A key factor for the performance of sensors is the speed at which the system returns to its initial state after a disturbance or measurement, similar to the taring of a balance. In the quantum sensor under investigation, this corresponds to the transition of electrons from an energetically excited state to the ground state. However, the electrons remain in a kind of "metastable intermediate state" for a short time. A team of physicists from Julius-Maximilians-Universität Würzburg (JMU) has now directly measured this "waiting time" in a two-dimensional material: it lasts exactly 24 billionths of a second.

This knowledge is particularly important for quantum technology. It can be used to significantly increase the accuracy of atomic sensors, paving the way for the medical diagnostics of the future, for example. Professor Vladimir Dyakonov, Head of the Chair of Experimental Physics VI (EPVI), was responsible for the study, which has been published in the journal Science Advances .

Background: In modern quantum technology, so-called atomic defects in solids form the basis for precise measuring instruments. For a long time, diamond was considered the standard material for quantum sensors, as its three-dimensional crystal structure effectively protects against external interference. If one of these atoms is missing in the normally perfect lattice of carbon atoms, this defect acts as a tiny quantum sensor whose properties can be controlled using lasers and microwaves.

The problem is that in the three-dimensional diamond lattice, the distance between the sensor defect and the object to be analysed is relatively large, which reduces the signal strength. The situation is different with the material that the Würzburg team investigated: hexagonal boron nitride (hBN) - a two-dimensional material made up of a single layer of atoms.

"In contrast to 3D crystals, hBN allows the positioning of spin defects with atomic precision within a very thin layer," says Vladimir Dyakonov, explaining the key advantage of this material. This enables a significantly smaller distance to the measurement object and therefore a stronger interaction. The negatively charged boron defects are particularly promising, as they can also be addressed optically at room temperature. However, to fully utilise the potential of these 2D sensors, mere spatial proximity is not enough; it is imperative to understand the internal "clock" and the dynamic processes of these defects in detail.

The speed at which the system returns to its ground state after optical excitation is a decisive factor. A so-called "metastable intermediate state", figuratively speaking a kind of car park or waiting room for electrons, plays an important role here: Before electrons can return to the ground state, they remain in this state for a short time, which limits the sequence of measurement cycles.

Until now, knowledge about this intermediate state was primarily based on theoretical simulations. The team at the University of Würzburg has now succeeded in directly measuring the lifetime of this state experimentally for the first time. "At room temperature, this is exactly 24 nanoseconds; at temperatures of liquid helium, this lifetime is almost doubled. To be able to observe this, we use a laser like a stroboscope and take snapshots of our system," explains Paul Konrad, PhD student at the EPVI Chair, who carried out the experiments. This direct experimental validation is highly relevant for experts, as it provides a reliable basis for precisely matching the control of the sensors to the natural dynamics of the material.

This is important because the sensitivity of a quantum sensor depends largely on what is known as "coherent control" - i.e. the ability to precisely control quantum states. The researchers were able to prove that optimised time management significantly increases the efficiency of this control. By inserting a targeted delay of around 150 nanoseconds between laser excitation and manipulation by microwaves, it was ensured that the "waiting room" of the intermediate state is completely emptied and all electrons are ready for measurement in the ground state.

The results of this optimisation: "The contrast of the measurement results increased by almost 26 percent. This results in an improvement in the sensitivity of the overall system by around eleven per cent," says Dyakonov. The reason for this increase is the effect that the adjusted timing allows more spins in the ensemble to be addressed at the same time. "As the sensitivity increases statistically with the number of spins involved, emptying the 'car park' leads directly to a more precise measurement result," summarises the physicist.

The insights gained are an important step for the further development of quantum sensor technology based on 2D materials. The knowledge about the lifetime of the intermediate state now enables the design of more complex measurement protocols or the use in novel 2D heterostructures.

Despite these advances, technological challenges remain. The magnetic environment in hBN is more complex than in diamond, as the material consists of 100 per cent magnetic isotopes whose nuclear spins shorten the coherence time of the sensors. Future research must therefore find ways to further minimise these environmental perturbations.