December 19, 2024 -- Quantum technologies are radically transforming our understanding of the universe. One emerging technology are macroscopic mechanical oscillators, devices that are vital in quartz watches, mobile phones, and lasers used in telecommunications. In the quantum realm, macroscopic oscillators could enable ultra-sensitive sensors and components for quantum computing, opening new possibilities for innovation in various industries.

Controlling mechanical oscillators at the quantum level is essential for developing future technologies in quantum computing and ultra-precise sensing. But controlling them collectively is challenging, as it requires near-perfect units, i.e. identical.

Most research in quantum optomechanics has centered on single oscillators, demonstrating quantum phenomena like ground-state cooling and quantum squeezing. But this hasn’t been the case for collective quantum behavior, where many oscillators act as one. Although these collective dynamics are key to creating more powerful quantum systems, they demand exceptionally precise control over multiple oscillators with nearly identical properties.

Scientists led by Tobias Kippenberg at EPFL have now achieved the long-sought goal: they successfully prepared six mechanical oscillators in a collective state, observed their quantum behavior, and measured phenomena that only emerge when oscillators act as a group. The research, published in Science, marks a significant step forward for quantum technologies, opening the door to large-scale quantum systems.

“This is enabled by the extremely low disorder among the mechanical frequencies in a superconducting platform, reaching levels as low as 0.1%,” says Mahdi Chegnizadeh, the first author of the study. “This precision allowed the oscillators to enter a collective state, where they behave as a unified system rather than independent components.”

To enable the observation of quantum effects, the scientists used sideband cooling, a technique that reduces the energy of oscillators to their quantum ground state—the lowest possible energy allowed by quantum mechanics.

Sideband cooling works by shining a laser at an oscillator, with the laser’s light tuned slightly below the oscillator’s natural frequency. The light’s energy interacts with the vibrating system in a way that subtracts energy from it. This process is crucial for observing delicate quantum effects, as it reduces thermal vibrations and brings the system near stillness.

By increasing the coupling between the microwave cavity and the oscillators, the system transitions from individual to collective dynamics. “More interestingly, by preparing the collective mode in its quantum ground state, we observed quantum sideband asymmetry, which is the hallmark of quantum collective motion. Typically, quantum motion is confined to a single object, but here it spanned the entire system of oscillators,” says Marco Scigliuzzo, a co-author of the study.

The researchers also observed enhanced cooling rates and the emergence of “dark” mechanical modes, i.e., modes that did not interact with the system's cavity and retained higher energy.

The findings provide experimental confirmation of theories about collective quantum behavior in mechanical systems and open new possibilities for exploring quantum states. They also have major implications for the future of quantum technologies, as the ability to control collective quantum motion in mechanical systems could lead to advances in quantum sensing and generation of multi-partite entanglement.

All devices were fabricated in the Center of MicroNanoTechnology (CMi) at EPFL.