“Superpositions” –The Cosmic Weirdness of Quantum Mechanics

Quantum Physics


“If quantum mechanics hasn’t profoundly shocked you , you haven’t understood it yet,” said physicist Niels Bohr. The more we delve into the cosmic weirdness of quantum mechanics the stranger the world becomes. A state of quantum superposition is being in more than one place, or more than one state, at the same time –a single event can be happening both here and there, or both today and tomorrow. Caltech’s great theoretical physicist, Nobel Laureate Richard Feynman, was fond of noting that the ‘paradox’ of quantum mechanics is only a conflict between reality and your feeling of what reality ‘ought to be’.

Questions Our Very Understanding of Space and Time

Quantum mechanics, which describes the behavior of subatomic particles, challenges common sense. Waves behave like particles; particles behave like waves. It’s like “sneaking a look at God’s cards,” said the Italian physicist Giancarlo Ghirardi.

Superpositions, however, are hard to create say scientists from EPFL, MIT, and CEA Saclay, “as they are destroyed if any kind of information about the place and time of the event leaks into the surrounding – and even if nobody actually records this information. But when superpositions do occur, they lead to observations that are very different from that of classical physics, questioning down to our very understanding of space and time.”

New research from the international team using a very short laser-pulse to trigger a specific pattern of vibration inside a diamond crystal demonstrates a state of vibration that exists simultaneously at two different times, and evidence this quantum superposition by measuring the strongest class of quantum correlations between light beams that interact with the vibration.

The Experiment –Classical vs Quantum

In their experiment, they reported that “each pair of neighboring atoms oscillated like two masses linked by a spring, and this oscillation was synchronous across the entire illuminated region. To conserve energy during this process, a light of a new color is emitted, shifted toward the red of the spectrum.”

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

The process described above reports the Ecole Polytechniqe Federale de Lausanne, “should therefore be seen as the fission of an incoming photon from the laser into a pair of photon and phonon – akin to 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 at the same time.”

Even More Counterintuitive

Even more counterintuitive supporting Feynman’s observation of a quantum reality: two particles can become entangled, losing their individuality. The only information that can be collected about them concerns their common correlations. Because both particles are described by a common state (the wavefunction), these correlations are stronger than what is possible in classical physics. It can be demonstrated by performing appropriate measurements on the two particles. If the results violate a classical limit, one can be sure they were entangled.

Entangled Light and Vibration

In the new study, EPFL researchers managed to entangle the photon and the phonon (i.e., light and vibration) produced in the fission of an incoming laser photon inside the crystal. To do so, the scientists designed an experiment in which the photon-phonon pair could be created at two different instants. Classically, it would result in a situation where the pair is created at time t1 with 50% probability, or at a later time t2 with 50% probability.

“The Trick” –Bridges Our Daily Reality and Quantum Mechanics

But here comes the “trick” played by the researchers to generate an entangled state. By a precise arrangement of the experiment, they ensured 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 phonon-photon pair becomes entangled, and exists in a superposition of time t1 and t2. This prediction was beautifully confirmed by the measurements, which yielded results incompatible with the classical probabilistic theory.

By showing entanglement between light and vibration in a crystal that one 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.

Source: 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.

The Daily Galaxy, Jake Burba, via Ecole Polytechniqe Federale de Lausanne and Science 

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