Anurag Mishra

Fast, High-Fidelity Erasure Detection of Dual-Rail Qubits with Symmetrically Coupled Readout

  1. Jimmy Shih-Chun Hung,
  2. Arbel Haim,
  3. Mouktik Raha,
  4. Gihwan Kim,
  5. Ziwen Huang,
  6. Ming-Han Chou,
  7. Mitch D'Ewart,
  8. Erik Davis,
  9. Anurag Mishra,
  10. Patricio Arrangoiz-Arriola,
  11. Amirhossein Khalajhedayati,
  12. David Hover,
  13. Fernando G.S.L. Brandão,
  14. Aashish A. Clerk,
  15. Alex Retzker,
  16. Harry Levine,
  17. and Oskar Painter
Erasure qubits are a promising platform for implementing hardware-efficient quantum error correction. Realizing the error-correction advantages of this encoding requires frequent mid-circuiterasure checks that are fast, high-fidelity, and scalable. Here, we realize erasure detection with a hardware-efficient circuit consisting of a single readout resonator dispersively and symmetrically coupled to both transmons of a dual-rail qubit. We use this circuit to demonstrate single-shot erasure detection in 384 ns with minimal impact on the dual-rail logical manifold, achieving a residual error per check of 6.0(2)×10−4, with only 8(3)×10−5 induced dephasing per check, and an erasure error per check of 2.54(1)×10−2. The high degree of matched dispersive readout coupling (χ-matching) within the dual-rail qubit code space also allows us to realize a new modality: time-continuous erasure detection performed in parallel with single-qubit gates. Here we achieve a median 7.2×10−5 error per gate with <1×10−5 error induced by erasure detection. This demonstrates a reduction in erasure detection overhead as well as a crucial ingredient for soft information quantum error correction. Together, these results establish symmetrically coupled dispersive readout as a fast, hardware-efficient, and scalable component for erasure-based quantum error correction using transmon dual-rail qubits.[/expand]
arxiv pdf

Distinguishing Classical and Quantum Models for the D-Wave Device

  1. Walter Vinci,
  2. Tameem Albash,
  3. Anurag Mishra,
  4. Paul A. Warburton,
  5. and Daniel A. Lidar
Recently the question of whether the D-Wave processors exhibit large-scale quantum behavior or can be described by a classical model has attracted significant interest. In this work
we address this question by studying a 503 qubit D-Wave Two device as a „black box“, i.e., by studying its input-output behavior. We examine three candidate classical models and one quantum model, and compare their predictions to experiments we have performed on the device using groups of up to 40 qubits. The candidate classical models are simulated annealing, spin dynamics, a recently proposed hybrid O(2) rotor-Monte Carlo model, and three modified versions thereof. The quantum model is an adiabatic Markovian master equation derived in the weak coupling limit of an open quantum system. Our experiments realize an evolution from a transverse field to an Ising Hamiltonian, with a final-time degenerate ground state that splits into two types of states we call „isolated“ and „clustered“. We study the population ratio of the isolated and clustered states as a function of the overall energy scale of the Ising term, and the distance between the final state and the Gibbs state, and find that these are sensitive probes that distinguish the classical models from one another and from both the experimental data and the master equation. The classical models are all found to disagree with the data, while the master equation agrees with the experiment without fine-tuning, and predicts mixed state entanglement at intermediate evolution times. This suggests that an open system quantum dynamical description of the D-Wave device is well-justified even in the presence of relevant thermal excitations and fast single-qubit decoherence.
arxiv pdf