Thread by @ngkabra

1/n In about a month, Google is expected to announce that they've achieved Quantum Supremacy. If confirmed, this will be a historic moment, similar to the Wright Brother's first flight.

Here's a thread with an over-simplified explanation of what that means.

2/n There's lots of media hype around this announcement (because a copy of the paper was leaked inadvertently). However, much of that is misleading.

Scott Aaronson's Quantum Supremacy FAQ is the best resource for sensible details about this announcement: scottaaronson.com/blog/?p=4317

3/n For those too lazy to read Scott's FAQ, here are the oversimplified highlights.

Google has managed to build a 53-qubit Quantum Computer which can solve a (very) contrived problem in a fraction of a second which will take months with huge data-centers full of normal computers

4/n (That's probably why the announcement hasn't yet gone out. Google is doing the massive normal-computer computation to confirm that their Quantum Computer actually produced the expected result)

5/n However, this is a contrived problem, solved using 53-qubits. We are nowhere close to solving a useful problem with Quantum Computers. That would require about 1000s of qubits, and we don't even know if we can build QCs with that many qubits.

6/n Even if we built a QC with 1000s of qubits, it is very likely that the qubits would lose the data before we could read the output, because of interference with the outside world ("decoherence").

A proposed solution to that is Quantum Error Correction.

7/n So far, we don't have practical algorithms for Quantum Error Correction for 1000s of qubits, and even if we did, that would increase the required number of qubits from 1000s to millions.

8/n In other words, notwithstanding this announcement, Quantum Computers are very far from being able to break all our encryption algorithms. Nobody is in any immediate danger yet.

9/n If we're so far from doing useful things with Quantum Computers, why is this announcement a big deal?

Let's go back to the Wright Brothers' first flights: they flew only 10 feet above the ground, and for less than a minute.

This was by no means "useful"

10/n It would take decades before airplanes would be able to do "useful" things that were difficult to do using conventional means.

But the first flight put a permanent end to all the critics claiming that heavier-than-air powered flight was impossible.

11/n Similarly, until now, there have been people claiming that Quantum Computers would never do any computation that couldn't be done efficiently using classical computers.

These kinds of criticisms can have serious impact on researchers and funding agencies.

That stops now.

12/end
I should repeat. Read Scott Aaronson's Quantum Supremacy FAQ. It is easy-to-understand and goes into many details that I skipped over. scottaaronson.com/blog/?p=4317

13/12 If I get time over the next few days, I'll try to do a thread with an Idiot's Introduction to Quantum Computing. Stay Tuned.

14/12 (Because people have been asking me this question)
Bitcoin/cryptocurrencies are safe for now. By the time Quantum Computers are able to break the cryptographic algorithms used in cryptocurrencies, they will have shifted over to Quantum Resistant Algorithms.
cc: @bidu676