[vamos_davai]

I don't really understand quantum computing, and hopefully someone would shed light and let me know if my understanding is wrong. Isn't a qubit similar to a transistor that can hold more than 2 states and the implication is that it'll be faster than current SIMD operations?

[roywiggins]

It's weirder than that. Some quantum algorithms offer a big-O speedup over classical ones. Shor's algorithm is O(n), which is categorically faster than any known classical algorithm for integer factoring. It's polynomial time, whereas the fastest classical algorithm is subexponential. For every subexponential algorithm, there will be some n at which Shor's algorithm will be faster, and some slightly higher n at which it's much, much, much faster.

https://en.wikipedia.org/wiki/Shor%27s_algorithm

[epicwhaleburger]

In no way am I qualified to talk about this with any sort of certainty, but I have been studying EE for the past 2 years and live in a house with a physicist who is probably far more well read in this topic. Classical computers are nice because they are predictable; if you want to order some product online, it would't be very helpful if the bit stream of data coming into your computer was fuzzy and muddled, making it hard to tell a 0 or a 1 apart. Quantum computing takes advantage of quantum properties such as the uncertainty principle and quantum entanglement to store bits in a state of being either a 0 or 1 until we "observe" the bit coming through (quantum bits -> qubit). This allows for algorithms, experiments, and other things quantum to be ran much faster than any classical computer since it can encode more data into a single bit. To minimize the fuzz in a q-computer, they freeze the sh*t out of it to keep the energy at a minimum, along with using conducting materials which excel at conducting heat away (like diamonds). I believe the spin of an electron is what determines the qubit value and when used in conjunction with quantum entanglement will affect other electrons that are already entangled. My little knowledge ends here but with this thought on quantum entanglement, you can see how flipping bits values respectively can allow for more ways to store information with the same amount of "stuff" remaining constant vs. a classical computer.

[jfengel]

A quantum computer doesn't work much like a classical computer at all, so any analogy to transistors fails pretty quickly.

One key aspect of that is that all of the qubits are entangled. The qubit isn't in more than two states, but in 2 states at once, and N qubits are in 2^N states at once. It's not quite as simple as that: you can't really just set up any algorithm in a quantum computer the way you would with a classical one. But for those problems amenable to quantum computation (including, notably, prime factoring), they can be solved very fast.

With 2^53 qubits we should be able to factor some very, very large prime numbers.

[scottlocklin]

You will find that this machine can't even factor the number 15 properly, as it's not error corrected, or a general purpose quantum computer. FWIIW nobody has factored the number 15 using quantum algorithms using the Shor algorithm yet; only using the subset of gates they know produces the numbers 3 and 5.

[acqq]

For those who are interested in the papers about this topic:

https://arxiv.org/abs/1301.7007

"Pretending to factor large numbers on a quantum computer (2013)"

"Of course this should not be considered a serious demonstration of Shor’s algorithm. It does, however, illustrate the danger in “compiled” demonstrations of Shor’s algorithm. To varying degrees, all previous factorization experiments have benefited from this artifice. While there is no objection to having a classical compiler help design a quantum circuit (indeed, probably all quantum computers will function in this way), it is not legitimate for a compiler to know the answer to the problem being solved. To even call such a procedure compilation is an abuse of language."

More references:

https://crypto.stackexchange.com/questions/59795/largest-int...

[cthalupa]

You keep repeating this, but as best as I can tell, it hasn't been factually accurate in a decade, unless you really want to quibble over the fact that they finished the calculation on a general purpose computer.

https://www.schneier.com/blog/archives/2009/09/quantum_compu...

>Instead, it came up with an answer to the "order-finding routine," the "computationally hard" part of Shor's algorithm that requires a quantum calculation to solve the problem in a reasonable amount of time.

They weren't the first team to do this, either, they just miniaturized parts of the hardware.

[scottlocklin]

The result you cite doesn't even say what you think it says; in fact it directly says they didn't factor a damn thing.

If I'm wrong and IBM's latest "quantum computer" can actually do the Shor algorithm on the number 15 using 4608 gate operations, I will publically eat one of my shoes like Werner Herzog. Assuming I can get someone else to take the other side of the bet, of course.

[gjm11]

It says:

> the chip itself didn't just spit out 5 and 3. Instead, it came up with an answer to the "order-finding routine"

O noes! The quantum computer only did the "order-finding" part of Shor's algorithm! ... But wait. Here's the Wikipedia page for Shor's algorithm:

> Shor's algorithm consists of two parts: 1. A reduction, which can be done on a classical computer, of the factoring problem to the problem of order-finding. 2. A quantum algorithm to solve the order-finding problem.

So, the article GP cited says that a quantum computer did the part of factoring the number 15 that actually uses a quantum computer. Nothing wrong with that.

The article's link to the actual paper is broken, which makes it harder to tell whether as you say they cheated somehow, but here's another article about factoring 15 with a quantum computer https://science.sciencemag.org/content/351/6277/1068 which so far as I can see claims to have actually done the whole of (the relevant part of) Shor's algorithm on actual QC hardware.

[scottlocklin]

Even assuming I humor you and refuse to notice they left out the gates to the wrong answer, and assuming I humor the authors in agreeing that this is a scalable form of the Shor algorithm (I deny both of these for the record) .... congratulations: by 2016 someone was able to factor the number 15. I guess quantum supremacy is right around the corner!

[vsef]

An explanation in comic form with text by quantum computing researcher Scott Aaronson: https://www.smbc-comics.com/comic/the-talk-3

QC is about setting up interference patterns between the qbits, it is fundamentally unlike classical computing. For some problems algorithms can be designed that use those interferences to compute things, like the famous Shor's algorithm for polynomial time prime factoring.

QC speeds things up only in the cases where an asymptotically faster algorithm can be designed this way, it is not a general purpose parallelization mechanism.

And we don't know many things it speeds up for sure! For example: we don't actually know that there isn't a classical polynomial time prime factors algorithm we haven't found. Here is a recent example of a quantum algorithm leading to the discovery of a classical algorithm that is equivalent to the quantum: https://www.quantamagazine.org/teenager-finds-classical-alte...

[seamyb88]

> Isn't a qubit similar to a transistor that can hold more than 2 states ...

Not really. From an information theory perspective, holding two states is the most efficient way of computing. Or at least, there is no speed-up gained from doing computations in any other base than 2.

Quantum computers can be in both states at the same time (as far as that interpretation of quantum mechanics goes). So, if you keep the qubits all in this superposition state, you can calculate multiple things at the same time.

[mrob]

>From an information theory perspective, holding two states is the most efficient way of computing.

Assuming the cost of operations on each digit is a multiple of its number of states, the most efficient base for computing with arbitrary numbers is 3, which is the closest integer to the optimal base e:

https://en.wikipedia.org/wiki/Radix_economy

https://en.wikipedia.org/wiki/Ternary_computer

[seamyb88]

Yes, you are correct. I said it all wrong.

From a physical perspective, holding two states is the most efficient way of computing. I.e. the electronics required to compute in bases other than 2 involve higher power and present difficult challenges.

[PeCaN]

Since other people have already explained how qubits are not at all similar to transistors, I'd like to mention that it is not comparable at all to SIMD and there is no indication that quantum computers will be better at vector math than classical computers.

[mattferderer]

I found this video to be a great explanation https://www.youtube.com/watch?v=OWJCfOvochA

Explains to 5 different levels of CS knowledge.

[grumpy8]

The gist of it is:

>> that can hold more than 2 states

--> that can hold more than 2 states AT THE SAME TIME (in weird quantum conditions)

So you're not trying 00, 01, 10, 11; you're trying [00|01|10 |11] at the same time, as well as other state in between 0 and 1.