Basil M. Smitham

Interaction-induced escape from an Aharonov-Bohm cage

  1. Jeronimo G.C. Martinez,
  2. Christie S. Chiu,
  3. Basil M. Smitham,
  4. and Andrew A. Houck
Advances in quantum engineering have enabled the design, measurement, and precise control of synthetic condensed matter systems. The platform of superconducting circuits offers two
particular capabilities: flexible connectivity of circuit elements that enables a variety of lattice geometries, and circuit nonlinearity that provides access to strongly interacting physics. Separately, these features have allowed for the creation of curved-space lattices and the realization of strongly correlated phases and dynamics in one-dimensional chains and square lattices. Missing in this suite of simulations is the simultaneous integration of interacting particles into lattices with unique band dispersions, such as dispersionless flat bands. An ideal building block for flat-band physics is the Aharonov-Bohm cage: a single plaquette of a lattice whose band structure consists entirely of flat bands. Here, we experimentally construct an Aharonov-Bohm cage and observe the localization of a single photon, the hallmark of all-bands-flat physics. Upon placing an interaction-bound photon pair into the cage, we see a delocalized walk indicating an escape from Aharonov-Bohm caging. We further find that a variation of caging persists for two particles initialized on opposite sites of the cage. These results mark the first experimental work where interacting particles circumvent an Aharonov-Bohm cage and establish superconducting circuits for studies of flat-band-lattice dynamics with strong interactions.
arxiv pdf

New material platform for superconducting transmon qubits with coherence times exceeding 0.3 milliseconds

  1. Alex P. M. Place,
  2. Lila V. H. Rodgers,
  3. Pranav Mundada,
  4. Basil M. Smitham,
  5. Mattias Fitzpatrick,
  6. Zhaoqi Leng,
  7. Anjali Premkumar,
  8. Jacob Bryon,
  9. Sara Sussman,
  10. Guangming Cheng,
  11. Trisha Madhavan,
  12. Harshvardhan K. Babla,
  13. Berthold Jäck,
  14. Andras Gyenis,
  15. Nan Yao,
  16. Robert J. Cava,
  17. Nathalie P. de Leon,
  18. and Andrew A. Houck
The superconducting transmon qubit is a leading platform for quantum computing and quantum science. Building large, useful quantum systems based on transmon qubits will require significant
improvements in qubit relaxation and coherence times, which are orders of magnitude shorter than limits imposed by bulk properties of the constituent materials. This indicates that relaxation likely originates from uncontrolled surfaces, interfaces, and contaminants. Previous efforts to improve qubit lifetimes have focused primarily on designs that minimize contributions from surfaces. However, significant improvements in the lifetime of two-dimensional transmon qubits have remained elusive for several years. Here, we fabricate two-dimensional transmon qubits that have both lifetimes and coherence times with dynamical decoupling exceeding 0.3 milliseconds by replacing niobium with tantalum in the device. We have observed increased lifetimes for seventeen devices, indicating that these material improvements are robust, paving the way for higher gate fidelities in multi-qubit processors.
arxiv pdf