21 research outputs found
Transient dynamics of a molecular quantum dot with a vibrational degree of freedom
We investigate the transient effects occurring in a molecular quantum dot
described by an Anderson-Holstein Hamiltonian which is instantly coupled to two
fermionic leads biased by a finite voltage. In the limit of weak
electron-phonon interaction, we use perturbation theory to determine the
time-dependence of the dot population and the average current. The limit of
strong coupling is accessed by means of a self-consistent time-dependent
mean-field approximation. These comple- mentary approaches allow us to
investigate the dynamics of the inelastic effects occurring when the applied
bias voltage exceeds the phonon frequency and the emergence of bistability.Comment: 7 pages, 4 figure
Discrete control of capacitance in quantum circuits
Precise in-situ control of system parameters is indispensable for all quantum
hardware applications. The capacitance in a circuit, however, is usually a
simple consequence of electrostatics, and thus quite literally cast in stone.
We here propose a way to control the charging energy of a given island by
exploiting recently predicted Chern insulator physics in common Cooper-pair
transistors, where the capacitance switches between discrete values given by
the Chern number. We identify conditions for which the discrete control
benefits from exponentially reduced noise sensitivity to implement protected
tunable qubits.Comment: 5 pages, 2 figures, supplementary material at the end of the
document. Comments and feedback are highly welcom
Optimal configurations for normal-metal traps in transmon qubits
Controlling quasiparticle dynamics can improve the performance of
superconducting devices. For example, it has been demonstrated effective in
increasing lifetime and stability of superconducting qubits. Here we study how
to optimize the placement of normal-metal traps in transmon-type qubits. When
the trap size increases beyond a certain characteristic length, the details of
the geometry and trap position, and even the number of traps, become important.
We discuss for some experimentally relevant examples how to shorten the decay
time of the excess quasiparticle density. Moreover, we show that a trap in the
vicinity of a Josephson junction can reduce the steady-state quasiparticle
density near that junction, thus suppressing the quasiparticle-induced
relaxation rate of the qubit. Such a trap also reduces the impact of
fluctuations in the generation rate of quasiparticles, rendering the qubit more
stable.Comment: 16 pages, 7 figures; to appear in Phys. Rev. Applie
Normal-metal quasiparticle traps for superconducting qubits
The presence of quasiparticles in superconducting qubits emerges as an
intrinsic constraint on their coherence. While it is difficult to prevent the
generation of quasiparticles, keeping them away from active elements of the
qubit provides a viable way of improving the device performance. Here we
develop theoretically and validate experimentally a model for the effect of a
single small trap on the dynamics of the excess quasiparticles injected in a
transmon-type qubit. The model allows one to evaluate the time it takes to
evacuate the injected quasiparticles from the transmon as a function of trap
parameters. With the increase of the trap size, this time decreases
monotonically, saturating at the level determined by the quasiparticles
diffusion constant and the qubit geometry. We determine the characteristic trap
size needed for the relaxation time to approach that saturation value.Comment: 11 pages, 5 figure
Efficient quasiparticle traps with low dissipation through gap engineering
Quasiparticles represent an intrinsic source of perturbation for superconducting qubits, leading to both dissipation of the qubit energy and dephasing. Recently, it has been shown that normal-metal traps may efficiently reduce the quasiparticle population and improve the qubit lifetime, provided the trap surpasses a certain characteristic size. Moreover, while the trap itself introduces new relaxation mechanisms, they are not expected to harm state-of-the-art transmon qubits under the condition that the traps are not placed too close to extremal positions where electric fields are high. Here we study a different type of trap, realized through gap engineering. We find that gap-engineered traps relax the remaining constraints imposed on normal metal traps. First, the characteristic trap size, above which the trap is efficient, is reduced with respect to normal metal traps, such that here, strong traps are possible in smaller devices. Second, the losses caused by the trap are now greatly reduced, providing more flexibility in trap placement. The latter point is of particular importance, since for efficient protection from quasiparticles, the traps ideally should be placed close to the active parts of the qubit device, where electric fields are typically high