Insider Brief:
- ETH Zürich researchers created a functional qubit from a tiny mechanical resonator, reimagining early mechanical computer design within quantum computing’s framework.
- They overcame challenges of evenly spaced energy states in mechanical oscillators by introducing “anharmonicity,” using a hybrid system of mechanical motion and superconducting quantum control.
- The hybrid mechanical qubit achieved 60% fidelity, demonstrating proof of concept but still trailing conventional qubits in performance.
- The team intends to explore applications for mechanical qubits as sensitive probes for detecting gravitational forces and to perform logical operations with multiple mechanical qubits.
Though oft mentioned most when lamenting the return of fashion from the ghost of one’s high school past, the cyclical nature of time is an opportunity for innovation to loop back and reimagine what once was to solve what is. The mechanical ingenuity that defined the earliest computers has found new relevance in quantum computing. Historically crafted from superconducting circuits, trapped ions, or photons, qubits have now taken an unexpected design turn. A recent article from Science covers a team at ETH Zürich that recently defied convention by creating a functional qubit from a tiny mechanical resonator, blending the legacy of mechanical computation with the futurism quantum tends to inspire.
Overcoming the Challenges of Vibrational Qubits
When we create something for the first time, its design risks becoming the blueprint for all that follows, shaping our imagination and unintentionally creating boundaries that future innovations may not always step beyond. A qubit need not exist only within golden chandeliers, but rather, can be any system with two isolated quantum states of differing energies. Superconducting qubits use circuits that carry persistent currents, with lower and higher energy states representing 0 and 1. These systems can transition smoothly between states using precisely tuned microwaves.
However, translating this principle to mechanical systems is no trivial feat According to the article, creating a mechanical qubit did not seem possible due to the even spacing of energy states in mechanical oscillators. Such “harmonic” spacing means stimulating one state risks triggering others, making it difficult to isolate just two states to function as a qubit.
The ETH Zürich team tackled this issue through a hybrid system combining mechanical motion and superconducting quantum control. Their solution introduces “anharmonicity,” altering the energy levels’ uniform spacing and enabling the isolation of two states. As the article notes, the central challenge lay in striking a balance between inducing sufficient anharmonicity while preserving the mechanical nature of the qubit.
Building the Hybrid Mechanical Qubit
The ETHZ researchers used a two-part system. The first component is a mechanical resonator made of aluminum nitride on a sapphire crystal. Vibrations induced by oscillating voltages could persist for millions of cycles, bouncing within the crystal. The second component is a superconducting qubit on a separate sapphire chip, stacked above the resonator. The superconducting circuit’s oscillating current induced quantum vibrations in the mechanical oscillator.
By tuning the superconducting qubit’s frequency just slightly off-resonance with the mechanical oscillator, the team created a hybrid system where the quantum states of both components melded. This interaction disrupted the evenly spaced energy levels, allowing two states to be isolated as the qubit’s 0 and 1. The system achieved 60% fidelity—far lower than the >99% fidelity of conventional qubits but still notable for a first-of-its-kind device.
A Mechanical Qubit’s Potential
While the hybrid mechanical qubit is unlikely to outcompete established designs anytime soon, its unique properties could make it more relevant in other contexts. For instance, mechanical systems might be useful as highly sensitive probes for detecting gravitational forces, which may elude other qubit technologies.
The ETHZ team is plans to push further, with hopes to demonstrate logical operations using two mechanical qubits. If successful, their work could be applied to further our understanding of quantum mechanics and its interface with gravity.
Looking Back To Get Ahead
The new mechanical qubit may not inspire full-size, steampunk quantum computers anytime soon, but it rekindles the ingenuity of early mechanical computers. By merging state-of-the-art superconducting circuits with vibrating mechanical components, the ETHZ team has contributed to the state of quantum research, blending classical inspiration with quantum innovation.
Contributing authors on the study include Yu Yang, Igor Kladarić, Maxwell Drimmer, Uwe von Lüpke, Daan Lenterman, Joost Bus, Stefano Marti, Matteo Fadel, and Yiwen Chu.
