GLOBAL CHALLENGES

NIST Crafts 'Schrodinger's Cat' from Light

R. Colin Johnson | Date: 09-30-10 | Comments

Schrodinger's Cat is a thought experiment in which the feline was balanced in a paradoxical quantum state�a superposition of being both dead and alive. Now the National Institute of Standards and Technology (NIST) claims to have created a cat made from photons that realizes Schrodinger's long predicted paradox.

Austrian physicist Erwin Schrodinger constructed his "cat" experiment as a "reductio ad absurdum" argument against one interpretation of quantum mechanics. Einstein had recently described quantum entanglement in an article suggesting that superposition of opposite states in atomic quanta could also become macroscopic. To illustrate the absurdity of that notion, Schrodinger constructed the situation where a cat could be both alive and dead simultaneously.

Now the National Institute of Standards and Technology (NIST) claims to have constructed just such a quantum cat, but made with lasers and light instead of flesh and fur.

NIST research associate Thomas Gerrits at the laser table used to create "quantum cats" made of light (source: NIST).

Quantum mechanics teaches that, on the atomic scale, many processes are stochastic�that is, non-deterministic superpositions of opposites poised on a knife's edge until forced to adopt one of the other resting state by perturbations from the environment. By removing all environmental influences inside a shielded box, such poised superpositions of opposites theoretically could be maintained indefinitely until perturbed.

Schrodinger's experiment proposed putting a cat inside a sealed box next to a flask of poison whose release was controlled by the radioactive decay of an isotope. Under Einstein's interpretation of quantum mechanics, the precise instant when a radioactive isotope decays cannot be calculated, only that it will decay at a certain rate. Further, in the absence of any environmental stimulation, the isotope enters a superposition of both states�emitting and not-emitting�until triggered by opening the box. The absurd conclusion, Schrodinger maintained, was that since the emission was indeterminate until the box was opened, that the cat must be both dead and alive too.

Since Schrodinger's original thought experiment, scientists extending the frontiers of quantum mechanics have searched for illustrations of the "cat state" in the real world, defining it arithmetically as a state where logical "1s" and "0s" are statistically balanced in the 50/50 state. Such superpositions of logical values, scientists argue, could allow future quantum computers to vault over all the sequential steps that today's computers must plod through to obtain answers.

Researchers at NIST now claim to have created that quantum "cat state" using lasers, a crystal, and an array of beam splitters and photon detectors. The setup, occupying a desktop laser table, could eventually allow NIST to improve the precision of its measurements, as well as to extend the frontiers of quantum computing and enable ultra-secure communications.

Colorized plots compare how closely the NIST quantum "cats" (left) compare with theoretical predictions for the cat state (right) (source: Gerrits/NIST).

Using ultra-fast laser pulses, NIST first squeezed the beams through a special crystal that converted each laser photon into pairs of entangled photons, which were in turn separated by a half-silvered mirror called a beam splitter. A single-photon detector subsequently helped verify that the resultant photons were in the cat state.