When light and atoms share 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, questioning our understanding of space and time.
EPFL scientists, WITH, and CEA Saclay, publishing in Scientific advances, demonstrates a state of vibration that exists simultaneously in two different moments and proves 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.
An illustration of the “common atmosphere” of light and atoms described in this study. Credit: Christophe Galland (EPFL)
This classic image is, however, incompatible with experiments. Instead, both light and vibration should be described as particles, or quanta: 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 seen as the fission of a photon entered from the laser into a pair of photons and phonons – similar to the nuclear fission of a atom in 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.
1. A laser generates a very short pulse of light 2. A fraction of this pulse is sent to a nonlinear device to change color 3. The two laser pulses overlap again in the same way, creating a pair of “write and reading ”of impulses. 4. Each pair is divided into a short and long path, 5. resulting in an “early” and “late” time interval, overlapping again 6. Inside the diamond, in the “early” time interval, a photon from the “write” pulse can generate a vibration, while a photon from the “read” pulse converts the vibration back into light. 7. The same sequence may occur during the “late” slot. But in this experiment, the scientists made sure that a single vibration is fully excited (both in the early and late time intervals). 8. By overlapping the photons again in time, it becomes impossible to discriminate the early moment from the latest moment of vibration. The vibration is now in a quantum overlap of early and late time. 9. In the detector, the ‘write’ and ‘read’ photons are separated according to their different colors and analyzed with single-photon counters to reveal their entanglement. Credit: Santiago Tarrago Velez (EPFL)
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 computing, communication, detection,” says Christophe Galland, head of the quantum and nano-optical laboratory at EPFL 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. “
Reference: “The correlations of the bell between light and vibration in ambient conditions” by Santiago Tarrago Velez, Vivishek Sudhir, Nicolas Sangouard and Christophe Galland, 18 December 2020, Scientific advances.
DOI: 10.1126 / sciadv.abb0260