A pair of quantum entangled photons sure makes a cute couple. Of course, the two might have opposite states–one might be spin up and another spin down, for example–but they promise they’ll always stay that way.

They’re also fiercely loyal, respecting their opposite-spin preferences no matter how long-distance their relationship. (That means that by checking the state of one entangled photon, you can instantly know the state of the other, distant photon, a handy way to “teleport” information.) Unfortunately, because the couple is merely two light particles, their shining example of old romance has been too dim for our eyes to see.

Until now. As announced in their recently published Arxiv.org paper, physicists led by Nicolas Gisin at the University of Geneva in Switzerland believe they have found a way to watch this love affair unfold: by boosting the light emitted by one member of a quantum entangled pair, they think they can make this quantum effect visible to a human eye.

Measuring quantum states such as spin up or spin down is like looking at whether a switch is on or off. This closely matches the concept of a bit, a single 1 or 0, in computing. With entangled photons, physicists call these on/off states quantum bits or “qubits.” What an observer would see while observing an entangled photon is really a choice between two states. The observer could then confirm entanglement by checking to see that the photon was loyal to its partner.

In the traditional set-up, two widely separated particle detectors are used to measure the entanglement of the two photons. But Gisin and his colleagues want to let the human eye do some of the work.

The researchers would send one photon to a standard detector and the other to a human observer in a dark room. The human would see a dim point of light in either the right or left field of view, depending on the photon’s quantum state. If those flashes of light correlate strongly enough with the output of the ordinary photon detector, then the scientists can conclude that the photons are entangled. [Wired]

But since the human brain won’t register the flash caused by one single photon, researchers need to increase the light coming to a person’s eye. More light requires more photons, but the original entanglement was a monogamous relationship. Gisin’s team proposes entangling a group of similar state photons with one member of the pair, creating enough light for a person to see.

First, Gisin and his colleagues will entangle a pair of photons, and then amplify these signals by entangling each of these photons with another ensemble of, say, 100 photons. In the arrangement they are currently developing, one pulse of photons would then be sent at a person, whereas the other would be sent at a conventional photon detector to test what the volunteer saw, Gisin says.[Scientific American]

The observer sees the group–what the researchers call a “macroscopic” qubit. One photon entangles with a second, and that second with the group. Though the observer won’t directly see the relationship between the first two photons, the second’s romantic indiscretions, it’s entanglement with the hoard of 100 or so photons, will be impossible to miss.

This probably won’t lead to any big scientific breakthroughs, Gisin admits.

“Why do we do it nevertheless?” he says. “We find entanglement fascinating.” [Wired]

Related content:
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Image: flickr / Katie Tegtmeyer