Physicists from the National Institute of Standards and Technology (NIST) have "frozen" a flat crystal of 150 beryllium ions (electrically charged atoms), opening up new possibilities for magnetism simulation at the scale quantum and signal detection from a mysterious dark matter.
For decades, many researchers have tried to cool vibrating objects large enough to be visible to the naked eye to the point where they have the minimum movement allowed by quantum mechanics, the theory that governs the behavior of matter. At the atomic scale. The colder it gets, the better, because it makes the device more sensitive, more stable and less distorted, and therefore more useful for practical applications. Until now, however, researchers have been able to reduce only a few types of vibrations.
In the NIST experiment, magnetic and electric fields cooled and trapped ions, so that they formed a disk less than 250 micrometers (millionths of a meter) in diameter. The disc is considered a crystal because the ions are arranged so as to be repeated regularly.
As described in Letters of physical examinationNIST researchers cooled the crystal in as little as 200 microseconds (one millionth of a second), so each ion contained about one third of the energy transported by a single phonon, a package of motion energy in the crystal. This is very close to the amount of energy in the lowest possible "fundamental" quantum state for crystal drumhead vibrations, which are similar to the back and forth movements of the crystal. a drum that beats.
The researchers cooled and slowed the 150 vibrations of the drum skin, one for each ion. (The simulation video below shows eight examples of drumhead vibrations.) Work has shown that hundreds of ions can be totally appeased with this technique, a significant advance over the previous demonstration by another group cooling a line of 18 ions.
For the vibrations at cooled frequencies in this demonstration, one-third of the energy transported by a phonon corresponds to 50 microKelvin, or 50 millionths of a degree above absolute zero (minus 459.67 ° F or less 273.15 ° C), John Bollinger, group leader, said. Even though it is not a record temperature, this level is close to the basic state of quantum mechanics for all modes of the battery head, which means that the thermal movement is weak for such a confined system, noted Bollinger.
To achieve as much cooling, the researchers targeted two lasers with crystal-specific frequencies and power levels. Lasers have coupled the energy levels of the ions to cause the ionic crystal to lose energy without increasing its motion. For most particles of laser light scattered by the crystal, the ions lost their motion, which cooled the crystal.
The method has not cooled other types of vibrations such as lateral movements of the disk-shaped crystal. But the movements of the drum skin have the most practical uses. Only drum skin vibrations are used in quantum simulations and quantum sensors.
More severe vibrations in the drum skin will make ionic crystal a more realistic simulator of quantum magnetism, which can be difficult to compute on conventional computers. Cooling of the ground state should also enable more complex entangled quantum systems, allowing better measurements for quantum detection applications.
"A quantum detection application on which we are excited about the research is the detection of very weak electric fields," said Bollinger. "With ground-state cooling, we are improving our ability to detect electric fields at a level that allows us to search for certain types of dark matter – axions (hypothetical subatomic particles) and hidden photons (non-force carriers). still seen). "
Future research will attempt to cool three – dimensional crystals with a much larger number of ions.
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Elena Jordan et al. Near-ground cooling of two-dimensional trapped-ion crystals of more than 100 ions, Letters of physical examination (2019). DOI: 10.1103 / PhysRevLett.122.053603