If the spins of phosphorus atoms in silicon are cleverly excited with microwave pulses, a so-called spin echo signal can be detected after a certain time. Surprisingly, this spin echo does not occur only once, but a whole series of echoes can be detected. Credit: C. Hohmann / MCQST

Spin is the total angular momentum of small particles that points in a certain direction, that can be manipulated using a magnetic field. This principle is the basic idea behind magnetic resonance imaging as used in hospitals.

An international research group has now found a fantastic impact in a system that is especially appropriate for processing quantum information: the spins of phosphorus atoms in a piece of silicon, coupled to a microwave resonator.

Simulating these spins with microwave pulses can detect a so-called spin-echo signal after a specific time. This signal is known as an injected pulse signal that re-emits as a quantum echo. Fascinatingly, this quantum echo doesn’t occur only once, but a whole series of echoes can be detected. This opens up new possibilities of how information can be processed with quantum systems.

Prof. Stefan Rotter from TU Wien (Vienna) said, “Spin echoes have been known for a long time, this is nothing unusual. First, a magnetic field is used to make sure that the spins of many atoms point in the same magnetic direction. Then the atoms are irradiated with an electromagnetic pulse, and suddenly their spins begin to change direction. However, the atoms are embedded in slightly different environments. Thus they are affected by slightly different forces.”

Dr. Hans Hübl, Deputy Director of the Walther-Meißner-Institute (WMI) at the Bavarian Academy of Sciences and Humanities, said, “As a result, the spin does not change at the same speed for all atoms. Some particles change their spin direction faster than others, and soon you have a wild jumble of spins with completely different orientations.”

Stefan Rotter said, “But it is possible to rewind this apparent chaos – with the help of another electromagnetic pulse. A suitable pulse can restore the previous spin rotation so that all spins fall back in sync once again.”[…]

Continue reading: A remarkable echo effect discovered