A state of vibration that exists simultaneously at two different times

Quantum mechanics has an exciting feature: a single event can exist in a state of superposition – happening both here and there or both today and tomorrow. 

Such superposition is quite challenging to create as they are easily destroyed if any information about the event’s place and time leaks into the surroundings – and even if nobody records this information. Once superposition is created, it leads to observations that are very different from that of classical physics, questioning down to our very understanding of space and time. 

Recently, scientists from EPFL, MIT, and CEA Saclay demonstrated a state of vibration simultaneously at two different times. They evidence this quantum superposition by measuring the strongest class of quantum correlations between light beams interacting with the vibration. 

Using a very short laser pulse, scientists triggered a specific vibration pattern inside a diamond crystal. They then oscillated pairs of neighboring atoms like two masses linked by a spring. This oscillation was synchronous across the entire illuminated region. 

A light of a new color was emitted during the process to conserve the energy. 

This classical picture, however, is inconsistent with the experiments. Instead, light and vibration should be described as particles or quanta: light energy is quantized into discrete photons. In contrast, vibrational energy is quantized into discrete phonons (named after the ancient Greek “photo = light” and “phono = sound”). 

Therefore, the process described above should be seen as the fission of an incoming photon from the laser into a pair of photons and phonons – akin 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 a superposition state, like the famous Schrödinger cat being alive and dead simultaneously. 

In this new study, scientists successfully entangled the photon and the phonon produced in an incoming laser photon’s fission inside the crystal. They did this by designing an experiment in which the photon-photon pair could be created at two different instants. Classically, it would result in a situation where the pair is created at time t1 with a 50% probability or at a later time t2 with a 50% probability. 

Here, scientists played a trick to generate an entangled state. They arranged the experiment in such a way that not even the faintest trace of the light-vibration pair creation time (t1 vs. t2) was left in the universe. 

In other words, they erased information about t1 and t2. Quantum mechanics then predicts that the photon-photon pair becomes entangled and exists in a superposition of time t1andt2. This prediction was beautifully confirmed by the measurements, which yielded results incompatible with the classical probabilistic theory. 

By showing the entanglement between light and vibration in a crystal that one could hold in one’s finger during the experiment, the new study creates a bridge between our daily experience and the fascinating realm of quantum mechanics. 

Christophe Galland, head of the Laboratory for Quantum and Nano-Optics at EPFL and one of the study’s main authors, said, “Quantum technologies are heralded as the next technological revolution in computing, communication. They are currently being developed by top universities and large companies worldwide, but the challenge is daunting. Such technologies rely on very fragile quantum effects surviving only at extremely cold temperatures or under high vacuum.”

“Our study demonstrates that even a common material at ambient conditions can sustain the delicate quantum properties required for quantum technologies. There is a price to pay, though: the quantum correlations sustained by atomic vibrations in the crystal are lost after only 4 picoseconds — i.e., 0.000000000004 of a second! This short time scale is, however, also an opportunity for developing ultrafast quantum technologies. But much research lies ahead to transform our experiment into a useful device — a job for future quantum engineers.” 

Journal Reference: 

1. Santiago Tarrago Velez, Vivishek Sudhir, Nicolas Sangouard, Christophe Galland. Bell correlations between light and vibration at ambient conditions. Science Advances 18 December 2020, 6: eabb0260. DOI: 10.1126/sciadv.abb0260