In a groundbreaking achievement, a team of scientists at the Max Planck Institute for the Science of Light has successfully cooled traveling sound waves in waveguides using laser light to a much greater extent than ever before. Led by Dr. Birgit Stiller, the team’s research brings us closer to achieving the quantum ground state of sound in waveguides and opens up new opportunities for quantum communication systems and future quantum technologies.
The Quest for the Quantum Ground State
The ultimate goal of cooling sound waves in waveguides is to reach the quantum ground state of sound. By completely cooling the system, the number of quantum particles causing disturbance to quantum measurements, known as acoustic phonons, can be reduced to almost zero. This bridging of the gap between classical and quantum mechanics provides deeper insights into the transition from classical to quantum phenomena of sound.
In their recent study published in Physical Review Letters, the Stiller Research Group reports a remarkable achievement in lowering the temperature of sound waves in an optical fiber. Through laser cooling, they were able to reduce the initial phonon number by 75% at a temperature of 74 K, equivalent to -199 Celsius. This decrease in temperature was made possible by utilizing the nonlinear optical effect of stimulated Brillouin scattering, where laser light efficiently couples to sound waves.
One of the advantages of using glass fibers is their excellent ability to conduct both light and sound over long distances. This strong interaction between light and sound in glass fibers allows for effective cooling of propagating sound waves. By creating an environment with less thermal noise, glass fibers provide a promising platform for quantum communication systems that require reduced disturbance from thermal noise.
Unlike most physical platforms that have been brought to the quantum ground state on a microscopic scale, the Stiller Research Group’s experiments demonstrate the cooling of sound waves over a length of 50 cm in an optical fiber. This significant achievement sets the stage for further experiments and applications in quantum technology. Cooling long acoustic phonons opens up possibilities for broadband applications and paves the way for a new landscape of experiments that deepen our understanding of the fundamental nature of matter.
In quantum mechanics, sound can be described as a particle known as the phonon. The phonon represents the smallest amount of energy that occurs as an acoustic wave at a certain frequency. To study a single quanta of sound and observe the transition from classical to quantum behavior, the number of phonons must be minimized. The quantum ground state is particularly significant for observing quantum effects, as the vibrations are almost frozen and quantum phenomena can be measured.
Waveguide systems have the advantage of allowing light and sound to propagate along the waveguide, rather than being confined between two mirrors. Acoustic waves in waveguides exist as a continuum and can have a broad bandwidth, making them suitable for high-speed communication systems. The ability to push fibers into the quantum ground state opens up new insights and applications in various fields, such as telecommunications and quantum computing.
The recent breakthrough in laser cooling of sound waves brings us one step closer to achieving the quantum ground state in waveguides. By significantly reducing the temperature of propagating sound waves in an optical fiber, the Stiller Research Group’s study opens up new possibilities for quantum communication systems and provides a deeper understanding of the fundamental nature of sound. This groundbreaking research lays the foundation for further experiments and applications in quantum technology, bringing us closer to a new era of quantum information processing.
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