Quasiparticles in Qubit Circuits

Advisor: Eli Levenson-Falk, PhD

Quasiparticles--single-particle excitations of the superfluid condensate--are a major source of decoherence in superconducting qubits. A quasiparticle tunneling across a Josephson junction can absorb energy from the qubit, causing relaxation; it can donate energy to the qubit, causing spurious excitation; and it can modify the qubit energy spectrum, causing dephasing. Quasiparticles can also cause loss during bulk transport, and can become trapped in sensitive circuit elements, causing further decoherence. While superconducting qubits are typically operated at temperatures low enough to completely eliminate all quasiparticles, experimental research has consistently shown significant non-thermal populations of quasiparticles.

These non-equilibrium quasiparticles may have many sources. Blackbody radiation from warmer stages of a cryostat can impact the circuit and excite quasiparticles. Likewise, cosmic radiation and nearby radioactive materials may generate quasiparticle populations that are significant. "Hot spots" in the circuit may be insufficiently thermalized and therefore generate thermal quasiparticles at a higher temperature. All of these mechanisms are likely significant and must be combatted in order to optimize qubit coherence.

In order to tackle the problem of quasiparticles, their behavior must be carefully studied. In LFL, we study quasiparticles through a few techniques. In one set of experiments, we trap quasiparticles in the internal Andreev states of weak-link Josephson junctions. These Andreev traps, when integrated into a resonant circuit, affect the resonant frequency of the circuit, enabling fast, single-shot detection of quasiparticle trapping. We use these traps to study the correlations between quasiparticles, their energy distribution, and their relaxation and recombination mechanisms. We also design qubits which are especially sensitive or insensitive to quasiparticle behavior, and measure qubit spectrum and lifetime in a variety of quasiparticle generation environments. These experiments allow us to better understand the mechanisms by which quasiparticles are created and destroyed, and to test strategies for mitigating quasiparticle-related decoherence.