Abstract : This is the final report for the IARPA CSQ project at Yale University, on the coherence limits of superconducting charge qubits. During this project, we performed the proposed work investigating the various sources of decoherence and their origins in different materials, using the participation ratio paradigm that we developed and disseminated in the community. The original project goal was to increase by an order of magnitude the coherence time of the state-of-the-art qubits (transmons and fluxonium designs) from a few microseconds to greater than 20 microseconds. By project end, we realized qubit and cavity devices with times in excess of a millisecond, or an improvement by almost three orders of magnitude. These results, especially improved 3D designs, have been widely adopted and successfully reproduced by many groups across the superconducting qubit community. At program end, we have an existence proof that qubits satisfying the quantum error correction threshold are possible with existing materials and fabrication techniques, leading to a growing belief that large-scale quantum computation with solid-state devices will eventually be realized.
Abstract : This is the final report for the IARPA CSQ project at Yale University, on the coherence limits of superconducting charge qubits. During this project, we performed the proposed work investigating the various sources of decoherence and their origins in different materials, using the participation ratio paradigm that we developed and disseminated in the community. The original project goal was to increase by an order of magnitude the coherence time of the state-of-the-art qubits (transmons and fluxonium designs) from a few microseconds to greater than 20 microseconds. By project end, we realized qubit and cavity devices with times in excess of a millisecond, or an improvement by almost three orders of magnitude. These results, especially improved 3D designs, have been widely adopted and successfully reproduced by many groups across the superconducting qubit community. At program end, we have an existence proof that qubits satisfying the quantum error correction threshold are possible with existing materials and fabrication techniques, leading to a growing belief that large-scale quantum computation with solid-state devices will eventually be realized.
Superconducting qubits and resonators with quality factors exceeding 107 are of great interest for quantum information processing applications. The improvement of present devices necessarily involves the consideration of participation ratios, which budget the influence of each physical component in the total energy decay rate. Experiments on compact resonators in which participation ratios were varied has demonstrated the validity of this method, yielding a two-fold improvement in quality factor. Similar experiments on compact transmon qubit devices led to a three-fold improvement over previous transmons, validating the method of participation ratios for qubits as well. Through the use of a 3D cavity, a further minimization of the participation of surface components combined with the removal of unnecessary components, produced an additional ten-fold increase in coherence times. Finally, the fluxonium qubit was redesigned in a similar minimalist environment with an improved superinductance, thus combining the advantages of the 3D architecture with the natural insensitivity to dissipation of the fluxonium, resulting in another tenfold increase in relaxation times. This large increase in relaxation and coherence times enables experiments that were previously impossible, thus preparing the field of quantum information to advance on other fronts.
Superconducting qubits are a leading platform for quantum computation, yet decoherence and scalability challenges continue to limit progress toward fault-tolerant systems. This review synthesizes research on architectures, noise sources, and error correction to clarify performance trade-offs and guide development. The transition from Transmon to Fluxonium has dramatically improved qubit performance, achieving millisecond-scale coherence times. However, this success reveals a fundamental limit of coherence now being primarily restricted by the physical properties of the materials, specifically dielectric losses occurring at the Josephson junction leads and within the surrounding materials. As overcoming these material limits becomes increasingly difficult, system-level engineering determines whether high-coherence qubits can scale.Scaling quantum systems is difficult because current cooling and control systems create too much heat and this forces a shift to integrated digital superconducting circuits and cryogenic light-based links. While these hardware challenges restrict scaling, experiments in error correction show that logical qubits can already outperform physical ones. For example, a distance-7 surface code improved logical qubit lifetime by 2.4×. Achieving practical fault tolerance, however, requires moving beyond simple error models. Future systems must combine techniques like Dynamical Decoupling and Machine Learning-based Optimal Control with dynamic classical control capable of handling complex, correlated faults such as Multi-Bit Burst Errors. Progress in materials, cryogenic system integration for control, and adaptive error management must advance together to realize scalable, high-fidelity quantum hardware.
We review progress at UCSB on understanding the physics of decoherence in superconducting qubits. Although many decoherence mechanisms were studied and fixed in the last 5 years, the most important ones are two-level state defects in amorphous dielectrics, non-equilibrium quasiparticles generated from stray infrared light, and radiation to slotline modes. With improved design, the performance of integrated circuit transmons using the Xmon design are now close to world record performance: these devices have the advantage of retaining coherence when scaled up to 9 qubits.
Improving control over physical qubits is a crucial component of quantum computing research. Here we report a superconducting fluxonium qubit with uncorrected coherence time T_{2}^{*}=1.48±0.13 ms, exceeding the state of the art for transmons by an order of magnitude. The average gate fidelity was benchmarked at 0.99991(1). Notably, even in the millisecond range, the coherence time is limited by material absorption and could be further improved with a more rigorous fabrication. Our demonstration may be useful for suppressing errors in the next generation quantum processors.
In the past years, superconducting quantum computation has received much attention and significant progress of the device design and fabrication has been made, which leads qubit coherence times to be improved greatly. Recently, we have successfully designed, fabricated, and tested the superconducting qubits based on the negative-inductance superconducting quantum interference devices (nSQUIDs), which are expected to have the advantages for the fast quantum information transfer and macroscopic quantum phenomenon study with a two-dimensional potential landscape. Their quantum coherence and basic physical properties have been demonstrated and systematically investigated. On the other hand, a new type of superconducting qubit, called transmon and Xmon qubit, has been developed in the meantime by the international community, whose coherence time has been gradually increased to the present scale of tens of microseconds. These devices are demonstrated to have many advantages in the sample design and fabrication, and multi-qubit coupling and manipulation. We have also studied this type of superconducting qubit. In collaboration with Zhejiang University and the University of Science and Technology of China, we have successfully fabricated various types of the coupled Xmon devices having the qubit numbers ranging from 4 to 10. Quantum entanglement, quantum algorithm of solving coupled linear equations, and quantum simulation of the many-body localization problem in solid-state physics have been demonstrated by using these devices. Also, we have made significant achievements in the studies of the macroscopic quantum phenomena, quantum dissipation, quantum microwave lasing, and some other quantum optics problems. In particular, Autler-Townes splitting under strong microwave drive, electromagnetically induced transparency, stimulated Raman adiabatic passage, microwave mixing, correlated emission lasing, and microwave frequency up-and-down conversion have been successfully studied, both experimentally and theoretically.
Abstract : This is the final report for the ARO/LPS project at Yale University, on the RF control and measurements of superconducting qubits. During this project, we performed the proposed research work, which was revolving around four major axes: I. The improvement of qubit coherence by investigating the nature of the various sources of decoherence, II. The improvement of readout fidelity by establishing the parameters influencing the efficiency and speed, III. The novel manipulation and entanglement of multiple qubits by developing gate hardware, protocols and room temperature electronics extendable to systems comprising more than two qubits, and IV. The development of a scalable architecture combining superconducting linear resonators and superconducting qubit. The overall goal of the project was to assess the potential of superconducting qubits for the construction of a quantum computer based on superconducting Josephson circuits.