1.4 million times the atmospheric pressure! Chinese scientists break working pressure record for quantum sensors
Recently, physicists at the Chinese Academy of Sciences have discovered that quantum sensors based on microscopic defects in the crystal structure of diamond can operate at pressures of up to 140 gigapascals (a unit of pressure; 1 gigapascal equals 1 billion parsecs), which is equivalent to 1.4 million standard atmospheres. This also sets a record for the operating pressure of quantum sensors based on nitrogen vacancy (NV) centers, whose newfound durability may benefit research in condensed matter physics and geophysics.
Measurement of photoluminescence of NV color centers at different pressures.
01NV color centers in diamond: fragility
NV color centers occur when two adjacent carbon atoms in diamond are replaced by a nitrogen atom and an empty lattice site. They are like tiny quantum magnets with different spins, and when excited with a laser pulse, they emit a fluorescent signal that can be used to monitor small changes in the magnetic properties of nearby material samples. This is because the intensity of the emitted NV center signal varies with the local magnetic field.
The problem is that such sensors are fragile and often do not work under harsh conditions. This makes them difficult to use to study the Earth's interior, where pressures of gigapascals (GPa) prevail, or to investigate materials like hydride superconductors, which are made at very high pressures.
02Magnetic resonance for optical detection
Diamond quantum sensor under gipa pressure
Optically detected magnetic resonance (ODMR) of diamond NV centers under gigapascal pressure
In this new work, a team led by Gangqin Liu at the National Research Center for Condensed Matter Physics and the Institute of Physics, Chinese Academy of Sciences, first created a microscopic high-pressure chamber called a diamond anvil cell (DAC) in which to place their sensors: these sensors consist of microdiamonds containing combinations of NV color centers. This type of sensor works thanks to a technique called optically detected magnetic resonance (ODMR), in which the sample is first excited with a laser (at a wavelength of 532 nm) and then manipulated by microwave pulses. The researchers applied the microwave pulses using a thin platinum wire that is robust to high voltage. The fluorescence emitted is then measured at the end.
"In the experiment, we first measured the photoluminescence of the NV center at different pressures," Liu explained, "and we observed fluorescence at nearly 100 gigapascals, which was an unexpected result that prompted us to make subsequent ODMR measurements."
While this result was surprising, Liu noted that the diamond lattice is very stable and does not undergo phase changes, even at a pressure of 100 gigapascals (1 Mbar, or nearly 1 million times the atmospheric pressure at Earth's sea level). While such high pressures do change the energy level and optical properties of the NV color center, the rate of change slows down at higher pressures, allowing the fluorescence to persist. Even so, Gangqin Liu explained that obtaining ODMR spectra at Mbar pressure is "not an easy task.
"We had to overcome many technical challenges, one in particular being that high pressure reduces the fluorescence signal of NV and introduces additional background fluorescence."
The researchers overcame these problems by using a large combination of NV color centers (about 5 × 105 in a single microdiamond) and optimizing the light collection efficiency of their experimental system. However, they also needed to avoid large pressure gradients across the sensor: as any inhomogeneity in the pressure distribution would broaden the OMDR spectrum and reduce the signal contrast.
"To meet this challenge, we chose potassium bromide (KBr) as the pressure medium and limited the detection volume to about 1um3, and we were able to use this method to obtain NV color-centered ODMR at a pressure of nearly 140 GPa."
Gangqin Liu added that the maximum pressure could have been higher because the pressure-induced modifications in the energy levels of the NV color centers turned out to be smaller than expected. "The key challenge in achieving this goal is to generate high pressures with little or no pressure gradient. This may be possible using an inert gas as a pressure transfer medium."
03High pressure limits for sensors being determined
Quantum control of diamond NV color centers and in situ sensing of strain and magnetic fields
Finally, these experiments show that NV color centers can be used as in-situ quantum sensors (situ quantum sensors) to study the magnetic properties of materials at Mbar pressure.
Now, the researchers are planning to optimize their sensor and determine its high pressure limit; they also hope to improve magnetic sensitivity (by optimizing fluorescence collection efficiency) and develop multi-mode sensing schemes to measure temperature and magnetic fields simultaneously.
Original paper：
https://arxiv.org/ftp/arxiv/papers/2204/2204.05064.pdf
Reference link：
https://physicsworld.com/a/quantum-sensor-survives-at-record-high-pressures/
