Stefan Filipp

Measurement of a Vacuum-Induced Geometric Phase

  1. Simone Gasparinetti,
  2. Simon Berger,
  3. Abdufarrukh A. Abdumalikov,
  4. Marek Pechal,
  5. Stefan Filipp,
  6. and Andreas J. Wallraff
Berry’s geometric phase naturally appears when a quantum system is driven by an external field whose parameters are slowly and cyclically changed. A variation in the coupling
between the system and the external field can also give rise to a geometric phase, even when the field is in the vacuum state or any other Fock state. Here we demonstrate the appearance of a vacuum-induced Berry phase in an artificial atom, a superconducting transmon, interacting with a single mode of a microwave cavity. As we vary the phase of the interaction, the artificial atom acquires a geometric phase determined by the path traced out in the combined Hilbert space of the atom and the quantum field. Our ability to control this phase opens new possibilities for the geometric manipulation of atom-cavity systems also in the context of quantum information processing.
arxiv pdf

A universal gate for fixed-frequency qubits via a tunable bus

  1. David C. McKay,
  2. Stefan Filipp,
  3. Antonio Mezzacapo,
  4. Easwar Magesan,
  5. Jerry M. Chow,
  6. and Jay M. Gambetta
A challenge for constructing large circuits of superconducting qubits is to balance addressability, coherence and coupling strength. High coherence can be attained by building circuits
from fixed-frequency qubits, however, leading techniques cannot couple qubits that are far detuned. Here we introduce a method based on a tunable bus which allows for the coupling of two fixed-frequency qubits even at large detunings. By parametrically oscillating the bus at the qubit-qubit detuning we enable a resonant exchange (XX+YY) interaction. We use this interaction to implement a 183ns two-qubit iSWAP gate between qubits separated in frequency by 854MHz with a measured average fidelity of 0.9823(4) from interleaved randomized benchmarking. This gate may be an enabling technology for surface code circuits and for analog quantum simulation.
arxiv pdf

Characterizing errors on qubit operations via iterative randomized benchmarking

  1. Sarah Sheldon,
  2. Lev S. Bishop,
  3. Easwar Magesan,
  4. Stefan Filipp,
  5. Jerry M. Chow,
  6. and Jay M. Gambetta
With improved gate calibrations reducing unitary errors, we achieve a benchmarked single-qubit gate fidelity of 99.95% with superconducting qubits in a circuit quantum electrodynamics
system. We present a method for distinguishing between unitary and non-unitary errors in quantum gates by interleaving repetitions of a target gate within a randomized benchmarking sequence. The benchmarking fidelity decays quadratically with the number of interleaved gates for unitary errors but linearly for non-unitary, allowing us to separate systematic coherent errors from decoherent effects. With this protocol we show that the fidelity of the gates is not limited by unitary errors, but by another drive-activated source of decoherence such as amplitude fluctuations.
arxiv pdf