© European Physical Society, EDP Sciences, 2025

Since its beginnings, optomechanics provides the fundamental principles and techniques needed to reach such sensitivity. Understanding the interaction of light with vibrations also enhanced laser interferometry, the core technology used in detectors like LIGO (Laser Interferometer Gravitational-Wave Observatory).

Fig. 1 Experimental setup in the laboratory.

Also in telecommunications, the interaction of optical signals with acoustic waves in optical fibers can be a strong nuisance for the transmission of a large amount of data: traveling sound waves at high frequency (GHz) can act like a mirror and back-reflect most of the incoming signals - so badly that no information arrives at the end of the optical fiber transmission link. The underlying physical effects are electrostriction and the photoelastic effect resulting in the strong nonlinear optical effect called stimulated Brillouin-Mandelstam scattering (SBS). Light-sound interactions in optical fibers are not all that bad though: they are the basis for practical long-range fiber sensors for structural health monitoring and they can be a very versatile tool for information processing, signal cleaning and very recently in our group even for neuromorphic computing.

Quantum interaction of optical signals with mechanical vibrations was largely explored in resonator configurations: optical resonances interacting with mechanical resonances – breathing modes – of tiny crystalline or glass spheres or drum-like structures [1]. These standing mechanical waves can be cooled down to the quantum ground state which means that the number of quanta of vibrations – phonons – is ideally close to 0. The significance of the ground state originates from its role as a reference point for all other energy states in a system, which determines the properties and behavior of a system. Thus, understanding a system’s quantum ground state is crucial for exploring its behavior and applications at a quantum level. In the quantum ground state of a mechanical vibration, where all noisy phonons are removed, one could incept quantum information via coherent transfer and use the mechanical mode as a quantum memory.

Fig. 2 Artist’s impression of a photonic crystal fiber taper.

Mechanical vibrations do not only exist as standing waves in a resonator configuration. There are also traveling acoustic waves, physically the same objects just without boundary conditions: periodic density changes which propagate freely through the medium instead of being localized. Traveling acoustic waves can be bulk acoustic waves or surface acoustic waves (or hybrids of both) and are for example used in acousto-optic modulators. As there are almost no boundary conditions in the direction of propagation, traveling acoustic waves exist as a continuum of modes, not only at resonant conditions like in a mechanical cavity.

Fig. 3 Record cooling rate in a waveguide configuration: cooling acoustic phonons from room temperature down to 74 k.

Traveling acoustic waves bring several unique ingredients to the table such as a very slow propagation speed (interesting for memory and repeater applications), a small footprint (interesting for integration) and sensitivity to the environment (interesting for quantum sensing). As their frequency differ by several orders of magnitude from optical frequencies, they are also a perfect candidate for frequency conversion required in hybrid quantum systems. In quantum technologies, traveling acoustic waves can transfer quantum information between solid-state qubits. In hybrid quantum systems, they can serve to couple superconducting circuits, quantum dots, and spin systems. Because of this, recently the interaction of light with traveling acoustic waves has gained more attention and experimentalists look into quantum effects such as entanglement, phonon cooling, squeezing and coherent transfer.

In terms of quantum technologies, several milestones using traveling acoustic waves have been reached. Bulk acoustic wave resonators coupled to superconducting qubits were recently used for building a mechanical qubit [2]. Phonons can be added and subtracted to quantum states in silica resonators [3]. Traveling waves interacting with telecom photons can create a traveling phononic qubit [4]. And these are only a few examples!

The more specific interaction of light with traveling acoustic waves via stimulated Brillouin-Mandelstam scattering is being studied for quantum interactions in a variety of platforms, such as crystalline optical resonators (the high-overtone bulk acoustic waves resonators – HBAR [5]), optical microresonators made out of silica or other materials [6] or waveguide systems such as integrated photonic chips [7] and optical fibers [8]. The aforementioned works [2-6] still contain an optical cavity which is boosting the efficiency of the optoacoustic coupling.

But what about experiments based on waveguides solely? In that case both optical and acoustic waves travel freely and do not bounce back. The most prominent example for optical (and acoustic) waveguides are optical fibers, which are especially interesting because of their low optical losses and long transmission lengths. They are therefore a very good candidate for quantum communications and as the connection link between two quantum systems, for example, quantum computers. Light in optical fibers is not restricted to discrete resonances and the interaction is basically limited by the optical transparency window of the material. What if one could use them for quantum applications themselves using an interaction with mechanical or acoustic vibrations?

Here, Brillouin-Mandelstam interactions are of great interest because photons are not only modified by existing vibrations, but light can also excite acoustic phonons without the need of actuators. This leads to a variety of applications where photonic signals can be processed and manipulated with acoustic waves. For instance, we were able to implement a memory for light in acoustic waves as sound waves travel at a 100000 times slower speed than light [9,10]. It is also possible to modify the acoustic waves all-optically and use them for signal processing. Because the frequency of the involved acoustic waves is GHz, the thermal phonon number is much reduced compared to kHz or MHz vibrations (following the Bose-Einstein distribution). Nevertheless, at room temperature there are still around one thousand phonons, even at GHz frequencies. Therefore, when thinking about quantum applications, active or passive cooling of phonons is important to be considered. We achieved active laser cooling using Brillouin-Mandelstam scattering itself [11] with a record cooling rate from room temperature by a difference of 220 K [8]. The laser light cools the acoustic vibrations and enables an environment with lower thermal noise which is “disturbing” noise for applications such as quantum memory. We recently also showed that stimulated Brillouin-Mandelstam scattering is a particularly efficient way in which photons could be entangled with traveling acoustic phonons [12]: while the two quanta travel along the same photonic structures, the phonons move at a much slower speed. The proposed entangling scheme can operate at temperatures in the tens of Kelvin. This is much higher than what standard approaches require. Those often employ expensive equipment such as dilution fridges. The possibility of implementing this concept in optical fibers or photonic integrated chips, renders the mechanism of stimulated Brillouin-Mandelstam scattering-based entanglement of particularly high interest for modern quantum technologies. Moreover, our Brillouin-based entanglement scheme is particularly resilient to external noise, the standard pitfall of any quantum technologies.

Fig. 4 Concept of entangling an optical photon with a traveling acoustic phonon via a strong optical pump in Brillouin-Mandelstam process.

To sum up, our steps towards quantum waveguide optoacoustics show promising avenues for all-optical control and manipulation of quantum states via traveling acoustic waves [13]. We think that entangling photons and phonons in optical fibers via Brillouin-Mandelstam scattering can be a versatile and resilient entanglement scheme for quantum communications. Being able to coherently transfer optical information to acoustic phonons and thus enabling a Brillouin quantum memory might prove a very practical and efficient in-fiber concept for quantum repeater.

But why stop at conventional quantum signal processing? Waveguide optoacoustics has more to offer, not only in the quantum domain, but also towards neuromorphic computing. Very recently, we experimentally demonstrated optoacoustic building blocks for optical neural networks that are based on traveling acoustic waves. We implemented several concepts such as a long short-term memory [14], an optoacoustic recurrent operator [15] and an optoacoustic activation function [16] that are compatible with existing physical neural networks. Using sound waves for optical neural networks has great potential to unlock a new class of photonic neuromorphic computing which can be reconfigured spontaneously. On-chip implementations of optical neural networks can also benefit from this approach without additional electronic controls. We imagine that combining both avenues – quantum interactions and neuromorphic architectures – can be a powerful tool box for quantum neuromorphic computing.

References

All Figures

	Fig. 1 Experimental setup in the laboratory.
In the text

	Fig. 2 Artist’s impression of a photonic crystal fiber taper.
In the text

	Fig. 3 Record cooling rate in a waveguide configuration: cooling acoustic phonons from room temperature down to 74 k.
In the text

	Fig. 4 Concept of entangling an optical photon with a traveling acoustic phonon via a strong optical pump in Brillouin-Mandelstam process.
In the text