/makingciscoconne0000bunn on the Internet Archive Open Library.

Nothing phenomenally new on the early days to add on top of Video "Nerds 2.0.1 excerpt about Cisco (1998)", but a few new good points:

Cisco at one point became the largest company by market capitalization. This wore off a bit as of 2020.
They used this overvalued stock in part to buy many other (often also overvalued) up and coming companies. This acquisition spree strategy was apparently not the norm at the time. rohitnair.wordpress.com/2011/12/13/cisco-history-cisco-systems-history-and-trivia-brand-history-and-trivia/ mentions they have bought more than 140 companies since, and that they call this strategy "Build, Buy And Partner"
a big part of what Cisco did was to allow cheap local communication in-campus. At that time, the ARPANET was already up and running, but their "routers", called Interface Message Processors were very expensive at about $100,000, and to send data across the campus you had to go through them, which meant expensive bandwidth. The routers sometimes failed, and the fallback was to send students around with disks: "sneakernet". They needed new local protocols and hardware to efficiently connect different campus networks.
Sandy Lerner nude photo
Cisco was a pioneer in having an Internet support forum. Customers could also help one another. This was fundamental in scaling support, as they grew so fast it would be impossible to hire a support team large enough without the help of the forum.
Cisco gave out source code to some customers who would then implement protocols they cared about, and Cisco would then merge it back