Quantum Superposition Evidenced by Measuring Interaction of Light with Vibration

An exclusively counterintuitive aspect of quantum mechanics is the fact that a single event can exist in a state of superposition—occurring both here and there, or both today and tomorrow.

It is challenging to create such superpositions because they are destroyed if any type of information related to the time and place of the event leaks into the surrounding—and even if nobody really records this information. However, when superpositions do happen, they result in observations that are highly distinct from that of classical physics, which questions down to the very understanding of time and space.

Researchers from EPFL, MIT and CEA Saclay demonstrate a state of vibration that occurs at two different times concurrently. They have proven this quantum superposition by quantifying the strongest family of quantum correlations between light beams that tend to interact with the vibration. The findings have been published in Science Advances.

The team triggered a particular pattern of vibration within a diamond crystal by using a very short laser pulse. Each pair of neighboring atoms oscillated similar to two masses connected by a spring, where the oscillation was found to be synchronous over the entire illuminated region. Energy is conserved during this process by the emission of light of a new color and shifting toward the red end of the spectrum.

But this classical picture is not consistent with the experiments. Rather, both vibration and light should be characterized as particles, or quanta—light energy is quantized into discrete photons, whereas vibrational energy is quantized into discrete phonons (which are named after the ancient Greek 'photo = light' and 'phono = sound').

The process illustrated above should hence be regarded as the fission of an incoming photon from the laser into a pair of photon and phonon—similar to nuclear fission of an atom into two smaller pieces.

However, this is not the only defect of classical physics. According to quantum mechanics, it is possible for particles to occur in a superposition state, such as the famous Schrödinger cat that is alive and dead simultaneously.

Much more counterintuitive is the fact that two particles can be entangled, thereby losing their individuality. The only information that can be gathered in relation to them is linked to their common correlations.

Since both particles are characterized by a common state or the wavefunction, these correlations are more robust compared to what is viable in classical physics. This can be demonstrated by carrying out suitable measurements on the two particles. In case a classical limit is violated by the results, then it can be said that they were entangled.

As part of the new study, researchers from EPFL were able to entangle the photon and the phonon (i.e. light and vibration) generated during the fission of an incoming laser photon within the crystal.

They achieved this by designing an experiment where the photon-phonon pair could be produced at two different instants. As per classical physics, it would lead to a condition where the pair is produced at time t₁ with 50% probability, or later at time t₂ with 50% probability.

However, here arrives the 'trick' played by the team to produce an entangled state. They performed an accurate arrangement of the experiment to ensure that not even the faintest trace of the light-vibration pair creation time (t₁ vs t₂) was left out in the universe.

Simply put, information related to t₁ and t₂ was erased. Then, quantum mechanics predicts whether the phonon-photon pair turns entangled and occurs in a superposition of time t₁ and t₂. The prediction was validated by the measurements, which produced results incompatible with the classical probabilistic theory.

The new study demonstrates entanglement between vibration and light in a crystal that can be held in the finger of a person during the experiment, thus forming a bridge between the daily experience and the enchanting world of quantum mechanics.

“Quantum technologies are heralded as the next technological revolution in computing, communication, sensing,” stated Christophe Galland, one of the main authors of the study, who is the head of the Laboratory for Quantum and Nano-Optics at EPFL.

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.

Christophe Galland, Head, Laboratory for Quantum and Nano-Optics, EPFL

“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,” added Galland.

1. A laser generates a very short pulse of light. 2. A fraction of this pulse is sent to a nonlinear device to change its color. 3. The two laser pulses overlap on the same path again, creating a “write & read” pair of pulses. 4. Each pair is split into a short and a long path, 5. yielding an “early” and a “late” time slot, overlapping once again. 6. Inside the diamond, during the “early” time slot, one photon from the “write” pulse may generate a vibration, while one photon from the “read” pulse converts the vibration back into light. 7. The same sequence may also happen during the “late” slot. But in this experiment, the scientists made sure that only one vibration is excited in total (in both early and late time slots). 8. By overlapping the photons in time again it becomes impossible to discriminate the early vs. late moment of the vibration. The vibration is now in a quantum superposition of early and late time. 9. In the detection apparatus, “write” and “read” photons are separated according to their different colors, and analyzed with single-photon counters to reveal their entanglement. Video Credit: Santiago Tarrago Velez (EPFL).

Journal Reference:

Velez, S. T., et al. (2020) Bell correlations between light and vibration at ambient conditions. Science Advances. doi.org/10.1126/sciadv.abb0260.