Identical photons generated at 150 million kilometers

Until the mid-20s^th century, the light was quite ordinary. Yes, it was both a particle and a wave, but it did not do anything very weird. Then, scientists, underemployed after the end of the Second World War, began to pay more attention to the properties of light. This was partly due to the availability of surplus projectors, which could be transformed into inexpensive light detector arrays to measure star properties.

This started the gold rush of photons, with scientists identifying all sorts of interesting potential behaviors. But to observe them, it would be necessary to have rather special sources of light, which did not exist. Scientists have shown that our own Sun could be transformed into one of these light sources.

A flock of identical photons

When two photons are indistinguishable, they can play unexpected turns. The diagram below shows an example: two identical photons strike a partially reflecting mirror at the same time. We can not predict where they will go, but wherever they go together. If the world were classic, we would expect everyone to behave independently and, half the time, they would choose different directions. But we are in a quantum world, so that does not happen.

This type of interference can only work with identical photons, this is where the special light sources come into play. The photons are distinguished by their color (wavelength), the purity of this color ( or, technically, by their coherence), the orientation of their oscillations. electric field (called polarization), their spatial shape and their time of arrival. Indeed, the creation of identical photons has always been so difficult that laboratory facilities and graduate students have been sacrificed to their creation.

But they were unique devices that, by their construction, could do nothing but generate identical pairs of photons. Could two independent devices emit identical single photons?

The development of quantum dots has made this possible. Quantum dots are what the label on the box indicates: tiny points of material that produce quantum behavior by confining a single electron. Containment limits electrons to specific energies; when the electron gets rid of energy, it does so by emitting a photon.

Materials scientists have discovered how to create these points so that they are almost identical, which means that the color of the emitted photon is the same. We also know how to place them in an environment that encourages them to have the same purity, the same spatial shape and the same polarization. The experiments revealed that two quantum dots can, in fact, emit identical photons.

However, that does not quite solve the problem. We basically made two copies of the same device and carefully controlled the environment to make sure they would naturally produce identical photons. But what about something outside our control, like the sun?

Cut out identical photons in a mixed flock

That's exactly what a team of researchers has tested. They used a tracking device to continuously capture sunlight. This light has been filtered to ensure that only photons of good color, purity, spatial shape and polarization are recorded. These were then sent into a partially reflective mirror with the light of a quantum dot.

But synchronization is also critical, as the experiment requires that only one photon of the Sun hits the partially reflecting mirror at the same time as the quantum dot photon. The Sun emits photons at random moments, but there are so many photons that there is always more than one available, whereas a quantum dot emits a photon that all about 10ns.

To eliminate this obstacle, the researchers threw away the photons of the Sun until the average photon rate was about the same as that of the quantum dot. Then the detectors were only turned on at specific time intervals, choosing only to detect the photons arriving with the quantum dot photon.

Under these conditions, the result was exactly as expected for the indistinguishable photons. Well almost. The quantum dot is not perfect, so sometimes it emits two photons instead of one. And the photon flux of the Sun will also occasionally have two photons, which means that researchers do not see the perfect interferences.

But it's pretty close. In fact, their results were so good that they went even further by showing that they could tangle the photons of the Sun with photons of a quantum dot.

On a plane, it is quite ordinary: quantum mechanics predicts that identical photons will behave in a certain way. This experiment actually takes the innumerable photons of the Sun and selects only those that are identical to the photons of the quantum dot. We did not expect any other results.

On another level, it's still incredible. Thinking that we now have the technology to take any old light bulb and make it a quantum resource is pretty amazing. The researchers also claim that the quantum properties of single photons of the Sun can be used to illuminate solar processes, such as the behavior of magnetic fields. This can be more interesting than any technological application.

Physical Review Letters, 2019, DOI: 10.1103 / PhysRevLett.123.080401 (About DOIs)