The Limits of Computation: Quantum Computers and Beyond. Scott Aaronson (MIT). GOLDBACH CONJECTURE: TRUE NEXT QUESTION. Things we never see…. Warp drive. Ü bercomputer. Perpetuum mobile.
The Limits of Computation:Quantum Computers and Beyond
Scott Aaronson (MIT)
GOLDBACH CONJECTURE: TRUE
The (seeming) impossibility of the first two machines reflects fundamental principles of physics—Special Relativity and the Second Law respectively
Does physics also put limits on computation?
Extrapolating: Robot uprising?
But even a killer robot would still be “merely” a Turing machine, operating on principles laid down in the 1930s…
And it’s conjectured that thousands of interesting problems are inherently intractable for Turing machines…
Is there any feasible way to solve NP-complete problems, consistent with the laws of physics?
(Why is it so hard to prove PNP? We know a lot about that today, most recently from algebrization [A.-Wigderson 2007])
Time Travel Computer
S. Aaronson and J. Watrous. Closed Timelike Curves Make Quantum and Classical Computing Equivalent, Proceedings of the Royal Society A 465:631-647, 2009. arXiv:0808.2669.
A quantum state of n “qubits” takes 2n complex numbers to describe:
Chemists and physicists knew that for decades, as a major practical problem!
In the 1980s, Feynman, Deutsch, and others had the amazing idea of building a new type of computer that could overcome the problem, by itself exploiting the exponentiality inherent in QM
Shor 1994: Such a machine could also factor integers
What we’ve learned from quantum computers so far:
21 = 3 × 7(with high probability)
The practical problem: decoherence.
A few people think scalable QC is fundamentally impossible ... but that would be even more interesting than if it’s possible!
[A. 2004]: Theory of “Sure/Shor separators”
[BBBV 1994] explained why quantum computers probably don’t offer exponential speedups for the NP-complete problems
[A. 2002] proved the first lower bound (~N1/5) on the time needed for a quantum computer to find collisions in a long list of numbers from 1 to N—thereby giving evidence that secure cryptography should still be possible even in a world with QCs
Limitations of Quantum Computers
4 2 1 3 2 5 4 5 1 3
Recent experimental proposal, which involves generating n identical photons, passing them through a network of beamsplitters, then measuring where they end up
Almost certainly wouldn’t yield a universal quantum computer—and indeed, it seems easier to implement
BosonSampling [A.-Arkhipov 2011]
Nevertheless, our experiment would sample a certain probability distribution, which we give strong evidence is hard to sample with a classical computer
Jeremy O’Brien’s group at the University of Bristol has built our experiment with 4 photons and 16 optical modes on-chip
The Information Content of Quantum StatesFor many practical purposes, the “exponentiality” of quantum states doesn’t actually matter—there’s a shorter classical description that works fine
Describing quantum states on efficient measurements only [A. 2004], “pretty-good tomography” [A. 2006]
10 Years of My Other Research in 1 Slide
Using quantum techniques to understand classical computing better [A. 2004] [A. 2005] [A. 2011]
Quantum Generosity … Giving back because we careTM
Quantum Money that anyone can verify, but that’s physically impossible to counterfeit [A.-Christiano 2012]
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