20120720, 07:26  #1 
Aug 2006
3·1,993 Posts 
What would you do with a small quantum computer?
Suppose you had a chance to use an 800qubit quantum computer. Yes, that's way beyond what we can make; maybe it's from a genie or an alien. You can't sell it or examine its inner workings (it's tamperproof); you can only use it on your own. What could you do? Suppose decoherence time is sufficiently large.
As far as I can tell you could factor smallish numbers fairly quickly, though I'm not sure how large or how fast. "How large" has to do with the amount of quantum error correction needed; if you don't need any you could factor numbers up to 398 bits (Beauregard's version of Shor's algorithm), or somewhat fewer with an errorcorrecting code (I'm guessing about 350 using CalderbankRainsShorSloane, by extrapolation from Table III, in the best case that you only need to correct a single error). What use could you find for lots of somewhatsmall factorizations? What other algorithms besides Shor's would be useful on a quantum computer of this size? (I can't imagine Grover's algorithm would be useful on any quantum computer that will be built in my lifetime, it seems to require too many qubits and give too little speedup.) 
20120720, 13:28  #2 
"Jason Goatcher"
Mar 2005
3·7·167 Posts 
I don't pretend to know the ins and outs of quantum computing, but couldn't the problem be stated by a regular computer, then solved by the quantum computer side?
Additionally, what about having leading bits, and then sieve each portion with the quantum part. So you could sieve in chunks of 2^398, or whatever, bits. Again, I don't understand quantum computers, maybe it's a requirement that the entire program be contained within the qubits. Edit: Alternately, couldn't any animal with an organic brain count as a quantum computer? If I carry a mouse in my pocket, am I not also carrying a quantum computer in my pocket? Not the way CRGreathouse intended, but, hey, it's that kind of forum. Last fiddled with by jasong on 20120720 at 13:31 
20120720, 15:48  #3 
Just call me Henry
"David"
Sep 2007
Cambridge (GMT/BST)
2^{5}·5·37 Posts 
How fast would it factorize small numbers of maybe 100 bits? Would it be fast enough to use factorizing large primes in NFS?

20120720, 16:12  #4 
Undefined
"The unspeakable one"
Jun 2006
My evil lair
61×103 Posts 
Me? Personally? Probably nothing. There would appear to be very little of value that one could do with an 800bit QC.
So nothing really except for purely research things. Like, perhaps, examining it with a view towards making a larger QC? 
20120720, 16:13  #5  
"Ben"
Feb 2007
2·1,789 Posts 
Quote:


20120720, 16:22  #6 
Romulan Interpreter
Jun 2011
Thailand
2×3×7×233 Posts 
I would certainly find all factors F with less then 402 bits of the mersenne numbers with prime exponent p smaller then 1G. That is feasible if one is able to factor really fast (instant) the odd D in D=(F1)/(2^s), then check for all factors p of D if F divides Mp. 800 (qu)bits is enough for this. Say that computer runs at 3GHz, it would need about 2*log2(10^9) clocks to clear one F (too see if this F is factor of any Mp with p smaller then 10^9) so this time is negligible comparing with primality prove. So, the task is reduced at enumerating all primes which are 1 or 7 mod 8 from 1 bit to 402 bits
(But what need eons of eons of eons on a sequential computer might be much fast on the QC, as every qubit has an infinite amount of states, all F's might be tested in parallel  hold your stone, that is me dreaming!) Last fiddled with by LaurV on 20120720 at 16:25 
20120720, 16:51  #7 
Sep 2009
7·311 Posts 
I'd use it to speed up the work I'm doing to factor the smaller composites in factordb. There are plenty to do.
Just how big a number it can factor makes a lot of difference. 398 bits is about 120 digits, there are about 135000 composites under that size. 350 bits is about 107 digits, but there are still a few thousand composites in that range. Chris 
20120720, 17:08  #8 
Bamboozled!
"πΊππ·π·π"
May 2003
Down not across
25313_{8} Posts 
Sell it on eBay?

20120720, 18:15  #9 
"Gang aft agley"
Sep 2002
2·1,877 Posts 
I think it would be best utilized for algorithmic development.
At first blush one might think that quantum algorithms can be developed well enough without experimental mathematical approaches or any actual quantum hardware (since we do that already), but looking at how we develop algorithms for conventional hardware, having actual machines and including tables of data and timings is really useful. Everyone goes on about factoring and database lookup but there is relations work that could be useful. First faster quantum algorithms for solving systems of linear equations (HHL). Second what PSLQ like algorithms would be possible? Last fiddled with by only_human on 20120720 at 18:28 Reason: grammar s/there is all kinds of relations work, there is relations work/ and it still looks wrong 
20120721, 02:22  #10 
Apr 2012
Gracie on alert.
110010000_{2} Posts 
Program it to play a scaled down version of GO.
Bind it to a simple cellular/biological process to determine if it can echo or resonate with this process in real time. Examine what exactly composes the geometry of space/time  may possibly require another quantum box to decipher and present the results in one's preferred formalism. An old Calvin and Hobbes comic had Calvin saying (when looking at a large dinosaur) "it's so big you have to look at it twice to see it once." Last fiddled with by jwaltos on 20120721 at 02:33 
20120721, 05:48  #11  
Romulan Interpreter
Jun 2011
Thailand
2·3·7·233 Posts 
You didn't pay attention!
Quote:
(from kungfu panda 1st: "what are you going to do big guy, sit on me?" "don't tempt me"..) 

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