Roberto Stassi

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

Quantum Nonlinear Optics without Photons

  1. Roberto Stassi,
  2. Vincenzo Macrì,
  3. Anton Frisk Kockum,
  4. Omar Di Stefano,
  5. Adam Miranowicz,
  6. Salvatore Savasta,
  7. and Franco Nori
Spontaneous parametric down-conversion is a well-known process in quantum nonlinear optics in which a photon incident on a nonlinear crystal spontaneously splits into two photons. Here
we propose an analogous physical process where one excited atom directly transfers its excitation to a pair of spatially-separated atoms with probability approaching one. The interaction is mediated by the exchange of virtual rather than real photons. This nonlinear atomic process is coherent and reversible, so the pair of excited atoms can transfer the excitation back to the first one: the atomic analogue of sum-frequency generation of light. The parameters used to investigate this process correspond to experimentally-demonstrated values in ultrastrong circuit quantum electrodynamics. This approach can be extended to realize other nonlinear inter-atomic processes, such as four-atom mixing, and is an attractive architecture for the realization of quantum devices on a chip. We show that four-qubit mixing can efficiently implement quantum repetition codes and, thus, can be used for error-correction codes.
arxiv pdf