New Qubit Design Sustains Information for Half a Millisecond

Scientists at University of Colorado Boulder, in collaboration with National Institute of Standards and Technology, Louisiana State University, University of Melbourne, have developed a new qubit based on a multimode rhombus circuit exhibiting improved resilience to noise. Pablo Aramburu Sanchez and colleagues intentionally modified the energy of a Josephson junction within the circuit to explore a ‘soft’ version of the rhombus qubit. This novel approach allows for direct probing of qubit transitions across several GHz and reduces potential drawbacks associated with traditional interferometer-based protection schemes, resulting in a biased-noise qubit. Measurements reveal an average relaxation time ($T_$1) of approximately 500μs in the biased-noise regime, alongside a Ramsey dephasing time ($T^{R}_\varphi$) of 90ns, demonstrating promising coherence properties.

Rhombus qubit design extends coherence time to 500 microseconds through asymmetric Josephson junctions

A substantial improvement in qubit coherence has been achieved, with the newly engineered “soft” rhombus qubit demonstrating an average $T_$1 relaxation time of 500μs. Previous frustrated designs managed only 27μs, representing a nearly twenty-fold increase in information maintenance duration. This extended coherence surpasses a critical threshold for executing complex quantum computations, now enabling algorithms previously limited by rapid signal decay. The design overcomes limitations inherent in earlier interferometer-based protection schemes by intentionally introducing asymmetry into the circuit’s Josephson junctions, which are tiny superconducting switches exhibiting non-dissipative switching behaviour. These junctions, formed from two superconducting materials separated by a thin insulating barrier, are fundamental building blocks in many superconducting quantum circuits.

The rhombus qubit, in its original conception, relies on encoding quantum information into the parity of charge states within the interferometer. However, this approach can be susceptible to charge noise. By deliberately altering the energy of one Josephson junction, effectively creating an ‘imbalance’, the researchers have created a ‘soft’ rhombus qubit. This asymmetry allows for direct probing of qubit transitions across several GHz, a significant departure from prior methods which often relied on indirect measurements or limited frequency ranges. The ability to directly observe these transitions is crucial for characterising the qubit’s behaviour and optimising its performance. Measurements revealed a Ramsey dephasing time of 90ns in the biased-noise regime, and a 670ns dephasing time when the qubit is frustrated, demonstrating enhanced stability. This advance builds upon previous frustrated designs and is vital for complex quantum calculations, particularly those requiring many quantum gates.

The significance of achieving a 500μs $T_$1 time lies in its implications for quantum error correction. Error correction codes require a certain number of physical qubits to encode a single logical qubit, and the effectiveness of these codes is directly related to the coherence time of the physical qubits. Longer coherence times reduce the rate of errors and therefore reduce the overhead required for error correction, bringing practical quantum computation closer to reality. Furthermore, the 90ns Ramsey dephasing time indicates the qubit’s ability to maintain phase coherence, which is essential for performing quantum interference and implementing many quantum algorithms.

Flux noise and quasiparticle tunneling limit coherence in advanced superconducting qubits

Increasingly precise control over delicate quantum states is required to create stable qubits within superconducting circuits. This work offers a promising pathway, demonstrating improved coherence times in a modified rhombus qubit designed to shield quantum information from disruptive environmental noise. However, flux noise and quasiparticle tunneling remain fundamental limitations to further gains, representing unwanted energy fluctuations and electron behaviour that degrade qubit performance. Flux noise arises from fluctuations in magnetic flux threading the superconducting loops of the qubit, while quasiparticle tunneling involves the unintended excitation of electrons across the superconducting gap, leading to energy loss.

Longer qubit coherence times, the duration quantum information remains stable, underpin the development of practical quantum computers, and even incremental gains matter sharply. Identifying flux noise and quasiparticle tunneling is important progress, as both represent sources of unwanted energy fluctuations within the superconducting material. Flux noise, originating from imperfections in the materials or external electromagnetic interference, manifests as random variations in the magnetic field experienced by the qubit. Quasiparticle tunneling, often triggered by cosmic rays or material defects, introduces energy dissipation and contributes to decoherence. Flux noise and quasiparticle tunneling are not immediately solvable, but pinpointing these issues directs future research towards materials science and circuit design improvements, focusing on mitigating the effects of unwanted magnetic field fluctuations and electron movement. Specifically, research is focusing on developing materials with lower magnetic susceptibility and improved surface quality to reduce flux noise, and on implementing techniques to suppress quasiparticle generation and recombination.

The ‘soft’ rhombus qubit design, while demonstrating significant improvements, is not immune to these limitations. Future work will likely focus on combining this design with materials and fabrication techniques that further minimise flux noise and quasiparticle tunneling. Exploring different superconducting materials, such as titanium nitride or tantalum, and implementing advanced surface treatments could potentially reduce these sources of decoherence. Furthermore, optimising the qubit geometry and circuit layout to minimise sensitivity to external noise is an ongoing area of research. The ultimate goal is to achieve coherence times long enough to enable fault-tolerant quantum computation, paving the way for powerful new computational capabilities.

Researchers demonstrated a rhombus qubit design exhibiting a 500 microsecond relaxation time in a biased-noise regime and a 27 microsecond relaxation time at frustration. These findings are important because longer qubit coherence times are essential for building stable quantum computers. The study identified flux noise and quasiparticle tunneling as limiting factors in qubit performance, providing direction for future materials science and circuit design improvements. Further research will likely focus on optimising materials and fabrication techniques to minimise these noise sources and extend coherence.