Reducing Quasiparticle Decoherence in Superconducting Quantum Circuits

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Nontechnical abstract:Quantum information science -the use of quantum mechanics to perform novel computing, simulation, communication, and sensing -is poised to revolutionize computing, biochemistry, condensed matter physics, cryptography, and astronomy, as well as a host of other fields. One of the most promising technology platforms is based on electrical circuits made of superconducting materials and operated at cryogenic temperatures. Quantum computers based on these quantum circuits have already been created and used for simple applications. However, these circuits are limited by decoherence, the loss or scrambling of quantum information due to environmental noise. A limiting source of this decoherence is the presence of excess quasiparticle excitations in the superconducting material. These quasiparticles are ubiquitous in quantum circuits, but their origins and behavior remain poorly understood. In this project, the PI is studying the mechanisms by which quasiparticles are created and destroyed, characterizing different techniques for trapping quasiparticles away from sensitive circuit elements, and using the knowledge gained in order to mitigate their harmful effects on quantum circuits. The project also trains one graduate student and one postdoctoral scholar in the rapidly expanding field of quantum information technology, growing the workforce for both academia and industry.Technical abstract:Nonequilibrium populations of quasiparticles exist in superconducting quantum circuits even at very low temperatures, limiting the coherence of these circuits and causing errors in quantum processors. The sources of these quasiparticles, their behavior in quantum circuits, and the best methods for mitigating their effects are all poorly understood. The goal of this project is to characterize these sources and reduce the quasiparticle generation rate, to better understand the mechanisms of quasiparticle annihilation and trapping and to increase the annihilation and trapping rates, and to generally reduce quasiparticle-mitigated decoherence. The experiments involved are based on high-fidelity measurements of coherent superconducting circuits engineered to be sensitive to quasiparticles. These include resonators incorporating phase-biased nanobridge Josephson junctions, whose internal Andreev states serve as quasiparticle traps, as sensitive non-saturating quasiparticle detectors. With these Andreev devices, temporal and spatial correlations between quasiparticles are measured and mechanisms of quasiparticle relaxation and excitation are characterized. Transmon qubits with tunable frequency are used to measure quasiparticle energy distributions; transmons engineered with quasiparticle-sensitive spectra are used to measure quasiparticle density and transport characteristics. Finally, tests of linear waveguide resonators' quality factors are used to measure quasiparticle density and transport. All these devices are used to test ways of mitigating quasiparticles, including radiation shielding, adding quasiparticle traps, changing circuit materials, altering circuit design, and novel modes of operation. The use of these circuits as quasiparticle-based detectors is also explored.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.