Groundbreaking steps towards achieving a quantum refrigerator

Researchers have moved closer towards accomplishing a quantum refrigerator with the invention of a novel cooling concept; computer simulations indicate that quantum fields could be utilised to break low-temperature records.

Alone, atoms are neither hot nor cold, and typically, temperature can only be classified for objects comprised of many particles.

However, researchers at TU Wein, FU Berlin, the University of Lisbon, and Nanyang Technological University, have now indicated it is feasible to highlight what possibilities arise when thermodynamics and quantum physics are merged: it is possible to precisely utilise quantum effects to cool a cloud of ultracold atoms even further.

Regardless of the advanced cooling methods that have previously been employed, with this novel method, it is possible to come closer to absolute zero. There is still a lot of research that must be done before this theory can be converted into an actual quantum refrigerator, but the researcher’s preliminary experiments have already demonstrated that the necessary steps to achieve this quantum refrigerator are hypothetically possible.

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,” explained Professor 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,” commented Professor 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,” said Professor Jörg Schmiedmayer of TU Wien.

Next, the group of researchers investigated the ways in which quantum heat machines could be designed. In this search, they were driven by the operating principle of a standard refrigerator: in the beginning, everything has the same temperature, including the inside of the refrigerator, the environment, and the coolant. However, once the coolant inside the fridge is evaporated, heat is extracted there. The heat is then released outside when the coolant is liquefied once more. So, by increasing and reducing the pressure, it is feasible to cool the inside and transfer the heat to the environment.

The question facing the researchers was whether there was also a quantum version of this process. “Our idea was to use a Bose-Einstein condensate for this, an extremely cold state of matter,” explained 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.”

Redistributing energy at an atomic level

A Bose-Einstein condensate is separated into three parts, which begin with 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,” explained 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,” added 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.”

Currently, this quantum refrigerator is merely a hypothetical idea, but research conducted so far has already indicated that the required steps are possible. “Now that we know that the idea basically works, we will try to implement it in the lab,” said Joao Sabino of TU Wien. “We hope to succeed in the near future.”

That would be a remarkable advancement in cryogenic physics, as regardless of what other techniques scientists employed to reach extremely low temperatures, it would always be possible to include this innovative quantum refrigerator at the end as a concluding 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,” added Schmiedmayer.

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