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Some of us have been reading Michael Nielsen's paper on a geometric approach to using quantum lower bounds (in brief, the construction of a Finsler metric on $SU(2^n)$ such that the geodesic distance from $I$ to an element $U$ is a lower bound on the number of gates in a quantum circuit that computes $U$).

I was wondering if there were concrete examples of problems where this program led to a lower bound that came close to, matched or beat prior lower bounds obtained by other means ?

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  • $\begingroup$ Also, how does this program compare to Ketan Mulmuley's on "Geometric Complexity Theory"? Mulmuley's program turns the problem of finding a lower bound to an upper bounding problem. But here we are looking for a lower bound on the geodesic as I understand from your question, right? $\endgroup$ – MCH Dec 6 '11 at 3:21
  • $\begingroup$ It's a different program: in some ways more concrete, and useful for specific lower bounds (or maybe - that's what the question is about) $\endgroup$ – Suresh Venkat Dec 6 '11 at 4:07
  • $\begingroup$ crossposted on theoretical physics (theoreticalphysics.stackexchange.com/questions/651/…) $\endgroup$ – Suresh Venkat Dec 10 '11 at 23:07
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    $\begingroup$ possible duplicate of Reading up on $BQP = BPP^{BQNC}$ $\endgroup$ – Greg Kuperberg May 17 '15 at 15:23
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Not exactly what you are looking for, I know, but geodesics have been used to prove optimal state transfer rates in Ising spin chains (see arXiv:0705.0378). I'm not sure how related this is to Nielsen's approach, as I haven't read that particular paper, but I remember thinking this was quite a neat result when it first came out. Basically this is the minimum time to transfer a quantum state from one end of a linear array of qubits to the other. It is a very simple problem, but in the above paper they show that the transfer can be achieved significantly faster than was previously believed (although of course there is still a linear scaling, with the speed-up in the constant).

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