When light and atoms have a common atmosphere

A particularly counter-intuitive feature of quantum mechanics is that a single event can exist in a state of overlap – happening both here and there, or both today and tomorrow.

Such overlaps are difficult to create because they are destroyed if any information about the place and time of the event leaks into the surroundings – and even if no one actually records this information. But when overlaps occur, they lead to observations that are very different from those of classical physics, raising questions that flow into our understanding of space and time.

Scientists from EPFL, MIT and CEA Saclay, publishing in Scientific advances, demonstrates a state of vibration that exists simultaneously at two different times and provides evidence of this quantum overlap by measuring the strongest class of quantum correlations between light beams that interact with vibration.

The researchers used a very short laser pulse to trigger a specific pattern of vibration inside a diamond crystal. Each pair of neighboring atoms oscillated like two masses connected by an arc, and this oscillation was synchronous throughout the illuminated region. To save energy during this process, a light of a new color is emitted, shifting to the red of the spectrum.

This classic image is, however, incompatible with experiments. Instead, both light and vibration should be described as particles or quantums: light energy is quantified in discrete photons, while vibrational energy is quantified in discrete phonons (named after the ancient Greek “photo = light” and “phono = sound “).

Therefore, the process described above should be viewed as the fission of a photon entering the laser into a pair of photons and phonons – similar to the nuclear fission of an atom into two smaller pieces.

But it is not the only shortcoming of classical physics. In quantum mechanics, particles can exist in an overlapping state, such as the famous Schrödinger cat being alive and dead at the same time.

Even more counterintuitive: two particles can become entangled, losing their individuality. The only information that can be collected about them refers to their common correlations. Because both particles are described by a common state (wave function), these correlations are stronger than what is possible in classical physics. It can be demonstrated by making appropriate measurements on the two particles. If the results violate a classic limit, we can be sure that they were confused.

In the new study, EPFL researchers were able to confuse the photon and phonon (ie light and vibrations) produced in the fission of a laser photon inside the crystal. To do this, the scientists designed an experiment in which the photon-phonon pair could be created at two different times. Classically, a situation would arise in which the pair is created at time t1 with 50% probability, or at a later time t2 with 50% probability.

But here comes the “trick” played by researchers to generate a confusing state. Through a precise arrangement of the experiment, they ensured that not even the faintest trace of the time of creation of the light-vibration pair (t1 vs. t2) remained in the universe. In other words, they deleted information about t1 and t2. Quantum mechanics then predicts that the phonon-photon pair becomes entangled and exists in an overlap of time t1 and t2. This prediction was nicely confirmed by measurements, which gave results incompatible with classical probabilistic theory.

Showing the tangle of light and vibration in a crystal that someone could hold in their finger during the experiment, the new study creates a bridge between our daily experience and the fascinating realm of quantum mechanics.

“Quantum technologies are heralded as the next technological revolution in computer science, communication, detection,” said Christophe Galland, head of EPFL’s Quantum and Nano-Optical Laboratory and one of the study’s lead authors. They are currently being developed by top universities and large companies around the world, but the challenge is daunting. Such technologies are based on very fragile quantum effects that survive only in extremely cold temperatures or high vacuum. Our study demonstrates that even a common material in environmental conditions can support the delicate quantum properties needed by quantum technologies. There is, however, a price to pay: the quantum correlations sustained by the atomic vibrations in the crystal are lost after only 4 picoseconds – that is 0.000000000004 second! This short time scale is, however, also an opportunity for the development of ultrafast quantum technologies. But much research is expected to turn our experiment into a useful device – a job for future quantum engineers. ”