Nord Quantique has successfully developed bosonic qubit technology with multimode encoding, outlining a path to a significant reduction in the number of qubits required for quantum error correction.
The result is an approach to quantum error correction that will deliver smaller yet more powerful systems, which consume a fraction of the energy.
These smaller systems are also simpler to develop for utility-scale due to their size and lower requirements for cryogenics and control electronics.
Multimode encoding: Removing the need for physical qubits
To carry out effective quantum error correction, the company has implemented an advanced bosonic code known as the Tesseract code.
This provides the system protection against many common types of errors, including bit flips, phase flips, and control errors. Another key advantage over single-mode encoding is that leakage errors, which remove the qubit from the encoding space, can now be detected and corrected.
In this demonstration, post-selection was used to filter out imperfect runs, resulting in the discarding of 12.6% of the data each round. This demonstrated excellent stability in quantum information, with no measurable decay observed through 32 error correction cycles.
The Tesseract code allows for increased error detection, and it is expected that this will translate into additional quantum error correction benefits as more modes are added.
These results are therefore a key stepping stone in the development of this hardware-efficient approach.
Julien Camirand-Lemyre, CEO of Nord Quantique, explained: “Multimode encoding allows us to build quantum computers with excellent error correction capabilities, but without the impediment of all those physical qubits.
“Beyond their smaller and more practical size, our machines will also consume a fraction of the energy, which makes them appealing, for instance, to HPC centres where energy costs are top of mind.”
Encoding individual qubits for better quantum error correction
The core concept of the multimode approach centres on simultaneously using multiple quantum modes to encode individual qubits. Each mode represents a different resonance frequency inside an aluminium cavity and offers additional redundancy, which protects quantum information.
The number of photons populating each mode can also be increased for even more protection, further escalating QEC capabilities.
This breakthrough enables additional quantum error correction capacity and extra means for detecting errors, while maintaining a fixed number of qubits.
It also delivers more benefits, which compound as they scale, opening new avenues for fault-tolerant quantum computing.
Examples include reducing the impact of auxiliary decay errors, enhancing logical lifetimes through the suppression of silent errors, and extracting confidence information to improve error detection and correction strategies further.
The path towards fault-tolerant quantum computing
“After years working on developing multimode operations on states encoded in superconducting cavities, I am pleased to see the progress made by the team at Nord Quantique,” stated Yvonne Gao, Assistant Professor at the National University of Singapore and Principal Investigator at the Centre for Quantum Technologies.
“Their approach of encoding logical qubits in multimode Tesseract states is a very effective method of addressing quantum error correction, and I am impressed with these results. They are an important step forward on the industry’s journey toward utility-scale quantum computing.”
Through this scientific advance, Nord Quantique now has a clear path to delivering fault tolerance at utility scale. The team will continue to improve its results by leveraging systems with additional modes to push the boundaries of quantum error correction.
The company’s first utility-scale quantum computers with more than a hundred logical qubits are expected by 2029.
