Scientists have learned to control quantum state of individual electrons: This could lead to a breakthrough in quantum computing

Physicists from the University of Regensburg have discovered a way to manipulate the quantum state of individual electrons using a microscope with atomic resolution. The results of the study have been published in the renowned journal Nature. Potentially, this finding could have significant implications for quantum computing.

As we know, the world around us is composed of molecules, and molecules are so tiny that even a speck of dust contains countless numbers of them. It is fascinating that there is now the capability to study not only molecules but even the atoms they consist of with high precision, thanks to a microscope. The latest invention by physicists is called an "atomic force microscope." Unlike an optical microscope, the atomic force microscope operates on different principles, relying on the sensitivity of the tiniest forces between the device's tip and the molecule under investigation. This approach allows obtaining an "image" of the internal structure of the molecule. However, merely observing the molecule in this way doesn't guarantee understanding all its properties with certainty. For instance, it is currently challenging to determine the atoms' composition within a molecule.

Fortunately, there are other tools available to determine the composition of molecules. One such method is electron spin resonance, based on the same principles as magnetic resonance imaging in medicine. However, in electron spin resonance, detecting a signal strong enough for detection typically requires an immense number of molecules. Thus, access to the properties of each molecule is not achievable, only their average values.

Researchers from the University of Regensburg, led by Professor Dr. Jascha Repp from the Institute of Experimental and Applied Physics, have now integrated electron spin resonance into atomic force microscopy. It is essential to note that electron spin resonance is recorded directly with the microscope's tip, so the signal originates from a single, individual molecule. Consequently, scientists can characterize individual molecules. This immediately allowed identifying the atoms comprising the molecule under investigation. "We even managed to distinguish molecules that differ not in the type of atoms they consist of but only in their isotopes, i.e., the composition of the atoms' nuclei," adds Lisanne Sellies, the first author of this study.

"However, another intriguing possibility brought by electron spin resonance has captured our attention even more," explains Professor Repp. "This technique can be used to control the spin quantum state of electrons present in a molecule."

This is illustrated in the figure by small colored arrows. But why is this interesting? Quantum computers store and process information encoded in a quantum state. To perform calculations, quantum computers need to manipulate the quantum state without losing information due to the so-called decoherence. It is worth noting that decoherence is the process of breaking the coherence, the connection between two quantum entangled particles, caused by the interaction of the quantum-mechanical system with the environment through an irreversible thermodynamic process.

Researchers from Regensburg have demonstrated that with their new technique, they can manipulate the spin quantum state in a single molecule multiple times before this state decays. Since the microscopy method allows obtaining images of individual molecular neighborhoods, the new methodology may help understand how decoherence in a quantum computer depends on the atomic environment and, ultimately, how to avoid it. This is a path to simpler and, most importantly, more accurate quantum computations.