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University of Basel

02 February 2022

Cooling matter from a distance

Illustration of a vibrating membrane and a cloud of atoms coupled by a laser loop
Light is used to couple a vibrating membrane to a cloud of atoms in order to form a control loop. The two different quantum systems — consisting of the membrane and the spins — therefore regulate one another’s temperature with no need for external measurement. (Image: Department of Physics, University of Basel)

Researchers from the University of Basel have succeeded in forming a control loop consisting of two quantum systems separated by a distance of one meter. Within this loop, one quantum system — a vibrating membrane — is cooled by the other — a cloud of atoms, and the two systems are coupled to one another by laser light. Interfaces such as this allow different kinds of quantum systems to interact with one another even over relatively large distances and will play a key role in quantum technologies of the future.

We’ve all experienced the principle of feedback — for example, when we use a thermostat in conjunction with a heating system to regulate indoor temperature. The thermostat measures the current temperature, compares it with the target value and regulates the flow of heat accordingly. Control loops of this kind appear in many areas of everyday life and technology.

They are also useful in the quantum world when it comes to bringing a system into a desired state. For example, it’s often necessary to work at very low temperatures — close to absolute zero — in order to observe the sensitive effects of the quantum world and to apply these effects to new technological applications. Classical feedback requires a measurement to be taken within a control loop and only works to a limited extent in the world of quanta, which differs from the macroscopic world we’re familiar with in many respects.

The reason for these limitations is that in quantum systems, the very act of taking a measurement causes a change in the system and therefore leads to uncontrolled backaction. With this in mind, researchers led by Professor Philipp Treutlein from the Department of Physics and the Swiss Nanoscience Institute of the University of Basel have used the principle of coherent feedback to cool a quantum system for the first time — and they have published their results in the journal Physical Review X.

Control without measurement

Coherent feedback describes a situation in which two quantum systems interact with one another. As one of the systems acts as a control unit for the other, no measurement is needed. Instead, the control system is configured to bring the target system into a desired state by means of coherent quantum mechanical interaction.

Specifically, the researchers used atoms as a quantum mechanical control system to control the temperature of a macroscopic but very thin vibrating membrane. This process first involves aligning the intrinsic angular momentum (spin) of the atoms in a well-defined direction, which corresponds to a very cold state close to absolute zero. In contrast, the high temperature of the membrane causes it to vibrate strongly.

Quantum mechanical interaction allows the atoms and membrane to swap states, causing the membrane to become cold as its energy is transferred to the atoms. Subsequently, however, the atoms can quickly be returned to their initial state using laser light in preparation for another energy transfer from the membrane.

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