The Time-Traveling Quantum Computer
New work by researchers indicates that in harnessing entanglement, NP-complete problems can be solved efficiently even when all time-traveling systems are completely isolated from the past.
(An open timelike curve)
Deutschian timelike curves.
(a) depicts a physical visualization of a CTC,
where an object entering one mouth of a wormhole
at some point tA may jump to a prior time tB
(with respect to an chronology respecting observer)
and interact with its past self via some unitary U.
(b) In the special case where no interaction occurs,
we obtain an open timelike curve. This naturally occurs,
for example, in instances where the wormhole mouths
are spatially separated.
Around ten years ago researcher Dave Bacon, now at Google, showed that a time-travelling quantum computer could quickly solve a group of problems, known as NP-complete, which mathematicians have lumped together as being hard.
The problem was, Bacon's quantum computer was travelling around 'closed timelike curves'. These are paths through the fabric of spacetime that loop back on themselves. General relativity allows such paths to exist through contortions in spacetime known as wormholes...
However, the new work shows that a quantum computer can solve insoluble problems even if it is travelling along "open timelike curves", which don't create causality problems. That's because they don't allow direct interaction with anything in the object's own past: the time travelling particles (or data they contain) never interact with themselves. Nevertheless, the strange quantum properties that permit "impossible" computations are left intact.
Stephen Baxter wrote about a CTC (Closed-Timeline-Curve Time-Travel Computer) in his 2004 novel Exultant:
But Nilis had assigned her to another part of the project, the development of his "CTC computer" as he called it, his closed-timeline-curve time-travel computing machine.
Via PhysOrg; thanks to Winchell Chung (@nyrath) of Project Rho for pointing this one out and providing an sf reference!
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