F. Lefloch

Coherence Limits in Interference-Based cos(2φ) Qubits

  1. S. Messelot,
  2. A. Leblanc,
  3. J.-S. Tettekpoe,
  4. F. Lefloch,
  5. Q. Ficheux,
  6. J. Renard,
  7. and É. Dumur
We investigate the coherence properties of parity-protected cos(2φ) qubits based on interferences between two Josephson elements in a superconducting loop. We show that qubit implementations
of a cos(2φ) potential using a single loop, such as those employing semiconducting junctions, rhombus circuits, flowermon and KITE structures, can be described by the same Hamiltonian as two multi-harmonic Josephson junctions in a SQUID geometry. We find that, despite the parity protection arising from the suppression of single Cooper pair tunneling, there exists a fundamental trade-off between charge and flux noise dephasing channels. Using numerical simulations, we examine how relaxation and dephasing rates depend on external flux and circuit parameters, and we identify the best compromise for maximum coherence. With currently existing circuit parameters, the qubit lifetime T1 can exceed milliseconds while the dephasing time Tφ remains limited to only a few microseconds due to either flux or charge noise. Our findings establish practical limits on the coherence of this class of qubits and raise questions about the long-term potential of this approach.
arxiv pdf

All silicon Josephson junctions

  1. F. Chiodi,
  2. J.-E. Duvauchelle,
  3. C. Marcenat,
  4. D. Débarre,
  5. and F. Lefloch
We have realised laser-doped all-silicon superconducting (S)/ normal metal (N) bilayers of tunable thickness and dopant concentration. We observed a strong reduction of the bilayers
critical temperature when increasing the normal metal thickness, a signature of the highly transparent S/N interface associated to the epitaxial sharp laser doping profile. We extracted the interface resistance by fitting with the linearised Usadel equations, demonstrating a reduction of one order of magnitude from previous superconductor/doped Si interfaces. In this well controlled crystalline system we exploited the low resistance S/N interfaces to elaborate all-silicon lateral SNS Josephson junctions with long range proximity effect. Their dc transport properties, such as the critical and retrapping currents, could be well understood in the diffusive regime. Furthermore, this work lead to the estimation of important parameters in ultra-doped superconducting Si, such as the Fermi velocity, the coherence length, or the electron-phonon coupling constant, fundamental to conceive an all-silicon superconducting electronics.
arxiv pdf