Quantum computing requires a good combination of order and disorder

Research conducted within the Matter and Light for Quantum Computing (ML4Q) cluster of excellence has analyzed the edge device structures of quantum computers to demonstrate that some of them do indeed operate dangerously close to a threshold of chaotic fusion. The challenge is to walk a thin line between too high, but also too low clutter to preserve device functionality. The study “The Transmon Platform for Quantum Computing Challenged by Chaotic Fluctuations” was published today in Nature Communications.

In the race for what could become a key future technology, tech giants like IBM and Google are investing huge resources in the development of quantum computing hardware. However, current platforms are not yet ready for practical applications. Multiple challenges remain, one of which is controlling device imperfections (“mess”).

It’s an old stability precaution: when large groups of people cross bridges, they should avoid walking in step to prevent the formation of resonances that destabilize the construction. Perhaps counterintuitively, the superconducting transmon qubit processor – a technologically advanced platform for quantum computing favored by IBM, Google and other consortia – relies on the same principle: intentionally introduced disorder blocks the formation of resonant chaotic fluctuations, thus becoming an essential part of the production of multi-qubit processors.

To understand this seemingly paradoxical point, consider a transmon qubit as a kind of pendulum. Qubits interconnected to form a computing structure define a system of coupled pendulums – a system which, like classical pendulums, can easily be excited by uncontrollable oscillations with disastrous consequences. In the quantum world, such uncontrollable oscillations lead to the destruction of quantum information; the computer becomes unusable. Intentionally introduced local “detunings” on simple pendulums keep such phenomena at bay.

“The transmon chip not only tolerates, but effectively requires random imperfections from qubit-to-qubit devices,” explained Christoph Berke, a final-year doctoral student in Simon Trebst’s group at the University of Cologne and first author of the paper. “In our study, we ask how reliable the principle of ‘stability by chance’ is in practice. By applying state-of-the-art diagnostics of disordered systems theory, we have found that at least some of the system architectures researched by industry are dangerously close to instability.

From the perspective of fundamental quantum physics, a transmon processor is a quantum many-body system with quantized energy levels. State-of-the-art digital tools calculate these discrete levels based on relevant system parameters, resulting in patterns superficially resembling a tangle of cooked spaghetti. Careful analysis of these structures for realistically modeled Google and IBM chips was one of many diagnostic tools applied in the paper to draw a stability diagram for transmon quantum computing.

“When we compared Google to IBM chips, we found that in the latter case, qubit states can be coupled to such a degree that controlled gate operations can be compromised,” said Simon Trebst, Computational Group Leader Condensed Matter Physics at the University. from Cologne. In order to secure controlled gate operations, one must therefore strike the fine balance between stabilizing qubit integrity and enabling inter-qubit coupling. In the language of pasta preparation, the quantum computer processor must be prepared to perfection, keeping the energy states “al dente” and avoiding tangling through overcooking.

The study of disorder in transmon material was carried out within the framework of the ML4Q cluster of excellence within the framework of a collaborative work between the research groups of Simon Trebst and Alexander Altland of the University of Cologne and the group of David DiVincenzo from RWTH Aachen University and Forschungszentrum Jülich. “This collaborative project is quite unique,” says Alexander Altland of the Institute for Theoretical Physics in Cologne. “Our complementary knowledge of transmon hardware, numerical simulation of complex many-body systems, and quantum chaos was the perfect prerequisite for understanding how quantum information with disorder can be protected. It also indicates how information obtained for small reference systems can be transferred to design scales relevant to the application.

David DiVincenzo, Founding Director of the JARA Institute for Quantum Information at RWTH Aachen University, concludes: “Our study demonstrates how important it is for hardware developers to combine device modeling with random quantum edge methodology and to integrate “chaos diagnostics” as part of the design of qubit processors in the superconducting platform. »


  1. Christoph Berke, Evangelos Varvelis, Simon Trebst, Alexander Altland, David P. DiVincenzo. Transmon platform for quantum computing challenged by chaotic fluctuations. Communication Nature, 2022; 13 (1) DOI: 10.1038/s41467-022-29940-y
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