Microwave-Activated Controlled-Z Gate for Fixed-Frequency Fluxonium Qubits

  1. Konstantin N. Nesterov,
  2. Ivan V. Pechenezhskiy,
  3. Chen Wang,
  4. Vladimir E. Manucharyan,
  5. and Maxim G. Vavilov
The superconducting fluxonium circuit is an artificial atom with a strongly anharmonic spectrum: when biased at a half flux quantum, the lowest qubit transition is an order of magnitude
smaller in frequency than those to higher levels. Similar to conventional atomic systems, such a frequency separation between the computational and noncomputational subspaces allows independent optimizations of the qubit coherence and two-qubit interactions. Here we describe a controlled-Z gate for two fluxoniums connected either capacitively or inductively, with qubit transitions fixed near 500 MHz. The gate is activated by a microwave drive at a resonance involving the second excited state. We estimate intrinsic gate fidelities over 99.9% with gate times below 100 ns.

Phonon-Mediated Quasiparticle Poisoning of Superconducting Microwave Resonators

  1. U. Patel,
  2. Ivan V. Pechenezhskiy,
  3. B. L. T. Plourde,
  4. M.G. Vavilov,
  5. and R. McDermott
Nonequilibrium quasiparticles represent a significant source of decoherence in superconducting quantum circuits. Here we investigate the mechanism of quasiparticle poisoning in devices
subjected to local quasiparticle injection. We find that quasiparticle poisoning is dominated by the propagation of pair-breaking phonons across the chip. We characterize the energy dependence of the timescale for quasiparticle poisoning. Finally, we observe that incorporation of extensive normal metal quasiparticle traps leads to a more than order of magnitude reduction in quasiparticle loss for a given injected quasiparticle power.