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    Un-computable quantum maze computed by quantum-maze computer / ArsTechnica · Monday, 7 December, 2020 - 20:10

Green lights illuminate what appears to be a glass box.

Enlarge / One beam enters, two beams leave. (credit: Melissa Meister / Flickr )

I am a great believer in solving problems with lasers. Are you suffering from a severely polarized society and a fast-growing population living below the poverty line? Well, I have the laser to solve all your problems.

OK, maybe not. But when it comes to quantum computing, I am of the belief that lasers are the future. I suspect that the current architectures are akin to the Colossus or the ENIAC: they are breakthroughs in their own right, but they are not the future. My admittedly biased opinion is that the future is optical. A new paper provides my opinion some support, demonstrating solutions to a mind-boggling 10 30 problem space using a quantum optical system. Unfortunately, the support is a little more limited than I'd like, as it is a rather limited breakthrough.

Photons flipping coins

The researchers have demonstrated something called a Gaussian boson sampling system. This is essentially a device designed to solve a single type of problem. It's based on devices called "beam splitters," so let's start with a closer look at how those work.

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    Quantum reality is either weirdly different or it collapses / ArsTechnica · Wednesday, 19 August, 2020 - 16:17 · 1 minute

Eugene Wigner.

Enlarge / Eugene Wigner. (credit: Denver Post Inc (Photo By David Cupp/The Denver Post via Getty Images))

Quantum mechanics, when examined closely, poses some deep questions about reality. These questions often take the form of thought experiments, which are later (usually much later) followed up by real experiments. One of the most difficult and deepest of these is a thought experiment proposed by Eugene Wigner in the 1960s, called "Wigner’s friend" (you don’t want to be Wigner’s friend). Now, much later, Wigner and his friend have been formalized and extended. The result sets us up with a contradiction: either reality is a lot stranger and less real at the level of quantum mechanics, or quantum states cannot possibly exist at large scales.

Don’t be Wigner’s friend

To understand why Wigner shouldn’t have any friends, we have to first dive into some details of quantum mechanics. Imagine measuring the spin of a single electron. Spin has an orientation in space, but it is not possible to measure that orientation. Instead, we have to choose an orientation and measure the spin along that orientation. So we might ask an electron if its spin is vertically upward or downward. The result (all else being equal) will be either up or down with 50 percent probability.

Let’s say we measure the spin and find that it is up. Any subsequent measurements will confirm it is up, as well. The measurement has defined the vertical spin component (a process often called wave function collapse). But it says nothing about the horizontal component of the spin—the horizontal component will remain in a superposition of left and right spins. That means that if we rotate our apparatus so that we are measuring spin left and right, the result will be random—the electron will be either spin-left or spin-right with 50 percent probability.

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