New possibilities for working with quantum information

Date:
September 24, 2020
Source:
Vienna University of Technology
Summary:
The spin of particles can be manipulated by a magnetic field. This
principle is the basic idea behind magnetic resonance imaging as
used in hospitals. A surprising effect has now been discovered in
the spins of phosphorus atoms coupled to microwaves: If the atoms
are excited, they can emit a series of echoes. This opens up new
ways of information processing in quantum systems.



FULL STORY ========================================================================== Small particles can have an angular momentum that points in a certain
direction -- the spin. This spin can be manipulated by 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 resonator. If these
spins are cleverly excited 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. Surprisingly, this spin echo does not
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 experiments were carried out at the Walther-Meissner-Institute
in Garching by researchers from the Bavarian Academy of Sciences and
Humanities and the Technical University of Munich, the theoretical
explanation was developed at TU Wien (Vienna). Now the joint work has
been published in the journal Physical Review Letters.

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. It is therefore possible that slightly different forces act on their spins. "As
a result, the spin does not change at the same speed for all atoms,"
explains Dr.

Hans Hu"bl from the Bavarian Academy of Sciences and Humanities. "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 reverse the previous
spin rotation so that the spins all come together 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 Hu"bl. "The remarkable thing is that we were not only able
to measure a single echo, but a series of several echoes." The spin
that influences itself At first, it was unclear how this novel effect
comes about. But a detailed theoretical analysis now made it possible
to understand the phenomenon: It is due to the strong coupling between
the two components of the experiment -- the spins and the photons in
a microwave resonator, an electrical circuit in which microwaves can
only exist at certain wavelengths. "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 Hu"bl.

The strong coupling between the atomic spins and the microwave
resonator is also responsible for the multiple echoes: If the spins
of the atoms all point in the same direction in the first echo, this
produces an electromagnetic signal. "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." The physics of the spin echo has great significance
for technical applications -- it is an important basic principle behind magnetic resonance imaging. The new possibilities offered by the multiple
echo, such as the processing of quantum information, will now be examined
in more detail. "For sure, multiple echos 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," says Rudolf Gross, co-author
and director of the Walther-Meissner-Institute.


========================================================================== Story Source: Materials provided
by Vienna_University_of_Technology. Original written by Florian
Aigner. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Stefan Weichselbaumer, Matthias Zens, Christoph W. Zollitsch,
Martin S.

Brandt, Stefan Rotter, Rudolf Gross, Hans Huebl. Echo Trains
in Pulsed Electron Spin Resonance of a Strongly Coupled
Spin Ensemble. Physical Review Letters, 2020; 125 (13) DOI:
10.1103/PhysRevLett.125.137701 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200924135346.htm

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