Shraddha Singh

Quantum Error Correction of Qudits Beyond Break-even

  1. Benjamin L. Brock,
  2. Shraddha Singh,
  3. Alec Eickbusch,
  4. Volodymyr V. Sivak,
  5. Andy Z. Ding,
  6. Luigi Frunzio,
  7. Steven M. Girvin,
  8. and Michel H. Devoret
Hilbert space dimension is a key resource for quantum information processing. A large Hilbert space is not only an essential requirement for quantum error correction, but it can also
be advantageous for realizing gates and algorithms more efficiently. There has thus been considerable experimental effort in recent years to develop quantum computing platforms using qudits (d-dimensional quantum systems with d>2) as the fundamental unit of quantum information. Just as with qubits, quantum error correction of these qudits will be necessary in the long run, but to date error correction of logical qudits has not been demonstrated experimentally. Here we report the experimental realization of error-corrected logical qutrits (d=3) and ququarts (d=4) by employing the Gottesman-Kitaev-Preskill (GKP) bosonic code in a circuit QED architecture. Using a reinforcement learning agent, we optimize the GKP qutrit (ququart) as a ternary (quaternary) quantum memory and achieve beyond break-even error correction with a gain of 1.82 +/- 0.03 (1.87 +/- 0.03). This work represents a new way of leveraging the large Hilbert space of a harmonic oscillator for hardware-efficient quantum error correction.
arxiv pdf

Scalable High-Performance Fluxonium Quantum Processor

  1. Long B. Nguyen,
  2. Gerwin Koolstra,
  3. Yosep Kim,
  4. Alexis Morvan,
  5. Trevor Chistolini,
  6. Shraddha Singh,
  7. Konstantin N. Nesterov,
  8. Christian Jünger,
  9. Larry Chen,
  10. Zahra Pedramrazi,
  11. Bradley K. Mitchell,
  12. John Mark Kreikebaum,
  13. Shruti Puri,
  14. David I. Santiago,
  15. and Irfan Siddiqi Singh
The technological development of hardware heading toward universal fault-tolerant quantum computation requires a large-scale processing unit with high performance. While fluxonium qubits
are promising with high coherence and large anharmonicity, their scalability has not been systematically explored. In this work, we propose a superconducting quantum information processor based on compact high-coherence fluxoniums with suppressed crosstalk, reduced design complexity, improved operational efficiency, high-fidelity gates, and resistance to parameter fluctuations. In this architecture, the qubits are readout dispersively using individual resonators connected to a common bus and manipulated via combined on-chip RF and DC control lines, both of which can be designed to have low crosstalk. A multi-path coupling approach enables exchange interactions between the high-coherence computational states and at the same time suppresses the spurious static ZZ rate, leading to fast and high-fidelity entangling gates. We numerically investigate the cross resonance controlled-NOT and the differential AC-Stark controlled-Z operations, revealing low gate error for qubit-qubit detuning bandwidth of up to 1 GHz. Our study on frequency crowding indicates high fabrication yield for quantum processors consisting of over thousands of qubits. In addition, we estimate low resource overhead to suppress logical error rate using the XZZX surface code. These results promise a scalable quantum architecture with high performance for the pursuit of universal quantum computation.
arxiv pdf