Daniele Lamberto

Noise Protected Logical Qubit in a Open Chain of Superconducting Qubits with Ultrastrong Interactions

  1. Roberto Stassi,
  2. Shilan Abo,
  3. Daniele Lamberto,
  4. Ye-Hong Chen,
  5. Adam Miranowicz,
  6. Salvatore Savasta,
  7. and Franco Nori
To achieve a fault-tolerant quantum computer, it is crucial to increase the coherence time of quantum bits. In this work, we theoretically investigate a system consisting of a series
of superconducting qubits that alternate between XX and YY ultrastrong interactions. By considering the two-lowest energy eigenstates of this system as a {\it logical} qubit, we demonstrate that its coherence is significantly enhanced: both its pure dephasing and relaxation times are extended beyond those of individual {\it physical} qubits. Specifically, we show that by increasing either the interaction strength or the number of physical qubits in the chain, the logical qubit’s pure dephasing rate is suppressed to zero, and its relaxation rate is reduced to half the relaxation rate of a single physical qubit. Single qubit and two-qubit gates can be performed with a high fidelity.
arxiv pdf

Circuit QED Emission Spectra in the Ultrastrong Coupling Regime: How They Differ from Cavity QED

  1. Samuel Napoli,
  2. Alberto Mercurio,
  3. Daniele Lamberto,
  4. Andrea Zappalà,
  5. Omar Di Stefano,
  6. and Salvatore Savasta
Cavity quantum electrodynamics (QED) studies the interaction between resonator-confined radiation and natural atoms or other formally equivalent quantum excitations, under conditions
where the quantum nature of photons is relevant. Phenomena studied in cavity QED can also be explored using superconducting artificial atoms and microwave photons in superconducting resonators. These circuit QED systems offer the possibility to reach the ultrastrong coupling regime with individual artificial atoms, unlike their natural counterparts. In this regime, the light-matter coupling rate reaches a considerable fraction of the bare resonance frequencies in the system. Here, we provide a careful analysis of the emission spectra in circuit QED systems consisting of a flux qubit interacting with an LC resonator. Despite these systems can be described by the quantum Rabi model, as the corresponding cavity QED ones, we find distinctive features, depending on how the system is coupled with the output port, which become evident in the ultrastrong coupling regime.
arxiv pdf