Multiple echoes as a result of a strong link between spins and microwave photons
2020-10-01 – News from the Physics Department
A team of researchers from the Technical University of Munich (TUM), TU Wien
(Vienna University of Technology) and the Bavarian Academy of Sciences and
Humanities have discovered a remarkable echo effect – this effect presents
exciting, new opportunities for working with quantum information.
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.
– Image: C. Hohmann / MCQST
Small particles can have an angular momentum that points in a certain direction
– this is known as spin. This spin can be manipulated using a magnetic field.
This principle, for example, is the basic idea behind magnetic resonance imaging
as used in hospitals.
An international research team has now discovered a surprising effect in a
system that is particularly well suited for processing quantum information: the
spins of phosphorus atoms in a piece of silicon, coupled to a microwave
If these spins are expertly stimulated with microwave pulses, a so-called spin
echo signal can be detected after a certain time – the injected pulse signal is
re-emitted as a quantum echo.
Amazingly, 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.
The echo of quantum spins
“Spin echoes have been known for a long time, this is nothing unusual”, says
Prof. Stefan Rotter
from TU Wien (Vienna). 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.
“As a result, the spin does not change at the same speed for all atoms,”
explains Dr. Hans Hübl, Deputy Director of the
at the Bavarian Academy of Sciences and Humanities and member of the Chair of
Technical Physics at the Technical University of Munich. “Some particles change
their spin direction faster than others, and soon you have a wild jumble of
spins with completely different orientations.”
But it is possible to rewind this apparent chaos – with the help of another
electromagnetic pulse. A suitable pulse can actually restore the previous spin
rotation, so that all spins fall back in sync once again.
“You can imagine it’s a bit like running a marathon,” says Stefan Rotter. “At
the start signal, all the runners are still together. As some runners are faster
than others, the field of runners is pulled further and further apart over time.
However, if all runners were now given the signal to return to the start, all
runners would return to the start at about the same time, although faster
runners have to cover a longer distance back than slower ones.”
In the case of spins, this means that at a certain point in time all particles
have exactly the same spin direction again - and this is called the “spin echo”.
“Based on our experience in this field, we had already expected to be able to
measure a spin echo in our experiments,” says Hans Hübl. “The remarkable thing
is that we were not only able to measure a single echo, but a series of several
The spin that influences itself
At first it was unclear how this new effect had come about, but more detailed
theoretical analyses made it possible to understand the phenomenon: it was down
to the strong link between the two elements involved in the experiment – the
spins and the photons in the microwave resonator, an electric circuit in which
microwaves can only exist with certain wave lengths.
“This coupling is the essence of our experiment: You can store information in
the spins, and with the help of the microwave photons in the resonator you can
modify it or read it out,” says Hans Hübl.
The strong coupling between the atomic spins and the microwave resonator is also
responsible for the multiple echoes: an electromagnetic signal is created when
the atom spins have all returned to the same direction at the first echo.
“Thanks to the coupling to the microwave resonator, this signal acts back on the
spins, and this leads to another echo, and on and on,” explains Stefan Rotter.
“The spins themselves cause the electromagnetic pulse, which is responsible for
the next echo.”
Great importance for technical applications
The physics behind spin echoes are of great importance for technical
applications - they constitute a fundamental principle that forms the basis of
magnetic resonance imaging, for example. It is now necessary to conduct more
thorough investigations into the new opportunities presented by the recently
discovered multiple echo, for the processing of quantum information, for
“For sure, multiple echoes in spin ensembles coupled strongly to the photons of
a resonator are an exciting new tool. It will not only find useful applications
in quantum information technology, but also in spin-based spectroscopy methods”,
director of the Walther-Meißner-Institute and
professor for technical physics at the Technical University of Munich.