Exploring spin coherence of a single atom: finding a (quantum needle) in a haystack

A team of scientist from the IBM Research Lab at Almaden (San Jose, USA), the IBS-QNS -Institute for Basic Science, Center for Quantum Nanoscience (Seoul, Korea) and the QuantaLab,  INL has been able to carry out a magnetic resonance experiment that probes an individual magnetic atom. This permits them to probe, at the atomic scale, “quantum coherence”, an essential resource in the so called quantum technologies.

Magnetic resonance is a technique that permits to obtain information about biological tissues from the measurement of the magnetic behaviour of atomic nuclei. In a conventional magnetic resonance measurement, such as those carried out at hospitals, it is necessary to integrate the signal coming from, at least, a trillion (1012) atoms.

Being able to probe a individual magnetic atom, is very exciting”, says Joaquín Fernández-Rossier (QuantaLab-INL , a theoretical physicist involved in the work, “this finding provides a deeper understanding of one of the main challenges that we are facing to build quantum computers, namely, to extend the lifetime of quantum coherence”.

These findings have been reported in the prestigious journal Science Advances, edited by the American Association for the Advancement of Science, on February 16 2018.

Magnetic resonance and quantum coherence. In a conventional magnetic resonance experiment, atoms are prepared in a quantum state in which their magnetic moments is pointing out, at the same time, in two opposite directions. The phenomenon that enables the existence of this type of state, impossible at the macroscopic scale, is known as quantum coherence. In their experiment, the research team was able to measure the lifetime of the quantum coherence of an individual atom.

Mechanism of loss of coherence. In this case, the resonance experiment entails electrical current going through the individual atom. In other words, the atom is being bombarded by electrons.The pace at which this happens determines the lifetime of the aforementioned coherent superposition state: the larger the current, the shorter the lifetime of the quantum coherence.

A microscope to “see” atoms. The tool that makes this experiment possible is the Scanning Electron Tunnelling Microscope, that contains a sharp metallic tip that is approached to the atoms of surface, in ultra-high vacuum and very low temperatures. This tool, used in many labs to visualise individual atoms, in a way that resembles the way visually impaired people uses a white cane, can in addition be used to excite and detect the magnetic resonance of individual atoms.

An ongoing collaboration.  This work is part of an ongoing collaboration between  the teams at IBM Almaden,  QNS-IBS and  the group of J. Fernández-Rossier at QuantaLab-INL.  A few months after another publication in Physical Review Letters, where the same team demonstrated the manipulation of couples of atoms on a surface, using the same technique

Probing quantum coherence in single-atom electron spin resonance
P. Willke, W. Paul, F. D. Natterer, K. Yang, Y. Bae, T.Choi, J. Fernández-Rossier,, A. J. Heinrich, and C. P. Lutz
SCIENCE  ADVANCES  : Vol. 4, no. 2, eaaq1543 FEB 16  2018

Engineering the Eigenstates of Coupled Spin-1/2 Atoms on a Surface
Yang, Kai; Bae, Yujeong; Paul, William; et al.
PHYSICAL REVIEW LETTERS Volume: 119 Issue: 22 Article Number: 227206 Published: NOV 29 2017




Scheme of the experimental setup. An STM tip (grey balls) is placed on top of an individual iron (Fe) atom, placed on a surface. Reproduced from Science Advances

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