Quantum algorithms bring ions to a standstill
Researchers overcome a major hurdle on the journey towards even more
accurate optical atomic clocks

Date:
December 13, 2021
Source:
Physikalisch-Technische Bundesanstalt (PTB)
Summary:
Laser beams can do more than just heat things up; they can cool
them down too. That is nothing new for physicists who have devoted
themselves to precision spectroscopy and the development of optical
atomic clocks. But what is new is the extremely low temperature that
researchers at the QUEST Institute at the Physikalisch-Technische
Bundesanstalt (PTB) have been able to reach with their highly
charged ions -- this type of ion has never been cooled down as far
as 200 myK before. The team working on this succeeded by combining
their established methods which include the laser cooling of
coupled ions and methods from the field of quantum computing.

The application of quantum algorithms ensured that ions that are
too dissimilar for traditional laser cooling to work effectively
could be cooled down together after all. This means that we are
getting closer to an optical atomic clock with highly charged ions,
and this clock might have the potential to be even more accurate
than existing optical atomic clocks.



FULL STORY ========================================================================== Laser beams can do more than just heat things up; they can cool them
down too.

That is nothing new for physicists who have devoted themselves to
precision spectroscopy and the development of optical atomic clocks. But
what is new is the extremely low temperature that researchers at the
QUEST Institute at the Physikalisch-Technische Bundesanstalt (PTB)
have been able to reach with their highly charged ions -- this type
of ion has never been cooled down as far as 200 myK before. The team
working on this succeeded by combining their established methods which
include the laser cooling of coupled ions and methods from the field
of quantum computing. The application of quantum algorithms ensured
that ions that are too dissimilar for traditional laser cooling to work effectively could be cooled down together after all. This means that we
are getting closer to an optical atomic clock with highly charged ions,
and this clock might have the potential to be even more accurate than
existing optical atomic clocks. The results have been published in the
current issue of Physical Review X.


==========================================================================
If you want to investigate particles -- such as ions -- extremely
accurately (say, using precision spectroscopy or for measuring their
frequency in an atomic clock), then you have to bring them as close as
you can to a standstill.

The most extreme standstill is the same as the lowest possible temperature
- - meaning you have to cool them down as efficiently as you can. One
of the established high-tech cooling methods is so-called laser
cooling. This method sees the particles being slowed down by lasers
that have been skillfully arranged. Not every particle is suited to
this method, however. That is why pairs of coupled ions have been used
at the QUEST Institute for a long time in order to overcome this: One
ion (called the "cooling ion" or the "logic ion") is cooled by lasers; simultaneously, its coupled partner ion is also cooled and can then be investigated spectroscopically (hence, it is called the "spectroscopy
ion"). But this method has previously always reached its limits when the
two ions have differed by too much in their charge-to-mass ratios - -
that is, when they have been very different in mass and very differently charged. "But it is now these very ions that are particularly interesting
for our research, for instance, for developing novel optical clocks,"
explains QUEST physicist Steven King.

As he and his team are naturally very experienced in applying the laws
of quantum mechanics (coupled cooling is, after all, based on quantum
laws), they have made use of the toolkit of the quantum computing
researcher. Quantum algorithms -- i.e. computer operations that are
based on manipulating individual quanta -- cannot only be used to perform calculations faster than ever before with a quantum computer. They can
also help to extract kinetic energy from the mismatched ion pair. During
the process of so-called algorithmic cooling, quantum operations are used
to do just that: to transfer the energy from the barely coolable motion
of the spectroscopy ion to the easily coolable motion of the logic ion.

And they managed to do this extremely well: "We were able to extract
so much energy from the pair of ions -- consisting of a singly charged beryllium ion and a highly charged argon ion -- that their temperature
finally dropped to only 200 myK," said one of QUEST's PhD students Lukas Spiess. Such an ensemble has never been so close to absolute zero (as in:
so motionless). "What is more, we also observed an unprecedentedly low
level of electric-field noise," he expanded. This noise normally leads
to the ions being heated when the cooling stops, but this turns out to be particularly low in their apparatus. Combining these two things means that
the final major hurdle in their way has now been overcome, and an optical atomic clock that is based on highly charged ions can be built. This
atomic clock could reach an uncertainty of less than 10-18. Only the
best optical atomic clocks in the world are currently able to reach this
kind of performance. These findings are also of great significance for
the development of quantum computers and for precision spectroscopy.

========================================================================== Story Source: Materials provided by Physikalisch-Technische_Bundesanstalt_(PTB). Original written by Erika
Schow. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Steven A. King, Lukas J. Spiess, Peter Micke, Alexander Wilzewski,
Tobias
Leopold, Jose' R. Crespo Lo'pez-Urrutia, Piet
O. Schmidt. Algorithmic Ground-State Cooling of Weakly Coupled
Oscillators Using Quantum Logic.

Physical Review X, 2021; 11 (4) DOI: 10.1103/PhysRevX.11.041049 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/12/211213121239.htm

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