Zero-point motion is an irrepressible wiggling that becomes visible at temperatures near absolute zero. Evidence of this quantum motion has previously been uncovered for trapped particles and for small resonators. Now researchers studying nanocrystals have identified a low-temperature emission effect, which they show is related to zero-point motion within the crystal lattice [1]. The effect may be useful in cooling down nanocrystals to lower temperatures than previously possible.

Quantum physics often shows up at ultracold temperatures. Normally, as an object becomes colder, it moves less and less. However, the Heisenberg uncertainty principle dictates that the motion can’t go exactly to zero—there will always be fluctuations. These quantum fluctuations have been studied in microscopic systems, such as trapped atoms and molecules [2]. But they’ve also been observed in macroscopic objects. Previous experiments have identified signatures of zero-point motion in small mechanical resonators, such as drums and beams (see Viewpoint: Seeing the “Quantum” in Quantum Zero-Point Fluctuations).

Those investigations focused on the whole object as it moves back and forth like a vibrating spring. But there are also internal vibrations—the object’s atoms wiggle around in their lattice structure. Xiaoyong Wang from Nanjing University in China and his colleagues have detected a signature of zero-point motion in the lattice of a nanocrystal. “As far as we know, this is the first time that this effect has been seen in a solid material,” says team member Zhi-Gang Yu from Washington State University. “Even we were surprised to observe it.”

The observed signature appeared in photoluminescence measurements, in which an object is excited with a laser and then subsequently relaxes back to its initial state by emitting light. If the outgoing emission has a frequency that is higher than that of the laser, the process is called up-conversion. The opposite case—emission at lower frequency—is called down-conversion. Up-conversion is especially interesting to researchers because the object gives up some of its internal energy and thus becomes colder.

Wang and his colleagues explored up-conversion in nanocrystals made from a lead-halide perovskite (CSPbI₃). This semiconductor has several exciton states, which are formed when an electron hops from the valence band to a higher-energy conduction band. When the electron subsequently falls back to the valence band, light is emitted at the telltale exciton frequency.

For their up-conversion study, the researchers targeted one of the perovskite’s exciton states by tuning their laser to a frequency just below the exciton frequency. In this case, the laser photons lack enough energy to excite electrons. However, the photons can get “help” from thermal fluctuations (or phonons) in the crystal. Indeed, at relatively high temperatures (above 10 K), Wang and his colleagues observed exciton emission from their nanocrystal—implying that phonons were supplying the additional energy needed for exciting the electrons.

This was all expected. The surprise came when the researchers lowered the temperature to 4 K. At this temperature, the phonons have insufficient energy to help the photons. “But we continued to see exciton emission,” Yu says. “It was a puzzle to us where the additional energy was coming from.” The answer was zero-point motion: The lattice continues to have energy in its quantum fluctuations.

Wang and his colleagues developed a model for how lattice vibrations at near zero temperature can affect the photoluminescence signal. They showed that zero-point motion creates an oscillating electric field within the material, which causes a “tilting” of the band structure. A similar effect happens when an external electric field is applied to a material. The tilting of the bands makes it easier for electrons to hop from the valence to the conduction band. The net result is that the zero-point motion supplies the additional energy needed for the up-conversion photoluminescence.

As mentioned, up-conversion removes energy from an object, so it might be possible to use the zero-point motion effect for cooling. Until now, it has been hard to cool objects below 4 K, as that is the limit set by helium-based cryostats. But if photoluminescence can harvest zero-point motion from a material, it could potentially reach sub-4-K temperatures. “These results open the door to a different approach to cooling at extreme temperatures,” Yu says.

“The primary novelty of this study is a departure from conventional descriptions of photoluminescence up-conversion,” says Masaru Kuno, a physical chemist at the University of Notre Dame in Indiana. The observed zero-point motion effect might offer a method for semiconductor optical refrigeration, which has been a long-standing holy grail in the laser-cooling community, Kuno says. But he says more thermodynamic measurements are needed to show that zero-point up-conversion can indeed lead to cooling of a nanocrystal. “Although the presented data are suggestive, further vetting is required to make the claims conclusive.”

–Michael Schirber

Michael Schirber is a Corresponding Editor for Physics Magazine based in Lyon, France.