Scientists Observe a Single Quantum Vibration Under Ordinary Conditions

Estimated reading time: 5 minutes

For the new study, the team looked to diamond as a test subject. In diamond, phonons naturally operate at high frequencies, of tens of terahertz — so high that, at room temperature, the energy of a single phonon is higher than the surrounding thermal energy.

“When this crystal of diamond sits at room temperature, phonon motion does not even exist, because there’s no energy at room temperature to excite anything,” Sudhir says.

Within this vibrationally quiet mix of phonons, the researchers aimed to excite just a single phonon. They sent high-frequency laser pulses, consisting of 100 million photons each, into the diamond — a crystal made up of carbon atoms — on the off chance that one of them would interact and reflect off a phonon. The team would then measure the decreased frequency of the photon involved in the collision — confirmation that it had indeed hit upon a phonon, though this operation wouldn’t be able to discern whether one or more phonons were excited in the process.

To decipher the number of phonons excited, the researchers sent a second laser pulse into the diamond, as the phonon’s energy gradually decayed. For each phonon excited by the first pulse, this second pulse can de-excite it, taking away that energy in the form of a new, higher-energy photon. If only one phonon was initially excited, then one new, higher-frequency photon should be created.

To confirm this, the researchers placed a semitransparent glass through which this new, higher-frequency photon would exit the diamond, along with two detectors on either side of the glass. Photons do not split, so if multiple phonons were excited then de-excited, the resulting photons should pass through the glass and scatter randomly into both detectors. If just one detector “clicks,” indicating the detection of a single photon, the team can be sure that that photon interacted with a single phonon.

“It’s a clever trick we play to make sure we are observing just one phonon,” Sudhir says.

The probability of a photon interacting with a phonon is about one in 10 billion. In their experiments, the researchers blasted the diamond with 80 million pulses per second — what Sudhir describes as a “train of millions of billions of photons” over several hours, in order to detect about 1 million photon-phonon interactions. In the end, they found, with statistical significance, that they were able to create and detect a single quantum of vibration.

“This is sort of an ambitious claim, and we have to be careful the science is rigorously done, with no room for reasonable doubt,” Sudhir says.

When sending in their second laser pulse to verify that single phonons were indeed being created, the researchers delayed this pulse, sending in into the diamond as the excited phonon was beginning to ebb in energy. In this way, they were able to glean the manner in which the phonon itself decayed.

“So, not only are we able to probe the birth of a single phonon, but also we’re able to probe its death,” Sudhir says. “Now we can say, ‘go use this technique to study how long it takes for a single phonon to die out in your material of choice.’ That number is very useful. If the time it takes to die is very long, then that material can support coherent phonons. If that’s the case, you can do interesting things with it, like thermal transport in solar cells, and interconnects between quantum computers.”