Particles in Love: Quantum Mechanics Explored in New Study

How ‘Alice and Bob’ Test Quantum Mechanics

The paper by Shalm, Marsili and colleagues was published in the journal Physical Review Letters, with the mind-bending title “Strong Loophole-Free Test of Local Realism.”

“Our paper and the other two published last year show that Bell was right: any model of the world that contains hidden variables must also allow for entangled particles to influence one another at a distance,” said Francesco Marsili of NASA’s Jet Propulsion Laboratory in Pasadena, California, who collaborated with Shalm.

This cartoon helps explain the idea of “entangled particles.” Alice and Bob represent photon detectors, which NASA’s Jet Propulsion Laboratory and the National Institute of Standards and Technology developed. (Credits: NASA/JPL-Caltech) Click image for larger version.

An analogy helps to understand the experiment, which was conducted at a NIST laboratory in Boulder:

Imagine that A and B are entangled photons. A is sent to Alice and B is sent to Bob, who are located 607 feet (185 meters) apart.

Alice and Bob poke and prod at their photons in all kinds of ways to get a sense of their properties. Without talking to each other, they then each randomly decide how to measure their photons, using random number generators to guide their decisions. When Alice and Bob compare notes, they are surprised to find that the results of their independent experiments are correlated. In other words, even at a distance, measuring one photon of the entangled pair affects the properties of the other photon.

“It’s as if Alice and Bob try to tear the two photons apart, but their love still persists,” Shalm said. In other words, the entangled photons behave as if they are two parts of a single system, even when separated in space.

Alice and Bob — representing actual photon detectors — then repeat this with many other pairs of entangled photons, and the phenomenon persists.

In reality, the photon detectors are not people, but superconducting nanowire single photon detectors (SNSPDs). SNSPDs are metal strips that are cooled until they become “superconducting,” meaning they lose their electric resistance. A photon hitting this strip causes it to turn into a normal metal again momentarily, so the resistance of the strip jumps from zero to a finite value. This change in resistance allows the researchers to record the event.

To make this experiment happen in a laboratory, the big challenge is to avoid losing photons as they get sent to the Alice and Bob detectors through an optical fiber. JPL and NIST developed SNSPDs with worldrecord performance, demonstrating more than 90 percent efficiency and low “jitter,” or uncertainty on the time of arrival of a photon. This experiment would not have been possible without SNSPDs.

How can this be used in “the real world?” Continue reading to learn about possible applications…