Ciro Santilli @cirosantilli  40

The CNOT gate is a controlled quantum gate that operates on two qubits, flipping the second (operand) qubit if the first (control) qubit is set.

This gate is the first example of a controlled quantum gate that you should study.

To understand why the gate is called a CNOT gate, you should think as follows.

First let's produce a generic quantum state vector where the control qubit is certain to be 0.

On the standard basis:we see that this means that only $|00⟩$ and $|01⟩$ should be possible. Therefore, the state must be of the form:where $x$ and $y$ are two complex numbers such that $|x|+|y|=1.0$

If we operate the CNOT gate on that state, we obtain:and so the input is unchanged as desired, because the control qubit is 0.

If however we take only states where the control qubit is for sure 1:

Therefore, in that case, what happened is that the probabilities of $|10⟩$ and $|11⟩$ were swapped from $x$ and $y$ to $y$ and $x$ respectively, which is exactly what the quantum NOT gate does.

So from this we understand more concretely what "the gate only operates if the first qubit is set to one" means.

Now go and study the Bell state and understand intuitively how this gate is used to produce it.

Toy/test/tought experiment algorithm.

TODO evaluate. No pip install???

Ciro Santilli's discord ID: cirosantilli#8921. See also: how to contact Ciro Santilli.

You gotta be born after the year 2000 to understand it.

This is becoming more and more popular as a group chat with channels and threads possibility as of 2020.

Very similar to Slack.

No user URLs? support.discord.com/hc/en-us/community/posts/360041519131-UserProfilesLinks

They force your username to have 4 random digits? www.reddit.com/r/discordapp/comments/43kjdl/whats_the_number_next_to_the_username/

Not possible to anonymously join just one server without creating a new account? What's the point of servers then! www.reddit.com/r/discordapp/comments/6gmjl7/changing_nick_before_joining_a_new_server/ Oh, also nicks don't hide your username from the server in any way, you can get the original username by just clicking on the person's username.

No proper threaded discussion without creating new channels? As of 2022 there is kind of a way, but it was a bit obtuse.

As of 2022 they also have a school hub: support.discord.com/hc/en-us/articles/4406046651927-Discord-Student-Hubs-FAQ which auto creates groups by university email access. Good idea, and shows popularity amongst that user group.

Servers don't have an ID to join them? www.reddit.com/r/discordapp/comments/b9zdt6/join_discord_server_from_id/

Can only make public servers if you have 1000 members?? support.discord.com/hc/en-us/articles/360023968311 Why so much bullshit?? www.reddit.com/r/discordapp/comments/6jouf8/how_do_i_make_my_server_public/

Also asked at: webapps.stackexchange.com/questions/163441/how-to-create-a-public-discord-server-that-anyone-can-join-without-an-invite

Ciro Santilli worked on it for a brief time in 2016, when it was still called Ring, before he got fired. :-)

The people were quite nice and the project idea is fine, Ciro hopes they succeed.

No chat only? .... community.jitsi.org/t/chat-function-only/79067

As of 2020: end-to-end encryption optional and turned off as default, and marked as experimental...

Appears to be based on XMPP: community.jitsi.org/t/jitsi-users-is-jitsi-a-regular-xmpp-server/13211

Basic must haves:

Other cool stuff:

TODO what's the fucking official discussion/feature request forum?

Feature overview:

Not end-to-end encrypted by default, WTF... you have to create "secret chats" for that:

You can't sync secret chats across devices, Signal handles that perfectly by sending E2EE messages across devices:This is a deal breaker because Ciro needs to type with his keyboard.

Desktop does not have secret chats: www.reddit.com/r/Telegram/comments/9beku1/telegram_desktop_secret_chat/ This is likey because it does not store chats locally, it just loads from server every time as of 2019: www.reddit.com/r/Telegram/comments/baqs63/where_are_chats_stored_on_telegram_desktop/ just like the web version. So it cannot have a private key.

Allows you to register a public username and not have to share phone number with contacts: telegram.org/blog/usernames-and-secret-chats-v2.

Has Reproducible builds on Android and iOS: core.telegram.org/reproducible-builds

Self deleting messages added to secret chats in Q1 2021: telegram.org/blog/autodelete-inv2

Can delete messages from the device of the person you sent it to, no matter how old.

Claimed to remove metadata from servers unless legally obliged to collect it: www.quora.com/Does-WhatsApp-store-messages-on-its-servers-or-is-all-deleted-after-delivery-and-only-stored-on-recipients-phones/answer/Ciro-Santilli

They've had a few breaches: www.whatsapp.com/security/advisories/

They claim to delete metadata: www.quora.com/Does-WhatsApp-store-messages-on-its-servers-or-is-all-deleted-after-delivery-and-only-stored-on-recipients-phones/answer/Ciro-Santilli