Experiments that reveal the weirdness of the quantum world usually involve precise and highly specialized equipment. But now physicists in Switzerland and the UK have proposed a way of using human vision to observe the purely quantum effect of "entanglement".

The experiment — which has yet to be performed in the lab — would involve entangling a pair of photons and then creating thousands of identical copies of one of the pair such that they could be seen by the human eye.

Entangled particles have a much stronger relationship than that allowed by classical mechanics. For example, the polarization of one photon is revealed instantly by measuring the polarization its entangled partner, regardless of the distance between the photons.

The new experiment, which has been proposed by Nicolas Gisin and colleagues at the University of Geneva and University of Bristol, would first involve creating a pair of entangled photons. This could be done, for example, by passing light through a non-linear crystal in which a higher-energy photon is absorbed followed by the emission of two lower-energy photons (arXiv:0902.2896).

Cloning photons

One of the photons is then “cloned” to create thousands of identical photons. This is done by stimulated emission — the same process behind a laser — whereby the original photon is sent through a pumped optical medium.

Because the clones are created in a coherent quantum process, it produces a pulse of light that is intense enough to be seen with the naked eye — yet is entangled with the second original photon. Measuring the polarization of the pulse will therefore reveal the polarization of the second photon.

The team proposes to measure the polarization of the pulse by passing it through a polarizing filter, which allows light with parallel polarization to pass through while deflecting light with perpendicular polarization by 90°. Two human observers — one looking along the parallel path and the other the perpendicular path — could then determine the polarization of each pulse.

Meanwhile, the polarization of the second photon of the pair would be determined by passing it through a similar polarizing filter that is monitored by two sensitive photon detectors.

Predicting the outcome

If the experiment is a success, the humans should be able to predict the outcome of the measurement on the second photon based on the observed polarization of the pulse. In other words, if the pulse is vertically polarized, then the second photon will be horizontally polarized.

While entanglement in photons was first observed over 30 years ago, Gisin is keen to point out an important distinction between this and previous experiments. In earlier work, the choice of measurement that forces the entangled pair into distinct polarization states is made before that state is amplified to a level where it can be perceived by a human observer. For example, a single photon is passed through a polarizing filter and then converted into an amplified electrical pulse by a detector.

By contrast, in this experiment the entangled state is amplified to the human level before the measurement is made — effectively bringing the observer one step closer to the weird world of quantum mechanics. Indeed, Gisin believes that, if successful, the experiment could be extended to clone the second entangled photon and use a total of four human observers to verify entanglement.

'Elegant experiment'

Seth Lloyd at the Massachusetts Institute of Technology told physicsworld.com that the proposal “does a considerable service by devising an elegant experiment where the human eye functions in a very efficient way as an entanglement detector”. However, he also points out that the eye is an extremely efficient detector of light, so it is not surprising that is could be used to detect entanglement.

Indeed, the challenges involved in actually doing the experiment are mostly related to the cloning process, according to Gisin. “Cloning cannot be perfect”, he explained, adding that unwanted spontaneous emission during cloning would create a significant number of photons that were not entangled.

This problem could be reduced using a technique called “phase covariant” cloning, but not eliminated. As a result, the experiment would have to be repeated many times over before the observers see enough entangled pulses to verify the effect.

Another challenge, according to Gisin, is producing cloned pulses of green light, which the eye is most sensitive to. Most cloning systems currently produce photons in the infrared.

“First we plan to amplify the photon so it can be seen”, said Gisin. “The rest should be relatively easy.”