When a guitar string is plucked, it vibrates like any vibrating object and rises and falls like a wave, as the laws of classical physics predict. According to the laws of quantum mechanics, which describe the functioning of physics at the atomic level, however, vibrations should behave not only as waves but also as particles. The same guitar string, when observed at the quantum level, should vibrate as individual energy units known as phonons.
Now scientists at MIT and at the Swiss Federal Institute of Technology have for the first time produced and observed a single phonon in a common material at room temperature.
Previously, single phonons were only found at ultracold temperatures and in precisely constructed microscopic materials that researchers must examine in a vacuum. In contrast, the team has generated and observed individual phonons in a piece of diamond sitting outside at room temperature. The findings, which the researchers write in an article published today in Physical Review X "bring quantum behavior closer to our daily lives."
"There is a dichotomy between our daily experience of what a vibration is ̵
The technique developed by the team can now be used to investigate other common materials on quantum vibration. This can help researchers characterize the atomic processes in solar cells and find out why certain materials are superconducting at high temperatures. From a technical point of view, the team's technique can be used to identify common phoneme-bearing materials that can make ideal connections or transmission lines between the quantum computers of the future.
"What our work means is that we now have access to Sudhir, one of the main authors of the paper.
Co-authors of Sudhir are Santiago Tarrago Velez, Kilian Seibold, the Swiss Nils Kipfer, Mitchell Anderson, and Christophe Galland Federal Institutes of Technology.
"Democratization of Quantum Mechanics"
Phonons, the individual vibrational particles described by quantum mechanics, are also associated with heat, for example when a crystal composed of ordered lattices Atoms are heated at one end, quantum mechanics predicts that heat in the form of phonons or single oscillations of bonds between molecules flows through the crystal.
Single-phonons were extremely difficult to detect, mainly because of their sensitivity to heat. Phonons are prone to heat energy larger than their own. When phonons of Na Being of low energy, exposure to higher thermal energies can cause tremendous excitation of the phonons of a material, making the detection of a single photon a project to observe individual phonons.
The First Attempts to Observe Individual Phonons This was done with materials specifically designed to accommodate very few phonons with relatively high energies. These researchers then dipped the materials into zero-point refrigerators that Sudhir described as "brutal, aggressively cold" to ensure that the surrounding heat energy was lower than the energy of the phonons in the material.
"If that's the case, then the [phonon] vibration can not absorb energy from the thermal environment to excite more than one phonon," explains Sudhir.
The researchers then shot a photon pulse (light particle) into the material, hoping that a photon would do so interacting with a single phonon. In this case, the photon should be reflected in a process known as Raman scattering with another energy imparted to it by the interacting phonon. In this way, the researchers were able to detect single phonons at ultracold temperatures and in carefully developed materials.
"We asked the question here how you can get rid of this complicated environment." We've created around this object and brought this quantum effect into our environment to see it in more general materials, "says Sudhir." It's sort of like a democratization of quantum mechanics. "
One in a million
For the new study, the team considered diamond as a test object: In diamonds, phonons work naturally at high frequencies of ten terahertz – so high that the energy of a single phonon at room temperature is higher than the surrounding heat energy.  "When this diamond crystal sits at room temperature, phonon motion does not even exist because there is no energy at room temperature to stimulate anything," says Sudhir.
Within this low-vibration mixture of phonons, researchers wanted to stimulate only a single phonon sent high-frequency laser pulses, each consisting of 100 million photons, into the diamond – a crystal au s carbon atoms – with the coincidence that one of them interacts and is reflected by a phonon. The team would then measure the reduced frequency of the photon involved in the collision – a confirmation that it had actually encountered a phonon, although this operation would not be able to determine if one or more phonons were excited.
To decode the number of excited phonons, the researchers sent a second laser pulse into the diamond as the phonon's energy gradually decreased. For each phonon excited by the first impulse, this second impulse can cancel the excitation and dissipate that energy in the form of a new photon of higher energy. If initially only one phonon was excited, a new, higher-frequency photon should be generated.
To confirm this, the researchers placed a semi-transparent glass through which this new, higher-frequency photon would emerge along with two detectors on both sides of the glass from the diamond. Photons do not divide. So if several phonons are excited and then de-energized, the resulting photons should pass through the glass and randomly scatter into both detectors. If only one detector "clicks" and displays the detection of a single photon, the team can be sure that this photon will interact with a single phonon.
"It's a clever trick to make sure we only observe one phonon," says Sudhir.
The probability of a photon interacting with a phonon is about one in ten billion, in their experiments The researchers blew up the diamond at 80 million pulses per second – which Sudhir describes as a "train of millions of billions of photons" over several hours to capture about 1 million photon-phonon interactions, and in the end they realized with statistical significance that they were
"This is an ambitious claim, and we must be careful that it is handled with the utmost care. No room for reasonable doubt," says Sudhir.
As the Researchers sent in their second laser pulse to verify that individual phonons were actually being generated, delaying that pulse and sending it to the Diama When the excited phonon gradually began to let go of energy. In this way they could understand the way in which the phonon itself fell.
"So we can not only study the birth of a single phonon, but also his death." Sudhir says. "Now we can say: & # 39; Use this technique to examine how long it takes for a single phonon to die in the material of your choice. & # 39; This number is very useful This material can support coherent phonons, in which case you can do interesting things with it, such as heat transport in solar cells and connections between quantum computers. "
Capturing the birth and death of a phonon
Preparation and Decay of a Single Quantum of Oscillation Under Ambient Conditions, Physical Review X (2019). journals.aps.org/prx/accepted/… 14863b2e9b18baeef2cf
This story was previously courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that features news on MIT research, innovation and teaching.
Scientists observe a single quantum vibration under normal conditions (2019, October 7)
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