Tunable Transmon Qubit Demonstrates Second Harmonic up to 0.2 of Fundamental, Enabling Customizable Microwave Devices

The ability to precisely control the nonlinear properties of superconducting circuits represents a significant challenge in quantum technology, but recent work by Ksenia Shagalov, David Feldstein-Bofill, and Leo Uhre Jakobsen, along with colleagues at the Niels Bohr Institute, University of Copenhagen, demonstrates a novel approach to achieving this control.

The team engineered a superconducting circuit incorporating a unique combination of tunnel junctions and a SQUID loop, allowing them to tune the harmonic content of the Josephson potential with exceptional precision. This innovative design enables strong manipulation of higher-order harmonics, reaching up to half the strength of the fundamental harmonic, and opens up exciting possibilities for creating more robust and versatile quantum devices, including protected qubits and customizable microwave components. The research establishes a pathway toward advanced quantum technologies by providing a highly tunable platform for manipulating the fundamental building blocks of superconducting circuits.

Double Junction Transmon Qubit Design and Characterisation Scientists are exploring a new type of transmon qubit constructed with two Josephson junctions instead of the traditional single junction. The primary goal of this research is to determine whether this double-junction design can improve qubit coherence and reduce unwanted harmonic content, potentially leading to better control and scalability of superconducting quantum circuits. The researchers hypothesize that the double junction creates an internal mode that helps isolate the qubit from noise, improving its stability.

The team employed the Born-Oppenheimer approximation to simplify the analysis of the double-junction system, allowing them to treat the internal mode as separate from the qubit mode. They developed a detailed Hamiltonian to account for interactions between the qubit, internal mode, and a resonator, a circuit element used to read out the qubit’s state. This theoretical framework allows them to predict and understand the harmonic content of the double-junction transmon, a critical factor in minimizing crosstalk between qubits. The researchers fabricated both double-junction and single-junction transmons using a precise fabrication process. They then used two-tone spectroscopy to characterize the energy levels and transitions of the qubits, and performed measurements of relaxation and dephasing times to assess their coherence properties. A standard, fixed-frequency transmon served as a benchmark for comparison. Experiments revealed that the double-junction transmon exhibits an internal mode not present in the single-junction design. Importantly, the double-junction transmon demonstrates significantly lower harmonic content compared to its single-junction counterpart. This finding suggests that the double-junction design can help mitigate crosstalk in multi-qubit circuits, a major limitation in scaling up quantum computers. The measured coherence times for the double-junction transmon are comparable to, or potentially better than, those of the single-junction transmon. The reduced harmonic content of the double-junction transmon represents a significant step towards building larger, more complex superconducting quantum circuits. The ability to tune the dispersive shift provides more precise control over qubit interactions, and the double-junction transmon represents a promising alternative to the traditional single-junction design, potentially offering improved performance and scalability.

Tunable Transmon Exhibits High Harmonic Content Scientists have achieved a breakthrough in superconducting circuit design by realizing a tunable double-junction transmon. This innovative configuration allows for precise control over the harmonic content of the Josephson potential, a critical aspect of qubit behaviour. Experiments reveal substantially higher harmonic content than previously observed, with a measured second harmonic reaching 10. 7% of the fundamental harmonic at zero external flux. Detailed spectroscopy of the first four transitions confirms the tunability of the harmonic content.

The team identified a “sweet spot” where the dispersive shift vanishes, achieved by carefully balancing the dispersive couplings to internal and external modes. This balance point results in no net change in the resonator frequency despite continued coupling, a remarkable demonstration of circuit control. Measurements of the total dispersive shift demonstrate a clear shift upwards at zero flux, cancellation at the balance point, and a downward shift near half a flux quantum. Furthermore, the research demonstrates the ability to extract the internal-mode frequency and its coupling to the resonator, revealing an avoided crossing near half a flux quantum. This precise control over the energy landscape opens pathways toward implementing advanced qubit designs, such as the cos(2φ) qubit, and customizable nonlinear microwave devices. The results confirm the potential of an all-superconducting platform for engineering qubit properties and advancing quantum technologies. Tunable Qubit with Enhanced Harmonic Control This research demonstrates a new superconducting circuit element incorporating a tunnel junction and a SQUID loop, enabling precise control over the harmonic content of the Josephson potential, a key feature in superconducting devices.

The team successfully created a tunable double-junction transmon and characterized its behaviour, identifying regimes where the circuit operates as a single effective mode. They observed a substantial second harmonic, exceeding previously reported levels, and extracted parameters describing the qubit’s energy spectrum. The researchers also investigated the dispersive coupling between the qubit and a readout resonator, revealing a critical balance point where the effects of the qubit and an internal circuit mode cancel each other out, resulting in no net shift in the resonator frequency. This achievement offers a pathway towards designing advanced qubit architectures, such as the cos(2φ) qubit, and other protected qubit designs. 👉 More information 🗞 Higher Josephson harmonics in a tunable double-junction transmon qubit 🧠 ArXiv: https://arxiv.org/abs/2512.08470 Tags: