Alessandro Bruno

Implementation and readout of maximally entangled two-qubit gates quantum circuits in a superconducting quantum processor

  1. Viviana Stasino,
  2. Pasquale Mastrovito,
  3. Carlo Cosenza,
  4. Anna Levochkina,
  5. Martina Esposito,
  6. Domenico Montemurro,
  7. Giovanni P. Pepe,
  8. Alessandro Bruno,
  9. Francesco Tafuri,
  10. Davide Massarotti,
  11. and Halima G. Ahmad
Besides noticeable challenges in implementing low-error single- and two-qubit quantum gates in superconducting quantum processors, the readout technique and analysis are a key factor
in determining the efficiency and performance of quantum processors. Being able to efficiently implement quantum algorithms involving entangling gates and asses their output is mandatory for quantum utility. In a transmon-based 5-qubit superconducting quantum processor, we compared the performance of quantum circuits involving an increasing level of complexity, from single-qubit circuits to maximally entangled Bell circuits. This comparison highlighted the importance of the readout analysis and helped us optimize the protocol for more advanced quantum algorithms. Here we report the results obtained from the analysis of the outputs of quantum circuits using two readout paradigms, referred to as „multiplied readout probabilities“ and „conditional readout probabilities“. The first method is suitable for single-qubit circuits, while the second is essential for accurately interpreting the outputs of circuits involving two-qubit gates.
arxiv pdf

Multi-mode ultra-strong coupling in circuit quantum electrodynamics

  1. Sal J. Bosman,
  2. Mario F. Gely,
  3. Vibhor Singh,
  4. Alessandro Bruno,
  5. Daniel Bothner,
  6. and Gary A. Steele
With the introduction of superconducting circuits into the field of quantum optics, many novel experimental demonstrations of the quantum physics of an artificial atom coupled to a
single-mode light field have been realized. Engineering such quantum systems offers the opportunity to explore extreme regimes of light-matter interaction that are inaccessible with natural systems. For instance the coupling strength g can be increased until it is comparable with the atomic or mode frequency ωa,m and the atom can be coupled to multiple modes which has always challenged our understanding of light-matter interaction. Here, we experimentally realize the first Transmon qubit in the ultra-strong coupling regime, reaching coupling ratios of g/ωm=0.19 and we measure multi-mode interactions through a hybridization of the qubit up to the fifth mode of the resonator. This is enabled by a qubit with 88% of its capacitance formed by a vacuum-gap capacitance with the center conductor of a coplanar waveguide resonator. In addition to potential applications in quantum information technologies due to its small size and localization of electric fields in vacuum, this new architecture offers the potential to further explore the novel regime of multi-mode ultra-strong coupling.
arxiv pdf