QUANTUM BOMB
A BOMB triggered by a single photon of light is a scary thought. If such a thing existed in the classical world, you would never even be aware of it.
Any photon entering your eye to tell you about it would already have set off the bomb, blowing you to kingdom come.
With quantum physics, you stand a better chance. According to a scheme proposed by the Israeli physicists
Avshalom Elitzur
Lev Vaidman in 1993, you can use quantum trickery to detect a light-triggered bomb with light-and stay safe a guaranteed 25 per cent of the time.
The secret is a device called an interferometer. It exploits the quantumly weird fact that, given two paths to go down, a photon will take both at once. We know this because, at the far end of the device, where the two paths cross once again, a wave-like interference pattern is produced in a suitably-placed detector.
To visualise what is going on, think of a photon entering the interferometer and taking e path while a ghostly copy one of itself goes down the other.
In Elitzur and Vaidman’s thought experiment, half the time there is a photon-triggered bomb blocking one path (see diagram, right).
Only the real photon can trigger the bomb, so if it is the ghostly copy that gets blocked by the bomb, there is no explosion-and nor is there an interference pattern at the other end.
In other words, we have “seen” the bomb without triggering it. Barely a year after Elitzur and Vaidman proposed their bomb-testing paradox, physicists at the University of Vienna, Austria, had brought it to life-not by setting off bombs, but by bouncing photons off mirrors.
In 2000, Shuichiro Inoue and Gunnar Björk of the Royal Institute of Technology in Stockholm, Sweden, used a similar technique to show that you could get an image of a piece of an object without shining light on it — something that could revolutionise medical imaging.
“It would be very useful for something like X-ray scanning, if there were no radiation damage to the tissue because no X-rays actually hit it,” says physicist Richard Jozsa of the University of Cambridge.
Josza is the brains behind perhaps the most eye-rubbing of such tricks: using a quantum computer to deliver the output of a program even when you don’t run the program.
As the team that implemented his idea in 2005 showed, quantum physics does at least retain some semblance of classical decency: to deliver a sensible answer, the computer does need to be switched on.
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Originally published at https://nohil21.blogspot.com on August 9, 2020.