@article{14517,
  abstract     = {State-of-the-art transmon qubits rely on large capacitors, which systematically improve their coherence due to reduced surface-loss participation. However, this approach increases both the footprint and the parasitic cross-coupling and is ultimately limited by radiation losses—a potential roadblock for scaling up quantum processors to millions of qubits. In this work we present transmon qubits with sizes as low as 36 × 39 µm2 with  100-nm-wide vacuum-gap capacitors that are micromachined from commercial silicon-on-insulator wafers and shadow evaporated with aluminum. We achieve a vacuum participation ratio up to 99.6% in an in-plane design that is compatible with standard coplanar circuits. Qubit relaxationtime measurements for small gaps with high zero-point electric field variance of up to 22 V/m reveal a double exponential decay indicating comparably strong qubit interaction with long-lived two-level systems. The exceptionally high selectivity of up to 20 dB to the superconductor-vacuum interface allows us to precisely back out the sub-single-photon dielectric loss tangent of aluminum oxide previously exposed to ambient conditions. In terms of future scaling potential, we achieve a ratio of qubit quality factor to a footprint area equal to 20 µm−2, which is comparable with the highest T1 devices relying on larger geometries, a value that could improve substantially for lower surface-loss superconductors. },
  author       = {Zemlicka, Martin and Redchenko, Elena and Peruzzo, Matilda and Hassani, Farid and Trioni, Andrea and Barzanjeh, Shabir and Fink, Johannes M},
  issn         = {2331-7019},
  journal      = {Physical Review Applied},
  number       = {4},
  publisher    = {American Physical Society},
  title        = {{Compact vacuum-gap transmon qubits: Selective and sensitive probes for superconductor surface losses}},
  doi          = {10.1103/PhysRevApplied.20.044054},
  volume       = {20},
  year         = {2023},
}

@article{13117,
  abstract     = {The ability to control the direction of scattered light is crucial to provide flexibility and scalability for a wide range of on-chip applications, such as integrated photonics, quantum information processing, and nonlinear optics. Tunable directionality can be achieved by applying external magnetic fields that modify optical selection rules, by using nonlinear effects, or interactions with vibrations. However, these approaches are less suitable to control microwave photon propagation inside integrated superconducting quantum devices. Here, we demonstrate on-demand tunable directional scattering based on two periodically modulated transmon qubits coupled to a transmission line at a fixed distance. By changing the relative phase between the modulation tones, we realize unidirectional forward or backward photon scattering. Such an in-situ switchable mirror represents a versatile tool for intra- and inter-chip microwave photonic processors. In the future, a lattice of qubits can be used to realize topological circuits that exhibit strong nonreciprocity or chirality.},
  author       = {Redchenko, Elena and Poshakinskiy, Alexander V. and Sett, Riya and Zemlicka, Martin and Poddubny, Alexander N. and Fink, Johannes M},
  issn         = {2041-1723},
  journal      = {Nature Communications},
  publisher    = {Springer Nature},
  title        = {{Tunable directional photon scattering from a pair of superconducting qubits}},
  doi          = {10.1038/s41467-023-38761-6},
  volume       = {14},
  year         = {2023},
}

@article{14032,
  abstract     = {Arrays of Josephson junctions are governed by a competition between superconductivity and repulsive Coulomb interactions, and are expected to exhibit diverging low-temperature resistance when interactions exceed a critical level. Here we report a study of the transport and microwave response of Josephson arrays with interactions exceeding this level. Contrary to expectations, we observe that the array resistance drops dramatically as the temperature is decreased—reminiscent of superconducting behaviour—and then saturates at low temperature. Applying a magnetic field, we eventually observe a transition to a highly resistive regime. These observations can be understood within a theoretical picture that accounts for the effect of thermal fluctuations on the insulating phase. On the basis of the agreement between experiment and theory, we suggest that apparent superconductivity in our Josephson arrays arises from melting the zero-temperature insulator.},
  author       = {Mukhopadhyay, Soham and Senior, Jorden L and Saez Mollejo, Jaime and Puglia, Denise and Zemlicka, Martin and Fink, Johannes M and Higginbotham, Andrew P},
  issn         = {1745-2481},
  journal      = {Nature Physics},
  keywords     = {General Physics and Astronomy},
  pages        = {1630--1635},
  publisher    = {Springer Nature},
  title        = {{Superconductivity from a melted insulator in Josephson junction arrays}},
  doi          = {10.1038/s41567-023-02161-w},
  volume       = {19},
  year         = {2023},
}