G. Catelani

Collective modes in the fluxonium qubit

  1. G. Viola,
  2. and G. Catelani
Superconducting qubit designs vary in complexity from single- and few-junction systems, such as the transmon and flux qubits, to the many-junction fluxonium. Here we consider the question
of wether the many degrees of freedom in the fluxonium circuit can limit the qubit coherence time. Such a limitation is in principle possible, due to the interactions between the low-energy, highly anharmonic qubit mode and the higher-energy, weakly anharmonic collective modes. We show that so long as the coupling of the collective modes with the external electromagnetic environment is sufficiently weaker than the qubit-environment coupling, the qubit dephasing induced by the collective modes does not significantly contribute to decoherence. Therefore, the increased complexity of the fluxonium qubit does not constitute by itself a major obstacle for its use in quantum computation architectures.
arxiv pdf

Parity switching and decoherence by quasiparticles in single-junction transmons

  1. G. Catelani
The transmon superconducting qubit is being intensely investigated as a promising approach for the physical implementation of quantum information processing, and high quality factors
of order 106 have been achieved both in two- and three-dimensional architectures. These high quality factors enable detailed investigations of decoherence mechanisms. An intrinsic decoherence process originates from the coupling between the qubit degree of freedom and the quasiparticles that tunnel across Josephson junctions. In a transmon, tunneling of a single quasiparticle is associated with a change in parity. Here we present the theory of the parity-switching rates in single-junction transmons and compare it with recent measurements. We also show that parity switching can have an important role in limiting the coherence time.
arxiv pdf

Ten Milliseconds for Aluminum Cavities in the Quantum Regime

  1. M. Reagor,
  2. Hanhee Paik,
  3. G. Catelani,
  4. L. Sun,
  5. C. Axline,
  6. E. Holland,
  7. I.M. Pop,
  8. N.A. Masluk,
  9. T. Brecht,
  10. L. Frunzio,
  11. M.H. Devoret,
  12. L.I. Glazman,
  13. and R. J. Schoelkopf
A promising quantum computing architecture couples superconducting qubits to microwave resonators (circuit QED), a system in which three-dimensional microwave cavities have become a
valuable resource. Such cavities have surface-to-volume ratios, or participation ratios a thousandfold smaller than in planar devices, deemphasizing potentially lossy surface elements by an equal amount. Motivated by this principle, we have tested aluminum superconducting cavity resonators with internal quality factors greater than 0.5 billion and intrinsic lifetimes reaching 0.01 seconds at single photon power and millikelvin temperatures. These results are the first to explore the use of superconducting aluminum, a ubiquitous material in circuit QED, as the basis of highly coherent (Q~10^7-10^9) cavity resonators. Measurements confirm the cavities‘ predicted insensitivity to quasiparticles (kinetic inductance fraction-5ppm) and an absence of two level dielectric fluctuations.
arxiv pdf

Decoherence of superconducting qubits caused by quasiparticle tunneling

  1. G. Catelani,
  2. Simon E. Nigg,
  3. S. M. Girvin,
  4. R. J. Schoelkopf,
  5. and L. I. Glazman
In superconducting qubits, the interaction of the qubit degree of freedom with quasiparticles defines a fundamental limitation for the qubit coherence. We develop a theory of the pure
dephasing rate Gamma_{phi} caused by quasiparticles tunneling through a Josephson junction and of the inhomogeneous broadening due to changes in the occupations of Andreev states in the junction. To estimate Gamma_{phi}, we derive a master equation for the qubit dynamics. The tunneling rate of free quasiparticles is enhanced by their large density of states at energies close to the superconducting gap. Nevertheless, we find that Gamma_{phi} is small compared to the rates determined by extrinsic factors in most of the current qubit designs (phase and flux qubits, transmon, fluxonium). The split transmon, in which a single junction is replaced by a SQUID loop, represents an exception that could make possible the measurement of Gamma_{phi}. Fluctuations of the qubit frequency leading to inhomogeneous broadening may be caused by the fluctuations in the occupation numbers of the Andreev states associated with a phase-biased Josephson junction. This mechanism may be revealed in qubits with small-area junctions, since the smallest relative change in frequency it causes is of the order of the inverse number of transmission channels in the junction.
arxiv pdf

Photon Shot Noise Dephasing in the Strong-Dispersive Limit of Circuit QED

  1. A. P. Sears,
  2. A. Petrenko,
  3. G. Catelani,
  4. L. Sun,
  5. Hanhee Paik,
  6. G. Kirchmair,
  7. L. Frunzio,
  8. L. I. Glazman,
  9. S. M. Girvin,
  10. and R. J. Schoelkopf
We study the photon shot noise dephasing of a superconducting transmon qubit in the strong-dispersive limit, due to the coupling of the qubit to its readout cavity. As each random arrival
or departure of a photon is expected to completely dephase the qubit, we can control the rate at which the qubit experiences dephasing events by varying textit{in situ} the cavity mode population and decay rate. This allows us to verify a pure dephasing mechanism that matches theoretical predictions, and in fact explains the increased dephasing seen in recent transmon experiments as a function of cryostat temperature. We investigate photon dynamics in this limit and observe large increases in coherence times as the cavity is decoupled from the environment. Our experiments suggest that the intrinsic coherence of small Josephson junctions, when corrected with a single Hahn echo, is greater than several hundred microseconds.
arxiv pdf