S/V Tavarua

A link to the paper behind the ‘quantum computers solve problems while off’ pseudo-science story that’s been circulating recently is here. (Kudos to Scott Aaronson for posting the reference in his blog.) Obviously it’s not that simple.

Here’s the abstract:

Suppose that we are given a quantum computer programmed ready to perform a computation if it is switched on. Counterfactual computation is a process by which the result of the computation may be learnt without actually running the computer. Such processes are possible within quantum physics and to achieve this effect, a computer embodying the possibility of running the computation must be available, even though the computation is, in fact, not run. We study the possibilities and limitations of general protocols for the counterfactual computation of decision problems (where the result r is either 0 or 1). If p(r) denotes the probability of learning the result r “for free” in a protocol then one might hope to design a protocol which simultaneously has large p(0) and p(1). However we prove that p(0)+p(1) never exceeds 1 in any protocol and we derive further constraints on p(0) and p(1) in terms of N, the number of times that the computer is not run. In particular we show that any protocol with p(0)+p(1)=1-epsilon must have N tending to infinity as epsilon tends to 0. These general results are illustrated with some explicit protocols for counterfactual computation. We show that “interaction-free” measurements can be regarded as counterfactual computations, and our results then imply that N must be large if the probability of interaction is to be close to zero. Finally, we consider some ways in which our formulation of counterfactual computation can be generalised.

Fantastic news on the Quantum Cryptography front:

A team of Los Alamos National Laboratory scientists, in collaboration with researchers from the National Institute of Standards and Technology in Boulder, Colo., and Albion College, in Albion, Mich., have achieved quantum key distribution (QKD) at telecommunications industry wavelengths in a 50-kilometer (31 mile) optical fiber.

The ESA has some great Cassini-Huygens images of their joint ESA/NASA mission.

“NASA’s Cassini spacecraft will orbit Saturn for four years, making an extensive survey of the ringed planet and its moons. The ESA Huygens probe will be the first to land on a world in the outer Solar System - on the surface of Titan, Saturn’s largest moon.”