The quantum refrigerator

Date:
July 29, 2021
Source:
Vienna University of Technology
Summary:
By combining quantum theory and thermodynamics, it is possible
to design a new kind of atomic refrigerator, which can cool down
extremely cold Bose-Einstein-condensates even further.



FULL STORY ==========================================================================
At first glance, heat and cold do not have much to do with quantum
physics. A single atom is neither hot nor cold. Temperature can
traditionally only be defined for objects that consist of many
particles. But at TU Wien, in collaboration with FU Berlin, Nanyang Technological University in Singapore and the University of Lisbon, it has
now been possible to show what possibilities arise when thermodynamics
and quantum physics are combined: One can specifically use quantum
effects to cool a cloud of ultracold atoms even further.


==========================================================================
No matter what sophisticated cooling methods have been used before --
with this technique, which has now been presented in the scientific
journal "Physical Review X-Quantum," it is possible to come a little
closer to absolute zero. A lot of work is still needed before this new
cooling concept can be turned into an actual quantum refrigerator, but
initial experiments already show that the necessary steps are possible
in principle.

A new field of research: quantum thermodynamics "For a long
time, thermodynamics has played an important role for classical
mechanical machines -- think of steam engines or combustion engines,
for example. Today, quantum machines are being developed on a tiny
scale. And there, thermodynamics has hardly played a role there so far"
says Prof. Eisert from the Free University of Berlin.

"If you want to build a quantum heat machine, you have to fulfil two requirements that are fundamentally contradictory," says Prof. Marcus
Huber from TU Wien. "It has to be a system that consists of many particles
and in which you cannot control every detail exactly. Otherwise you
cannot speak of heat. And at the same time, the system must be simple
enough and sufficiently precisely controllable not to destroy quantum
effects. Otherwise, you can't talk about a quantum machine." "Back in
2018, we came up with the idea of transferring the basic principles of
thermal machines to quantum systems by using quantum field descriptions of many-body quantum systems," says Prof. Jo"rg Schmiedmayer (TU Wien). Now
the research team from TU Wien and FU Berlin examined in detail how such quantum heat machines can be designed. They were guided by the operating principle of an ordinary refrigerator: initially, everything has the same temperature -- the interior of the refrigerator, the environment and the coolant. But when you evaporate the coolant inside the refrigerator,
heat is extracted there. The heat is then released outside when the
coolant is liquefied again. So by raising and lowering the pressure it
is possible to cool the inside and transfer the heat to the environment.

The question was whether there could also be a quantum version of such
a process. "Our idea was to use a Bose-Einstein condensate for this, an extremely cold state of matter," says Prof. Jo"rg Schmiedmayer. "In recent years, we have gained a lot of experience in controlling and manipulating
such condensates very precisely with the help of electromagnetic fields
and laser beams, investigating some of the fundamental phenomena at the borderline between quantum physics and thermodynamics. The logical next
step was the quantum heat machine." Energy redistribution at the atomic
level A Bose-Einstein condensate is divided into three parts, which
initially have the same temperature. "If you couple these subsystems in
exactly the right way and separate them from each other again, you can
achieve that the part in the middle acts as a piston, so to speak, and
allows heat energy to be transferred from one side to the other," explains Marcus Huber. "As a result, one of the three subsystems is cooled down."
Even at the beginning, the Bose-Einstein condensate is in a state of very
low energy -- but not quite in the lowest possible energy state. Some
quanta of energy are still present and can change from one subsystem
to another -- these are known as "excitations of the quantum field."
"These excitations take on the role of the coolant in our case," says
Marcus Huber. "However, there are fundamental differences between our
system and a classical refrigerator: In a classical refrigerator, heat
flow can only occur in one direction -- from warm to cold. In a quantum
system, it is more complicated; the energy can also change from one
subsystem to another and then return again. So you have to control very precisely when which subsystems should be connected and when they should
be decoupled." So far, this quantum refrigerator is only a theoretical
concept -- but experiments have already shown that the necessary steps
are feasible. "Now that we know that the idea basically works, we will
try to implement it in the lab," says Joao Sabino (TU Wien). "We hope
to succeed in the near future." That would be a spectacular step forward
in cryogenic physics -- because no matter what other methods you use to
reach extremely low temperatures, you could always add the novel 'quantum refrigerator' at the end as a final additional cooling stage to make one
part of the ultracold system even colder. "If it works with cold atoms,
then our ideas can be implemented in many other quantum systems and lead
to new quantum technology applications," says Jo"rg Schmiedmayer.

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


========================================================================== Journal Reference:
1. Marek Gluza, Joa~o Sabino, Nelly H.Y. Ng, Giuseppe Vitagliano, Marco
Pezzutto, Yasser Omar, Igor Mazets, Marcus Huber, Jo"rg
Schmiedmayer, Jens Eisert. Quantum Field Thermal Machines. PRX
Quantum, 2021; 2 (3) DOI: 10.1103/PRXQuantum.2.030310 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/07/210729122150.htm

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