Quantum computers promise vast processing power, but their building blocks have a serious flaw: qubits mostly talk to their nearest neighbors. Now scientists at the University of Warwick have found a way around that problem, using sound waves to link qubits across a chip. The result, published June 15 in APL Quantum, could finally make large-scale quantum computing possible.
The team built a prototype quantum processing unit (QPU) that pairs qubits with phonons — quasiparticles that carry vibration energy. They call the approach Quantum Phononic Links, or QPLs. Instead of relying on bulky surface acoustic waves, which require complex designs and extra hardware, the Warwick researchers use the chip’s own crystal lattice to let distant qubits exchange quantum information.
The Problem With Neighboring Qubits
Quantum computers work by manipulating qubits, which hold quantum information in superposition states. But qubits are fragile. Move them too far apart, and they lose coherence — their quantum information fades away.
The conventional solution is to keep qubits close together. That works for small machines, but it fails for large ones. A machine with a million qubits needs a way for qubits on opposite sides of the chip to talk to each other.
“One of the key challenges in quantum computing is long-range qubit connectivity,” Maksym Myronov, an associate professor of semiconductor materials and devices at the University of Warwick and first author of the study, said in a statement. “Our work introduces a new concept in which phonons act as a quantum bus, enabling distant qubits to exchange quantum information while remaining fully compatible with semiconductor technology.”
Phonons As Quantum Buses
The phonon approach borrows ideas from condensed matter physics. Phonons are quasiparticles that represent collective vibration in a solid. They naturally exist in semiconductors, and they travel through the crystal lattice.
The Warwick team’s QPLs are engineered waveguides and cavities that confine and guide acoustic modes within a compressively strained germanium “quantum well.” A quantum well is an ultrathin layer that acts as a guide for particles.
The researchers focused on qubits made from valence-band holes — the absence of electrons at absolute zero — hosted in a specialized material they call compressively strained germanium on silicon, or cs-GoS. Germanium is particularly useful for quantum computing because it reduces decoherence from external interference.
How Hole Spin Qubits Work
A hole is the absence of an electron, but it behaves like a particle with a spin state. That spin state can encode quantum information, with 1s and 0s represented by the direction of the spin.
Hole spin qubits have long coherence times — they can hold quantum information for a while. They also communicate using electron impulses in control systems. But they struggle to bond and share quantum information with qubits they don’t directly neighbor.
That’s a big problem for error correction. Large systems need distant qubits to couple effectively, and conventional methods fall short.
Comparing The Approaches
Other long-range communication proposals exist. Some redirect capacitors to reduce sensitivity to charge noise. Others use surface acoustic waves. Both have fundamental limitations on size and coherence.
QPLs avoid those problems. By using phonons as the connections, the technique offers a versatile interface for hybrid quantum systems — conventional computers with quantum computing functionality. That could enable coherent interconnection of semiconductor spin qubits with other quantum platforms, such as cloud-based quantum computing.
The Prototype Chip
To demonstrate the technology, the team created a QPU with silicon and a thin germanium crystal layer. The cs-GoS material hosts the valence-band holes.
The researchers sent sound-like vibrations through the specialized material to carry quantum information between the distant qubits. They linked qubits separated by less than a micrometer — one-thousandth of a millimeter — and those up to 300 mm — 11.8 inches — apart.
The slow wave velocity and short wavelength of the acoustic excitations were key to making this work. The spin states in the material are highly sensitive to lattice deformations, which enable precise control and coupling of spin states with vibrational energy.
What Comes Next
The combination of phononic engineering and hole-spin physics makes cs-GoS a promising platform for next-generation quantum architectures, the researchers said. It could achieve both long-range coherent coupling and large-scale integration.
The work builds on earlier advances in quantum computing. Other recent developments include a “most accurate” quantum computing chip and Schrödinger’s cat-inspired quantum computing that is 160 times more reliable.
The Warwick team’s approach is distinct. It uses the chip’s own lattice to move information, rather than adding external components.
The Bottom Line
Long-range qubit connectivity is one of the hardest problems in quantum computing. The Warwick team has found a clever workaround, using phonons as a quantum bus.
The approach is promising, and the prototype demonstrates the underlying physics. Whether it scales to a million qubits remains to be seen. But the path is clearer now.
The paper is titled “Quantum phononic links for on-chip long-range coupling of hole spin qubits in compressively strained germanium on silicon.” It was authored by Myronov, Yonjali Yonjali, and Studenikin. The DOI is 10.1063/5.0332643.
The research is still developing. A quantum computer with a million qubits could push the boundaries of what computing can do. The phonon bus is a step in that direction.
- The slow wave velocity of the acoustic excitations
- The short wavelength of the acoustic excitations
- The high sensitivity of the spin states to lattice deformations
- The precise control and coupling of spin states with vibrational energy
Source material: “New quantum chip taps into weird quasiparticles to get qubits to communicate over long distances,” Live Science.
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