Single-photon emitters are an important building block for the applications of quantum technologies. Photons are an excellent means of transmitting data in a fast and secure manner. However, it is necessary to have a sound physical under­standing of the structure of the single-photon emitter and how to control them. Therefore, a team of physicists from the Univer­sity of Münster in Germany and Wrocław University of Science and Technology in Poland has undertaken the first systematic study of the ultrafast control of single-photon emitters in the two-dimensional material hexagonal boron nitride (hBN) using laser pulses.

Such two-dimen­sional materials continue to be the focus of many scientific studies. Graphene is one prominent example and, in 2010, a Noble Prize was awarded for the discovery and the initial analysis of its properties. “Hexagonal boron nitride – hBN in short – is a 2D material with parti­cularly interesting properties”, explains Daniel Wigger from the Theo­retical Physics Department of Wrocław Tech. “Among other things, hBN hosts single-photon emitters, which even work at room tempera­ture, in contrast to many other systems that require extremely low tempera­tures.” Experts are convinced, that these single-photon emitters originate from impurity atoms, that is atomic defects or color centers within the hBN crystal.

The scientists took a closer look at these color centers, whose exact atomic structure is still unknown. They developed a compre­hensive under­standing of the dynamics within the color center inside the hBN crystal by combining their experiments with theoretical modelling. One of their focuses was on the detri­mental impact of the environment the quantum dynamics. Micro­scopic systems are affected by different inter­actions with the environment, which manifest as external noise on varying time scales, for example, as slight colour fluc­tuations of the emitted photons. The quantum proper­ties of such systems are, in particular, very sensitive to this noise. This can lead to deco­herence, which results in the loss of quantum information stored in the system.

The team used ultra­fast laser pulses to prepare and read out the quantum state of the atomic defect. “In simple terms, the applied technique works like a strobo­scope,” says physicist Steffen Michaelis de Vasconcellos from the Institute of Physics and the Center for Nanotechnology at the Univer­sity of Münster. “A first pulse creates a quantum state which, after a brief delay, is read out by a second pulse. By varying the time between the two pulses, it is possible to measure the change in the quantum state and thereby the decoherence.” In addition to this key experiment, the physicists undertook a detailed investi­gation into the spectrum of the emitters, that is, which color the emitter is generating.

They supplemented the experi­ments with computer simulations, which were run with the same parameters as the experiments. Special attention was paid to phonons – sound waves in the crystal – that can have a parti­cularly detri­mental influence. “Experiment and theory have provided a consistent picture in our study,” says Wigger. The scientists have, for the first time, not only considered the dynamic character of the emitter system but also the light spectrum, in order to understand the impact of these external influences on different time scales. Based on these results, pertur­bations can potentially be avoided in future appli­cations and phonons can be integrated into techno­logical appli­cations as an additional type of quantum excitation. (Source: WWU)

Reference: J. A. Preuss et al.: Resonant and phonon-assisted ultrafast coherent control of a single hBN color center, Optica 9, 522 (2022); DOI: 10.1364/OPTICA.448124

Link: Quantum Optics & Nanophotonics, Westfälische Wilhelms-Universität Münster WWU, Münster, Germany