Peter L. McMahon

Fabrication-free assessment of microwave losses in germanium-based dielectrics and superconductors

  1. Haoran Lu,
  2. Kushagra Aggarwal,
  3. Xiangqin Wang,
  4. Pauline Drexler,
  5. Daniel Tong,
  6. Maciej W. Olszewski,
  7. Anand Ithepalli,
  8. Lingda Kong,
  9. Debdeep Jena,
  10. Peter L. McMahon,
  11. David A. Muller,
  12. Dominique Bougeard,
  13. and Valla Fatemi
We present a flip-chip-based sensing scheme to measure effective microwave losses associated with target materials for quantum technologies, without requiring any device fabrication
on the material under test. Using this approach, we quantify the microwave losses of a strain-engineered Ge/SiGe quantum well heterostructure and investigate losses arising from its Ge substrate and intermediate layers. The quality factors of the fabricated microwave resonators agree with the losses of dielectric materials independently extracted from flip-chip sensing measurements. We further study the superconductor platinum silicon germanide (PtSiGe) prepared by thermal reaction with a deposited Pt film, finding high microwave losses that limit the suitability of the films studied here as the sole superconductor for high-quality resonator applications. By coating Pt with Nb prior to the reaction, we observe a substantial reduction in microwave loss and a nearly three-fold enhancement of the transport critical temperature. The temperature dependence of the microwave loss is consistent with gap inhomogeneity in both superconducting films. These results identify constraints on material choices, provide design guidance for microwave circuits on planar Ge heterostructures, and demonstrate a fast-turnaround testing method for new materials for superconducting quantum circuits.
arxiv pdf

Exponential quantum advantage for learning signals with a single qubit

  1. Ishaan Kannan,
  2. Sridhar Prabhu,
  3. Saeed A. Khan,
  4. Mandar M. Sohoni,
  5. Xingrui Song,
  6. Saswata Roy,
  7. Alen Senanian,
  8. Valla Fatemi,
  9. Peter L. McMahon,
  10. and Jordan Cotler
Quantum technology has the potential to transform scientific discovery, but quantum advantages often require processing capabilities well beyond the reach of experimental platforms.
We show that coupling a single controllable qubit to an otherwise conventional sensor can exponentially reduce the number of measurements required to learn classical signals. These rigorous quantum advantages apply to fundamental sensing tasks, including learning Fourier coefficients, extracting temporal correlations from time-varying signals, and estimating transformations of physical observables. Using a superconducting cavity–qubit architecture, we experimentally demonstrate 107-fold reductions in the number of measurements required for Fourier-amplitude and time-varying signal learning. Our quantum feature sensing algorithms further enable orders-of-magnitude improvements in simulations of weak-signal dark matter detection and wireless communication applications. These quantum advantages are derived from Quantum Phase-Space Inference (QΨ), a unifying theory of quantum-enhanced experiments that simultaneously converts a set of experimental objectives and constraints into tight lower bounds and optimal quantum-enhanced learning algorithms while producing a certificate of quantum advantage. QΨ extends beyond the regimes captured by quantum Fisher information and provides a framework for systematically identifying rigorous quantum advantages in practical experimental tasks. Together, our results establish that near-term quantum technology can exponentially enhance our ability to learn from classical signals.
arxiv pdf