The Quantum Ground State of Sound Achieved in Waveguides Using Laser Cooling

In a groundbreaking achievement, a team of scientists at the Max Planck Institute for the Science of Light, led by Dr. Birgit Stiller, has successfully cooled traveling sound waves in waveguides to a much greater extent than previously possible using laser light. This significant step brings us closer to achieving the quantum ground state of sound in waveguides, which has important implications for quantum communication systems and future quantum technologies.

Eliminating Unwanted Noise and Bridging the Gap Between Classical and Quantum Sound Phenomena

At room temperature, acoustic waves generate unwanted noise that can interfere with quantum measurements. By cooling the system and reaching the quantum ground state of an acoustic wave, the number of disruptive quantum particles, known as acoustic phonons, can be reduced to nearly zero. This cooling process allows us to bridge the gap between classical sound behavior and quantum mechanics, providing a deeper understanding of the transition from classical to quantum phenomena of sound.

Over the past decade, significant technological advancements have been made in cooling various systems to the quantum ground state. For instance, mechanical vibrations oscillating between two mirrors in a resonator can be cooled to extremely low temperatures. However, achieving the same state in optical fibers, where high-frequency sound waves propagate, has been a challenge. Now, the Stiller Research Group has made progress in this area, as they were able to cool a sound wave in an optical fiber at room temperature by an astounding 219 K using laser cooling. This corresponds to a reduction of 75% in the initial phonon number at a temperature of 74 K.

The researchers achieved this drastic reduction in temperature by utilizing the nonlinear optical effect of stimulated Brillouin scattering. Through this effect, the laser light efficiently couples with sound waves, leading to the cooling of the acoustic vibrations. The result is an environment with reduced thermal noise, which is crucial for quantum communication systems. Glass fibers, in addition to their strong interaction capability, have the advantage of conducting light and sound effectively over long distances, making them an ideal platform for this research.

Achieving the Quantum Ground State in Waveguides

Typically, most physical platforms that have reached the quantum ground state have been microscopic in nature. However, the Stiller Research Group’s experiment utilized a 50 cm long optical fiber, where a sound wave spanning the entire length was cooled to extremely low temperatures. This achievement paves the way for broader applications in quantum technology, particularly in the manipulation of long acoustic phonons.

From Classical Density Wave to Quantum Particle

In the classical world, sound is commonly understood as a density wave in a medium. However, quantum mechanics provides an alternative perspective – sound can also be described as a particle called a phonon. Phonons represent the smallest amount of energy that occurs as an acoustic wave at a specific frequency. To study a single quantum of sound, the number of phonons needs to be minimized. The transition from classical to quantum behavior of sound is most easily observed in the quantum ground state, where the average number of phonons is close to zero, enabling the measurement of quantum effects.

The Advantages of Waveguide Systems

Waveguide systems offer unique advantages over traditional mirror-bound systems. In waveguides, both light and sound propagate along the waveguide instead of being confined between two mirrors. Acoustic waves in waveguides exist as a continuum, allowing for a broad bandwidth and making them highly promising for applications such as high-speed communication systems. The achievement of reaching the quantum ground state in waveguides opens up new possibilities for deeper insights into the fundamental nature of matter.

The successful cooling of traveling sound waves in waveguides using laser light represents a significant milestone in achieving the quantum ground state of sound. By reducing the number of disruptive quantum particles and eliminating unwanted noise, this research contributes to a deeper understanding of the transition from classical to quantum sound phenomena. The implications of this advancement extend to quantum communication systems and pave the way for broader applications in quantum technology. With waveguide systems offering unique advantages, such as broad bandwidth and long-distance propagation, the manipulation of acoustic phonons in these systems holds great potential for future advancements in quantum research.