Reaching the ground state of quantum sound

A groundbreaking study by scientists at the Max Planck Institute for the Science of Light, spearheaded by Dr. Birgit Stiller, has marked a significant advancement in the field of quantum mechanics. The team, including prominent researcher Laura Blázquez Martínez, has remarkably cooled sound waves in optical fibres far beyond previous achievements, using laser light. This progress edges closer to reaching the elusive quantum ground state of sound in waveguides.

Quantum ground state, a key concept in quantum mechanics, refers to a state with minimal energy and disturbance. By cooling the system, scientists aim to reduce the acoustic phonons – particles of sound that can disrupt quantum measurements – to nearly zero. This approach bridges the gap between classical and quantum mechanics, enhancing the precision of quantum observations.

In the last decade, technological leaps have enabled various systems to approach this ground state. However, the complexity of cooling high-frequency sound waves in optical fibres remained a challenge until now. The Stiller Research Group's latest study, published in Physical Review Letters, reports a significant temperature reduction of a sound wave, initially at room temperature, by 219 Kelvin using laser cooling. This method, ten times more effective than prior attempts, reduced the initial phonon number by 75%, achieving a low temperature of 74 Kelvin (-194 Celsius).

Uniquely, this experiment extended beyond the microscopic scale. The team cooled a sound wave along the entire 50 cm length of an optical fibre, an unprecedented achievement in quantum optoacoustics. The versatility of glass fibres, which effectively conduct light and sound over long distances, further bolsters the study's significance. Laura Blázquez Martínez noted, “An interesting advantage of glass fibres, in addition to this strong interaction, is the fact that they can conduct light and sound excellently over long distances.”

Classically, sound is a density wave in a medium. Quantum mechanics, however, views sound as a particle - the phonon. Minimising phonons is crucial for observing quantum behaviour in sound. The quantum ground state, where phonons are nearly absent, allows for the observation of quantum effects. The transition from classical to quantum behaviour of sound becomes more discernible in this state.

The potential of this research extends beyond fundamental understanding. Its applications in quantum technology, particularly in quantum communications, are promising. Dr. Stiller’s team is enthusiastic about the future insights and applications that this step towards the quantum ground state in waveguides will bring. As Dr. Stiller asserted, “We are very enthusiastic about the new insights that pushing these fibres into the quantum ground state will bring”, highlighting the dual benefit of fundamental research and practical applications in quantum technologies.

You can find the full report here: Laura Blázquez Martínez, Philipp Wiedemann, Changlong Zhu, Andreas Geilen, and Birgit Stiller (2024). “Optoacoustic Cooling of Traveling Hypersound Waves”. Physical Review Letters 132, 023603 (2024) DOI: https://doi.org/10.1103/PhysRevLett.132.023603