Riju Banerjee

A Cascaded Random Access Quantum Memory

  1. Ziqian Li,
  2. Eesh Gupta,
  3. Fang Zhao,
  4. Riju Banerjee,
  5. Yao Lu,
  6. Tanay Roy,
  7. Andrew Oriani,
  8. Andrei Vrajitoarea,
  9. Srivatsan Chakram,
  10. and David I. Schuster
Dynamic random access memory is critical to classical computing but notably absent in experimental quantum computers. Here we realize an 8-bit cascaded random access quantum memory
using superconducting circuits and cavities and showcase the ability to perform arbitrary gate operations on it. In addition to individual error channels such as photon loss, quantum memories can also experience decoherence from many-body self-interaction. We characterize the origin and contributions of many-body infidelity throughout the memory cycle. We find that individual modes can be accessed with ≲1.5% infidelity per mode and that the entire memory can be accessed in arbitrary order with an error rate below the depolarization threshold of the surface code, paving the way for fault-tolerant quantum memories.
arxiv pdf

Niobium coaxial cavities with internal quality factors exceeding 1.5 billion for circuit quantum electrodynamics

  1. Andrew E. Oriani,
  2. Fang Zhao,
  3. Tanay Roy,
  4. Alexander Anferov,
  5. Kevin He,
  6. Ankur Agrawal,
  7. Riju Banerjee,
  8. Srivatsan Chakram,
  9. and David I. Schuster
Group-V materials such as niobium and tantalum have become popular choices for extending the performance of circuit quantum electrodynamics (cQED) platforms allowing for quantum processors
and memories with reduced error rates and more modes. The complex surface chemistry of niobium however makes identifying the main modes of decoherence difficult at millikelvin temperatures and single-photon powers. We use niobium coaxial quarter-wave cavities to study the impact of etch chemistry, prolonged atmospheric exposure, and the significance of cavity conditions prior to and during cooldown, in particular niobium hydride evolution, on single-photon coherence. We demonstrate cavities with quality factors of Qint≳1.4×109 in the single-photon regime, a 15 fold improvement over aluminum cavities of the same geometry. We rigorously quantify the sensitivity of our fabrication process to various loss mechanisms and demonstrate a 2−4× reduction in the two-level system (TLS) loss tangent and a 3−5× improvement in the residual resistivity over traditional BCP etching techniques. Finally, we demonstrate transmon integration and coherent cavity control while maintaining a cavity coherence of \SI{11.3}{ms}. The accessibility of our method, which can easily be replicated in academic-lab settings, and the demonstration of its performance mark an advancement in 3D cQED.
arxiv pdf