Mollie E. Schwartz

Silicon Hard-Stop Spacers for 3D Integration of Superconducting Qubits

  1. Bethany M. Niedzielski,
  2. David K. Kim,
  3. Mollie E. Schwartz,
  4. Danna Rosenberg,
  5. Greg Calusine,
  6. Rabi Das,
  7. Alexander J. Melville,
  8. Jason Plant,
  9. Livia Racz,
  10. Jonilyn L. Yoder,
  11. Donna Ruth-Yost,
  12. and William D. Oliver
As designs for superconducting qubits become more complex, 3D integration of two or more vertically bonded chips will become necessary to enable increased density and connectivity.
Precise control of the spacing between these chips is required for accurate prediction of circuit performance. In this paper, we demonstrate an improvement in the planarity of bonded superconducting qubit chips while retaining device performance by utilizing hard-stop silicon spacer posts. These silicon spacers are defined by etching several microns into a silicon substrate and are compatible with 3D-integrated qubit fabrication. This includes fabrication of Josephson junctions, superconducting air-bridge crossovers, underbump metallization and indium bumps. To qualify the integrated process, we demonstrate high-quality factor resonators on the etched surface and measure qubit coherence (T1, T2,echo > 40 {\mu}s) in the presence of silicon posts as near as 350 {\mu}m to the qubit.
arxiv pdf

Superconducting Qubits: Current State of Play

  1. Morten Kjaergaard,
  2. Mollie E. Schwartz,
  3. Jochen Braumüller,
  4. Philip Krantz,
  5. Joel I-Jan Wang,
  6. Simon Gustavsson,
  7. and William D. Oliver
Superconducting qubits are leading candidates in the race to build a quantum computer capable of realizing computations beyond the reach of modern supercomputers. The superconducting
qubit modality has been used to demonstrate prototype algorithms in the `noisy intermediate scale quantum‘ (NISQ) technology era, in which non-error-corrected qubits are used to implement quantum simulations and quantum algorithms. With the recent demonstrations of multiple high fidelity two-qubit gates as well as operations on logical qubits in extensible superconducting qubit systems, this modality also holds promise for the longer-term goal of building larger-scale error-corrected quantum computers. In this brief review, we discuss several of the recent experimental advances in qubit hardware, gate implementations, readout capabilities, early NISQ algorithm implementations, and quantum error correction using superconducting qubits. While continued work on many aspects of this technology is certainly necessary, the pace of both conceptual and technical progress in the last years has been impressive, and here we hope to convey the excitement stemming from this progress.
arxiv pdf

Observation of measurement-induced entanglement and quantum trajectories of remote superconducting qubits

  1. Nicolas Roch,
  2. Mollie E. Schwartz,
  3. Felix Motzoi,
  4. Christopher Macklin,
  5. Rajamani Vijay,
  6. Andrew W. Eddins,
  7. Alexander N. Korotkov,
  8. K. Birgitta Whaley,
  9. Mohan Sarovar,
  10. and Irfan Siddiqi
The creation of a quantum network requires the distribution of coherent information across macroscopic distances. We demonstrate the entanglement of two superconducting qubits, separated
by more than a meter of coaxial cable, by designing a joint measurement that probabilistically projects onto an entangled state. By using a continuous measurement scheme, we are further able to observe single quantum trajectories of the joint two-qubit state, confirming the validity of the quantum Bayesian formalism for a cascaded system. Our results allow us to resolve the dynamics of continuous projection onto the entangled manifold, in quantitative agreement with theory.
arxiv pdf