An Atomic Quantum Bit Made Switchable

One bit per atom: Augsburg physicists, together with US colleagues, achieve the ultimate limit for a nanoscale data memory

superconducting quantum bits
The quantum tunneling of the magnetization allows a targeted freezing or folding of a magnetic moment, depending on which direction an external magnetic field is applied. © University of Augsburg / IfP / EKM

The increasing miniaturization of electronic circuits and storage media is progressing steadily. But how small can you actually make a bit of information? Is a single atom sufficient to write 0 and / or 1 and freeze their state? The magnetic moment of single atoms or of small clusters actually allows this. It acts like a tiny bar magnet that allows only two possible orientations: either the magnetic north pole points upwards or downwards. Between both states there is a high energy barrier, which prevents easy switching and prohibits orientations in the middle.

But now quantum mechanics allows a shortcut: Instead of laboriously climbing the energy barrier, you can tunnel through them. However, there are some things to keep in mind: the energies of the two states between which such a tunneling process takes place must be exactly the same, which is also called degeneration. With an externally applied magnetic field, this can be canceled, resulting in a blockade of the tunnel path. The orientation of the magnetic moment is frozen.

Instead of only 0.003 Tesla

That this can be achieved even with very small magnetic fields, results of the junior research group around the Augsburg physicist Dr. Ing. Anton Jesche (Chair of Experimental Physics VI / EKM), who have now appeared in an article in the journal Physical Review Letter. Together with colleagues from the University of Central Florida and the Ames National Laboratory, the quantum tunneling of the magnetization of individual iron atoms was investigated, which were introduced into a crystalline matrix of lithium nitride. The fact that quantum tunneling can be weakened in magnetic fields has been known for some time and has been intensively studied on so-called molecular magnets. However, a very strong magnetic field in the range of one tesla had to be generated in order to have a noticeable effect on the switchability of the magnetic bit. In contrast, less than half a percent of this value is sufficient to completely suppress the tunnel effect in the newly developed iron system. “Even with a simple coil that you can wrap around your little finger, you can create a field of this size,” reports Jesche, “but above all, it can be switched on or off almost instantaneously, ie without the slightest time delay. ”

This extraordinary behavior is based, on the one hand, on the low defect density of the crystals grown in Augsburg. On the other hand, the chemical environment plays a crucial role: the iron atoms are held in place by only two nearest neighbors. As a result, a high anisotropy, ie a high directional dependence of the atomic properties is generated, which prevents accidental folding of the magnetic moments.

Freeze the magnetic moment or fold it down

However, not only has the suppression of the quantum mechanical tunneling effect been successful, the opposite has also proved possible: If one sets the external magnetic field along certain directions, namely perpendicular to the imaginary line between iron and its two neighboring atoms, the tunneling rate can even be significantly increased. You can either freeze the magnetic moment or promote its folding specifically.

Technically easily realizable 10 Kelvin above absolute zero

With one bit per atom, this seems to be the ultimate limit for a nanoscale data store. “In principle, one can also carry out mathematical operations with these states,” says Jesche, “although it is still a long way to a possible quantum computer.” However, the relatively high temperatures at which the transition from the classical one are promising are in any case already promising to quantum mechanical behavior: 10 Kelvin above absolute zero can be technically quite easily realized, they are more than a hundred times higher than in current computer architectures based on superconducting quantum bits.

Source : University of Augsburg