A quantum thermometer for measuring ultra-cold temperatures
Researchers show that a quantum sensor using a single atom can accurately measure the coldest places in the universe

Date:
September 16, 2020
Source:
Okinawa Institute of Science and Technology (OIST) Graduate
University
Summary:
In everyday life, measuring temperature is pretty
straightforward. But in the quantum world, which deals with
the super small and the ultra-cold, determining how hot or cold
something is starts to get more challenging.

Now researchers have described a quantum process that uses a single
atom as a thermometer to sensitively measure the temperature of
an ultra-cold gas.



FULL STORY ==========================================================================
In everyday life, measuring temperature is pretty straightforward. But
in the quantum world, which deals with the super small and the
ultra-cold, determining how hot or cold something is starts to get more challenging. Now, in a collaboration between the Okinawa Institute of
Science and Technology Graduate University (OIST), University College
Dublin and Trinity College Dublin, researchers have described a quantum
process that uses a single atom as a thermometer to sensitively measure
the temperature of an ultra-cold gas.


==========================================================================
"As quantum physicists, our ultimate goal is to create and measure systems
as close as possible to absolute zero. This is the lowest temperature
limit, around -273DEGC or zero on the Kelvin temperature scale, and
it's when particles stop moving. These ultra-cold systems are important
for successfully harnessing quantum technologies or reducing noise in
quantum experiments," said Professor Thomas Busch, head of the Quantum
Systems Unit at OIST and co-author of the study, published as an Editor's Suggestion in Physical Review Letters.

"So being able to detect minute changes in temperature, at only tens of billionths of a degree above zero kelvin, is critical." Typically at
room temperature there are more than a hundred billion trillion atoms
whizzing about at speeds of up to 300 -- 400 meters per second. "When we measure the temperature in a room, we don't try and measure the movement
of all these atoms, we just take measurements from a thermometer," said
Dr. Thoma's Fogarty, a scientist in the Quantum Systems Unit. "Although
one could, in principle, try to measure the velocity of all the atoms
in a quantum system, we wanted to design a simpler and better method
that uses a quantum thermometer." But using a thermometer to measure a
quantum system isn't simple. These systems are colder than any place that exists naturally in the universe. They are also very small, containing
only about 100,000 atoms in the gas. If the thermometer is too large or
too warm it would heat up the gas being measured and destroy the system's quantum properties. So instead, the approach this collaboration took was
to use a thermometer that was also very small and very cold -- a single, super-cooled atom.

When first added into the system, this thermometer atom exists in two
different energy states at the same time -- a unique, counter-intuitive property of quantum systems. But as the thermometer atom interacts with
the ultra-cold gas, the quantum features of this combined energy state
decay. The rate at which this decay occurs is directly related to the temperature of the ultra-cold gas being probed, so as the scientists
measure the state of the thermometer atom, they can accurately infer
the temperature.

"This process essentially destroys the 'quantumness' of the thermometer
atom through interactions with the gas, making it truly a quantum
thermometer," Dr.

Fogarty explained.

The researchers also reported the optimal timings for when the
measurements should be taken, as well as the ideal strength of the
interactions between the single atom and the gas, to get the best
sensitivity and the least amount of noise. The colder the gas, the slower
the decay process occurs, as the single atom interacts more slowly and
less often with the gas. "Therefore, in order to measure temperature
at the lowest extremes we need to wait a long time before measuring and
we require weaker interactions to maximize the signal and minimize the
noise," added Dr. Fogarty.

The team is now exploring numerous paths to improve the method's
sensitivity, such as by using machine learning to optimize
the interactions between the thermometer atom and the gas, or by
introducing more thermometer atoms into the system so more complex
quantum interactions can occur.

"This new method has pushed the bounds of thermometry, which has important applications for quantum technology," concluded Prof. Busch. "I expect
that we are going to see it being used very soon in experiments."

========================================================================== Story Source: Materials provided by Okinawa_Institute_of_Science_and_Technology_(OIST)
Graduate_University. Original written by Dani Ellenby. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Mark T. Mitchison, Thoma's Fogarty, Giacomo Guarnieri, Steve
Campbell,
Thomas Busch, John Goold. In Situ Thermometry of a Cold Fermi Gas
via Dephasing Impurities. Physical Review Letters, 2020; 125 (8)
DOI: 10.1103/PhysRevLett.125.080402 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200916113404.htm

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