Max Planck Team Achieves Major Breakthrough in Cooling Sound Waves for Quantum Tech

Scientists at the Max Planck Institute for the Science of Light, led by Dr. Birgit Stiller, have made significant progress in cooling sound waves in waveguides using laser light. This achievement brings us closer to reaching the quantum ground state of sound, which could eliminate unwanted noise generated by acoustic waves at room temperature. The research provides a deeper understanding of the transition from classical to quantum phenomena of sound and is relevant to quantum communication systems and future quantum technologies. The team managed to lower the temperature of a sound wave in an optical fibre by 219 K, reducing the initial phonon number by 75%.

Quantum Ground State of Sound: A Leap Forward

A team of scientists at the Max Planck Institute for the Science of Light, led by Dr. Birgit Stiller, has made a significant advancement in the field of quantum acoustics. They have successfully cooled traveling sound waves in waveguides to a much lower temperature than previously achieved using laser light. This accomplishment is a significant step towards reaching the quantum ground state of sound in waveguides, which could have implications for quantum communication systems and future quantum technologies.

The quantum ground state of an acoustic wave can be reached by completely cooling the system. This process reduces the number of quantum particles, known as acoustic phonons, which cause disturbance to quantum measurements, to almost zero. This achievement bridges the gap between classical and quantum mechanics. Over the past decade, technological advances have made it possible to put a wide variety of systems into this state. However, this has not yet been possible for optical fibers in which high-frequency sound waves can propagate. The Stiller Research Group has now moved a step closer to this goal.

Laser Cooling of Sound Waves

In their study, published in Physical Review Letters, the researchers report that they were able to lower the temperature of a sound wave in an optical fiber initially at room temperature by 219 K using laser cooling. This is ten times further than had previously been reported. The initial phonon number was reduced by 75%, at a temperature of 74 K, -194 Celsius. This drastic reduction in temperature was made possible by the use of laser light. The cooling of the propagating sound waves was achieved via the nonlinear optical effect of stimulated Brillouin scattering, in which light waves are efficiently coupled to sound waves. This effect allows the laser light to cool the acoustic vibrations and create an environment with less thermal noise, which is beneficial for a quantum communication system.

Quantum Ground State in Waveguides

Most physical platforms previously brought to the quantum ground state were microscopic. However, in this experiment, the length of the optical fiber was 50 cm and a sound wave extending over the full 50 cm of the core of the fiber was cooled to extremely low temperatures. “These results are a very exciting step towards the quantum ground state in waveguides and the manipulation of such long acoustic phonons opens up possibilities for broadband applications in quantum technology,” according to Dr. Birgit Stiller, head of the quantum optoacoustics group.

Sound as a Particle: The Phonon

Sound, in the day-to-day classical world, can be understood as a density wave in a medium. However, from the perspective of quantum mechanics, sound can also be described as a particle: the phonon. This particle, the sound quantum, represents the smallest amount of energy which occurs as an acoustic wave at a certain frequency. In order to see and study single quanta of sound, the number of phonons must be minimized. The transition from the classical to quantum behavior of sound is often more easily observed in the quantum ground state, where the number of phonons is close to zero on average, such that the vibrations are almost frozen and quantum effects can be measured.

Implications for Quantum Communication Systems

The advantage of using a waveguide system is that light and sound are not bound between two mirrors, but propagating along the waveguide. The acoustic waves exist as a continuum – not only for certain frequencies – and can have a broad bandwidth, making them promising for applications such as high-speed communication systems. “We are very enthusiastic about the new insights that pushing these fibers into the quantum ground state will bring”, emphasizes Dr. Stiller. “Not only from the fundamental research point of view, allowing us to peek into the quantum nature of extended objects, but also because of the applications this could have in quantum communications schemes and future quantum technologies”.

“An interesting advantage of glass fibers, in addition to this strong interaction, is the fact that they can conduct light and sound excellently over long distances,” – Laura Blázquez Martínez

“These results are a very exciting step towards the quantum ground state in waveguides and the manipulation of such long acoustic phonons opens up possibilities for broadband applications in quantum technology,” – Dr. Birgit Stiller

Quick Summary

Scientists at the Max Planck Institute for the Science of Light have made significant progress in cooling sound waves in waveguides using laser light, reducing the temperature of a sound wave in an optical fibre by 219 K, ten times further than previously achieved. This advancement not only enhances understanding of the transition from classical to quantum phenomena of sound, but also holds potential for quantum communication systems and future quantum technologies.

• A team of scientists at the Max Planck Institute for the Science of Light, led by Dr. Birgit Stiller, has made significant progress in cooling sound waves in waveguides using laser light.
• This achievement is a step towards reaching the quantum ground state of sound in waveguides, which could eliminate unwanted noise generated by acoustic waves at room temperature.
• The researchers were able to lower the temperature of a sound wave in an optical fiber by 219 K, ten times further than previously reported. This was achieved through the nonlinear optical effect of stimulated Brillouin scattering, where light waves are coupled to sound waves.
• The experiment involved cooling a sound wave extending over the full 50 cm of the core of an optical fiber to extremely low temperatures.
• The research provides a deeper understanding of the transition from classical to quantum phenomena of sound and has implications for quantum communication systems and future quantum technologies.
• The research team includes Laura Blázquez Martínez, Andreas Geilen, Changlong Zhu, and Philipp Wiedemann.
• The findings were published in the Physical Review Letters.