# What type of algorithms are faster with a quantum computer?

I am a beginning CS student and I am learning algorithms. I heard that even with quantum computers, that general sorting algorithms can never have better than $n\log n$ time. However, I also know that factoring algorithms would be a lot faster. In general terms what kind of algorithms would become substantially faster with quantum computers?

• I suggest you rephrase your question. You are really asking which problems can be solved faster with quantum computers. Apr 20, 2015 at 3:59
• Scott Aaronson (the quantum computing guru) has (two versions of a) talk about this subject exactly, titled *When exactly do quantum computers provide a speedup?". You can find it (or rather, them) here: scottaaronson.com/talks. Apr 20, 2015 at 4:03
• As far as I know, none. We need new algorithms. (cf Yuval's first comment.)
– Raphael
Apr 20, 2015 at 8:46
• it is not actually proven that factoring is faster, only conjectured, etc., its an open question P ?= BQP etc
– vzn
Apr 20, 2015 at 15:17
• Jul 18, 2016 at 21:45

• Shor's algorithm which puts FACTORING in BQP (bounded error quantum polynomial-time, effectively the quantum equivalent of P) also can be used to solve the DISCRETE LOGARITHM problem, where we want to find an integer $k$ such that $a^{k} = b$ where $a$ and $b$ are given, in (quantum) polynomial time. The DISCRETE LOGARITHM problem has the same status as the FACTORING problem in that we don't know whether they are polynomial-time solvable, so this might not be a speed up.