Alexey Bezryadin

A Dayem Loop Qubit Based on Interfering Superconducting Nanowires

  1. Cliff Sun,
  2. and Alexey Bezryadin
We propose a qubit design based on two parallel superconducting nanowires (i.e., a „Dayem loop qubit“). The inclusion of two nanowires instead of one leads to the Little-Parks
effect, which provides an oscillator behavior for the qubit frequency as well as anharmonicity. Our key result is that even if the nanowires have an increasingly linear CPR at low supercurrents, the quantum interference between two condensates, induced by a magnetic field, leads to a restoration of cubic nonlinearity, which is predicted to be sufficient to create a functional transmon qubit based on thin superconducting wires. We consider both generic (cubic) current-phase relationships (CPR) as well as more realistic microscopic CPR, having higher-order nonlinearities. For higher-order CPRs, we propose a simple power-law phenomenological approximation valid at very low temperatures, at which superconducting qubits normally operate.
arxiv pdf

Meissner transmon qubit – architecture and characterization

  1. Jaseung Ku,
  2. Zack Yoscovits,
  3. Alex Levchenko,
  4. James Eckstein,
  5. and Alexey Bezryadin
We present a new type of transmon split-junction qubit which can be tuned by Meissner screening currents in the adjacent superconducting film electrodes. The best detected relaxation
time was of the order of 50 {\mu}s and the dephasing time about 70 {\mu}s. The achieved period of oscillation with magnetic field was much smaller than in usual SQUID-based transmon qubits, thus a strong effective field amplification has been realized. This Meissner qubit allows an efficient coupling to superconducting vortices. We present a quantitative analysis of the radiation-free energy relaxation in qubits coupled to Abrikosov vortices. The observation of coherent quantum oscillations provides strong evidence that vortices can exist in coherent quantum superpositions of different position states. According to our suggested model, the wave function collapse is defined by Caldeira-Leggett dissipation associated with viscous motion of the vortex cores.
arxiv pdf