Roger Penrose’s Idea on Testing Quantum Superposition

Feline conundrums

By Davide Castelvecchi

Things can be in two places at once. That is one of the bizarre facts of atomic world life, but according to four Oxford physicists it may soon be found to happen at much larger scales.

Nature’s elementary constituents seem to defy our intuition. A photon doesn’t know where it is until you ask it: as quantum theory predicts, a physical system is in a “superposition,” or co-existence, of states, until measurement makes it decide. That forced decision is called decoherence.

Quantum physics pioneer Erwin Schroedinger pointed out that that law ought to apply in the macroscopic realm as well, with “ridiculous” consequences. A cat could be alive and dead at the same time, he described in a celebrated thought experiment.

William Marshall, Christoph Simon, Dik Bouwmeester and Roger Penrose claim that a mirror the size of a bacterium — smaller than a cat, but still very large for quantum standards — could be shown to be in a superposition of states. “Quantum mechanics has not yet been tested at this regime,” says Marshall.

The experiment they propose would shoot a laser beam, one photon at a time, and split it in two with a semi-transparent glass. The two beams would aim at two mirrors. One of the mirrors would be about a micron wide and mounted on a mechanical oscillator — a “mini-version of a diving board,” as Marshall says. That would allow it to move if hit by the laser.

In superposition, each photon in the beam would take both paths, hit both mirrors, and bounce back to interfere with itself, with observable effects. The moveable mirror would then be simultaneously displaced and left alone, living in two places a billionth of a micron apart. But any contact with the outside world would trigger decoherence. The interference would disappear, so one would know that the photon decided on a definite path. either hitting the diving board or not (Physical Review Letters vol 91, p 130401).

Bouwmeester and Marshall now plan to do this in practice. In order to prevent decoherence, the oscillator will have to be cooled to within one-tenth of a degree from absolute zero, to dampen thermal motion. It will also have to be pliable enough, so that it can vibrate at low frequencies. That will make it is easier to displace with a single photon, but harder to keep from conducting heat.

Keith Schwab, who is designing a different superposition experiment at the University of Maryland, is skeptical. “Do I think it can ever be done? Probably not,” Schwab says. “But if they find a way to work at higher frequencies, there is a shot.”

Quantum theory is arguably the most accurately verified model in all of science. So far, it has never failed. Testing the limits of its applicability may hold the key to the ultimate version of the theory, which should include gravity. “If the experiment works,” Marshall says, “it may just confirm quantum mechanics. But unexpected results would be even more interesting.”

Note: This was my first real science reporting job, in one of my classes in the Science Communication program at UCSC. The assignment was to generate a story idea, interview scientists, and write a short news story, following the style of New Scientist magazine.

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