Researchers have developed a quantum chip architecture that uses vibrations called phonons to connect qubits across long distances, potentially addressing a key obstacle to building large-scale quantum computers.
The technology, Quantum Phononic Links (QPLs), is designed to allow qubits that are physically separated to exchange quantum information. The researchers say the approach can support connections ranging from less than a micrometer to distances spanning a 300mm semiconductor wafer.
This level of connectivity could be important in the push toward quantum systems with one million qubits. Today’s semiconductor qubits can communicate effectively with nearby qubits, but extending those connections across a large processor is far more difficult. Fully functional quantum error correction requires communication between qubits that are not closely adjacent.
QPLs address this problem using phonons, which carry vibrational energy through a material. In effect, the phonons provide a communications channel that transports quantum information between separated qubits.
Maksym Myronov, associate professor of semiconductor materials and devices at the University of Warwick and first author of the research, described phonons as a “quantum bus” that allows distant qubits to exchange information while remaining compatible with semiconductor technology.
Allowing Quantum Information to Remain Usable
The researchers built the architecture around hole spin qubits hosted in compressively strained germanium on silicon. Germanium offers properties that make it attractive for quantum computing, including support for long coherence times, meaning quantum information can remain usable for longer periods before interference causes it to degrade.
A hole is created by the absence of an electron in a semiconductor and can behave like a particle. Its spin state can be used to store quantum information. More important for QPLs, hole spin states in strained germanium respond to physical changes in the semiconductor’s crystal structure.
The researchers designed tiny phononic waveguides and cavities to control these vibrations inside a thin germanium quantum well. Because acoustic waves move relatively slowly, the links can be made compact while connecting qubits across widely varying distances.
This approach also avoids the need for piezoelectric transducers used by surface acoustic wave technology. Other proposed methods for connecting remote qubits include microwave resonators and moving quantum information through semiconductor structures. But these techniques face challenges involving physical size and scalability.
The QPL architecture is also compatible with CMOS fabrication, an important consideration for eventually manufacturing quantum processors using technology derived from the semiconductor industry’s existing production infrastructure.
However, there’s still a large gap between a proposed design and a commercially useful million-qubit computer. Solving the connectivity problem is essential because adding qubits alone will not produce useful large systems. Those qubits must also exchange information while preserving fragile quantum states.
Researchers see another potential use for QPLs beyond individual processors. Phonon-based connections could provide an interface between semiconductor spin qubits and other quantum technologies, supporting hybrid computer systems and quantum networks.
