[quantum-info] Fwd: [Iqcmembers] Today's 12:30 lunch colloquium: Charles H. Bennett - "Time travel for nerds: would closed timelike curves speed up hard computations or crack codes?" - RAC1 2009

[William Matthews]

Begin forwarded message:

> From: Will Matthews <w2matthe at iqc.ca>
> Date: Mon 7 May 2012 10:19:45 EDT
> To: iqcmembers <iqcmembers at iqc.ca>, iqc-seminar at maillist.math.uwaterloo.ca
> Subject: [Iqcmembers] Today's 12:30 lunch colloquium: Charles H. Bennett - "Time travel for nerds: would closed timelike curves speed up hard computations or crack codes?" - RAC1 2009
> 
> Charles H. Bennett - IBM
> 
> "Time travel for nerds: would closed timelike curves speed up hard computations or crack codes?"
> 
> We study discuss various models of closed timelike curves (CTCs), and their
> utility (assuming they exist) for distinguishing nonorthogonal quantum states
> or speeding up hard computations. CTCs are notorious for giving rise to the
> "grandfather paradox"---initial conditions admitting no consistent future. One
> widely used model of CTCs, Deutsch's 1991 mixed-state-fixed-point model,
> abolishes the grandfather paradox, and was formerly thought to have dramatic
> consequences for quantum computation and cryptanalysis. However, we show that
> when the tasks of computation and state discrimination are properly formulated,
> such CTCs provide no help in state discrimination and have not been shown to
> help speed up hard computations. An alternate "post-selected" CTC model,
> exploits the grandfather paradox instead of avoiding it, using it as a putative
> physical mechanism for enforcing the otherwise abstract mathematical idea of
> post-selection. We discuss whether the existence of such CTCs would have major
> consequences for computation, cryptography and ordinary notions of causality.
> 
> Joint work with Debbie Leung, Graeme Smith, and John Smolin.
> 
> http://iqc.uwaterloo.ca/news-events/calendar/generated/charles-h-bennett-2012-5-7
> _______________________________________________