Researchers have developed a silicon-based integrated device that can controllably split and route individual packets of sound at the quantum level. This component, known as a phonon splitter, overcomes a significant obstacle in quantum engineering by providing a crucial building block for connecting different types of quantum hardware. The achievement promises to accelerate the development of hybrid quantum networks, a powerful approach that combines the strengths of various quantum platforms to create more robust and scalable quantum computers and communication systems.
The new device functions as a microscopic traffic controller for phonons, which are quantized units of vibrational energy, or sound. By enabling these sound particles to serve as on-chip messengers, the technology creates a viable pathway for linking processors that excel at fast calculations, such as superconducting qubits, with systems better suited for long-term information storage, like spin-based memories. This integration is key to building quantum circuits that are both powerful and reliable. The work, led by researchers at Delft University of Technology, fills a critical gap in the toolkit required for building practical phononic circuits, moving the field closer to functional, large-scale quantum information processing on a single chip.
The Quantum Interconnect Challenge
One of the central difficulties in advancing quantum technology is the lack of a universal platform. Different quantum systems possess unique advantages. For instance, superconducting circuits can perform thousands of operations in a fraction of a second but are notoriously fragile, losing their quantum information in a very short time. Other systems, such as those based on electron spin or special crystal defects, can store quantum states for much longer periods but are slower to manipulate. To build a truly effective quantum computer, engineers need a way to combine these disparate systems, leveraging the speed of one and the stability of another.
This requires the creation of a hybrid network, where information can be seamlessly transferred between different physical platforms. For years, the primary candidate for this role was the photon, the quantum particle of light. Optical fibers are excellent at transmitting quantum information over long distances. However, converting quantum states between microwave-based superconducting qubits and optical photons is inefficient and adds complexity. This has driven researchers to explore alternative intermediaries that can operate more naturally within the physical environment of a chip. Phonons have emerged as a leading contender because they are mechanical vibrations and can couple more easily to a wide variety of quantum devices, including qubits, atoms, and nanomechanical systems, all within a compact, solid-state environment.
Acoustic Quantum Highways
Using phonons as quantum messengers is not a new idea, but previous efforts have faced significant technical hurdles. Early approaches often relied on surface acoustic waves (SAWs), which are vibrations that travel along the surface of a material. While functional, SAW-based devices are difficult to scale down. Their two-dimensional nature makes them prone to losses and interference from surface imperfections, which degrades the delicate quantum information they carry. Furthermore, the relatively short lifetimes of phonons in these systems limited the complexity of any potential circuit, as the information would decay before it could be routed and processed effectively.
To overcome these limitations, the Delft team turned to a more robust design using phononic-crystal waveguides. These are engineered nanostructures built into the silicon chip that create a highly controlled path for sound waves. By creating a periodic pattern of holes in the material, the waveguide confines high-frequency gigahertz phonons to a narrow channel. This confinement acts like an acoustic tunnel, protecting the phonons from environmental noise and minimizing cross-talk between adjacent communication channels. This advanced design dramatically extends the phonon’s lifetime, giving it enough time to travel, interfere, and be routed across a chip, which is essential for performing complex quantum operations.
Designing the Phonon Splitter
The core of the new technology is a chip-based four-port directional coupler. Fabricated on a silicon chip, the device is analogous to fiber optic couplers used in telecommunications but is designed to manage mechanical vibrations instead of light. It consists of two input ports and two output ports connected by the phononic-crystal waveguides. The precise geometry of these waveguides allows a single phonon entering one of the input ports to be controllably split between the two output ports. The splitting ratio can be precisely tuned, allowing the phonon to be directed entirely to one output or divided between them.
This controllable splitting is the key function that was previously missing from the quantum acoustics toolkit. While scientists had already developed methods to generate and guide phonons, they lacked a compact, on-chip component that could reliably route them. The device operates at cryogenic temperatures, a requirement for most quantum hardware, to prevent thermal vibrations from overwhelming the single-phonon quantum states. At these temperatures, the vibrations can be treated as discrete, reliable units of quantum information, or “flying qubits,” that shuttle information between stationary components on the chip. The silicon-based fabrication is also a major advantage, as it leverages mature semiconductor manufacturing techniques, paving the way for scalable production.
Precision Routing at the Quantum Level
In practice, the device functions as a junction in a quantum information network. Research team leader Simon Gröblacher described the coupler as acting “like a junction in a quantum ‘postal route.'” This allows an excitation, or piece of quantum information, created in one quantum processor to be reliably sent to another processor on the same chip. It can also be split and sent to multiple recipients simultaneously, enabling more complex and flexible quantum information processing architectures.
The experiments demonstrated that the splitter operates with quantum-level precision. It can split, route, and recombine single quantum vibrations without destroying the fragile quantum state. This capability is foundational for creating more sophisticated phononic circuits, including routers and switches, that can direct quantum information with high fidelity. The ability to manipulate single phonons in this manner opens the door to creating intricate on-chip networks that can manage quantum data flow between different parts of a larger quantum device, much like how data is managed in a classical computer chip but governed by the laws of quantum mechanics.
Paving the Way for Hybrid Systems
The most immediate impact of this phonon splitter is its potential to unite different quantum technologies. It could form the basis of a microscopic router that links fast superconducting qubits with long-lived spin-based quantum memories. In such a system, a superconducting qubit would perform a rapid calculation, then transfer the result to a phonon. The phonon would travel through a waveguide to the splitter, which would route it to a spin-based memory for storage. This would allow the system to preserve the quantum information for extended periods, overcoming the short coherence times that currently limit superconducting processors.
This synergy is expected to catalyze the development of hybrid quantum systems that harness the best features of each physical platform. By overcoming longstanding barriers posed by incompatible hardware, integrated phononic circuits could lead to quantum devices that are more powerful and less prone to errors. Beyond computing, the technology could also enable new types of ultra-sensitive sensors that use mechanical vibrations to detect minute forces or fields. The precise control over phonons could also open up new avenues for fundamental quantum physics experiments, allowing scientists to explore the interactions between sound and matter at the single-particle level.