#### Theory v Practice

Can someone please explain how to reconcile the slick theory with the grubby practice, when it comes to 'quantum' and 'computing'. I really want to understand...

The University of New South Wales, working with Sandia National Laboratories in New Mexico, is celebrating what it hopes will be another step towards large-scale quantum computing: a technique that can address single electron qubits separated by mere nanometres. Quantum bits - qubits - are the quantum-physics counterpart to the …

Can someone please explain how to reconcile the slick theory with the grubby practice, when it comes to 'quantum' and 'computing'. I really want to understand...

All you need to know is many forms of unbreakable encryption will be able to be brute forced in a matter of seconds and that we are a decade away from the technology. Of course that is also what they said five years ago and been saying about fusion as a controllable energy source for decades. My guess is the technology is so disruptive that if someone does get it working for general problems their national government will confiscate the thing immediately.

Seconded. I've googled quantum computing many times but I've yet to find any detailed explanation of how qubits could exactly find 2 prime co-factors of a large prime number. A 'dummys guide' if you will, for those of us who are literate in conventional architectures and programming. Even if the explanation is dependent on a well behaved and practical qubit. Has anyone invented some sort of programming language for quantum computing? Would be nice to see a (presumably theoretical) code fragment.

etc.

Someone out there in El Reg land must know about this!

Hey, I'm an undergraduate in computer science and applied physics studying quantum computing. For factoring large semiprime numbers, you want to look at Shor's algorithm: http://en.wikipedia.org/wiki/Shor's_algorithm

As a heads up, even the wikipedia article is nasty stuff. That page will be pretty much hieroglyphics unless you have taken quantum mechanics, matrix algebra, and understand BraKet notation.

This page is pretty advanced too, but they link to a lot of resources and appear to have a working applet: http://users.telenet.be/nicvroom/shor.htm

I found this but know nothing about it/not a recommendation: http://www.dwavesys.com/en/dev-tutorial-intro.html

I'll have a go. (If your question is WHY is quantum reality like (or somewhat like) this, no-one has the faintest idea. It's like asking why gravity exists. At present, just assume that $DEITY thought it was a good idea).

A quantum bit is in both states 0 and 1 at once, with a probability of 50% that when it is observed, it will be a 1 or a 0.

Entangled bits are in a state where their properties are mathematically correlated, so for example two qubits represent 0,1,2 and 3 at the same time. Call an ensemble of entangled quantum bits a quantum register. Start so that its N bits represent all humbers in the range 0 to (2^N)-1, with equal probability of 1/(2^N), at once.

Now perform arithmetic operations on your N-bit quantum register. For example if you add a quantum register to itself and then observe it, you are guaranteed an even number. The self-addition operator causes bit 0 to assume a zero probability of it being a one. So far, somewhat trivial.

It gets really interesting if they can make a quantum register with a large number of bits, and perform operations on it such that the probability of it representing any number that is not a factor of a specified much larger number is zero. If the specified number is the product of two large prime numbers, when you observe the quantum register you will obtain one factor or the other. The probability of any other number (pattern of observed bits) is zero. Suddenly not nearly so trivial!

You obtain your other factor, and also check that the quantum entanglement had not failed during your quantum computation, by conventional long division on a conventional computer. Entanglement failure during a quantum computation is akin to a hardware failure in a conventional CPU except far more likely and (on the positive side) not irreversible. For the factorisation application (i.e. cracking N-bit PK cryptography) even an unreliable quantum computer is useful, just as long as it occasionally spits out a right answer with a much higher probability than 1/(2^N) (i.e. guesswork).

Last time I read up on it, they'd managed a four-bit quantum computer and were able to factorize 15 (in other words, when observed after mathematical operators applied, their 4-bit register almost always said 3 or 5, rarely anything else). Of course that's trivial, but a 2048-bit quantum computer would be anything but. I don't know what's the latest N qubits? 30-plus would start getting practically very useful, and probably very secret.

My money is on quantum computing breaking down somewhere between four bits and 2048 bits, and that this will prove to be a gateway to some new physics. (Hopefully not of the Laundry variety, involving a takeover of our universe by beings best not even thought about). Philosophically, I'm not prepared to take on board the implications of a working Megabit quantum computer (or Giga - Tera - Zetta- ...!)

BTW if you are doing research in this field and make a sudden breakthrough of huge magnitude, **your only chance of staying alive and at liberty is to spam the details to as much of the world as you can as fast as possible**. There are also some mathematical theorems (unproven but believed true by almost all mathematicians), for which a **disproof** would have similarly vast real-world implications.

While I don't fully understand it either, I can tell you this about Theory v Practice:

In theory, there is no difference between theory and practice.

In practice, there is a great deal of difference between theory and practice.

Mine's the one I borrowed from the Emperor's new collection, with the cat in the pocket...

The Dwave link was exactly what I was looking for - I feel like I know more about quantum computing than I did. I note that the Dwave tutorials have nothing about cryptography but does have something about quant finance! Not sure what the message is there.

So now just expecting a 'knock on the door' from the NSA.

Thanks for the answers.

Due to the lack of an icon for my brain dribbling out of my ears I choose Paris I feel intellctually equal to her after reading this

Um, what? OK! Is that like a whole new meaning to "quantify"? So will we need regular deliveries of silicon atoms to replace those "long-lived" ones? Hey, a Smartphone app for reordering silicon control atoms! Maybe they could even teleport them to your home. Why is Paris looking so super-smart right now? My head's quantum state hurts.

You've got to marvel at a computing technology that relies on hot phosphene gas.

I prefer silane gas myself (not to be around of course). Anything with a flash point lower than room temperature is be scared and awed. The only other material I was more afraid of in my former career was Hydrofluoric acid. You get a drop on your skin and not know it, you will 12 hours later when you are screaming in agony as it eats your bones from the inside.

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##### Welcome, stranger: Inside Microsoft's command line shell

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