Raymond Mencia

The high-coherence fluxonium qubit

  1. Long B. Nguyen,
  2. Yen-Hsiang Ling,
  3. Aaron Somoroff,
  4. Raymond Mencia,
  5. Nicholas Grabon,
  6. and Vladimir E. Manucharyan
We report superconducting fluxonium qubits with coherence times largely limited by energy relaxation and reproducibly satisfying T2 > 100 microseconds (T2 > 300 microseconds in one
device). Moreover, given the state of the art values of the surface loss tangent and the 1/f flux noise amplitude, coherence can be further improved beyond 1 millisecond. Our results violate a common viewpoint that the number of Josephson junctions in a superconducting circuit — over 100 here — must be minimized for best qubit coherence. We outline how the unique to fluxonium combination of long coherence time and large anharmonicity can benefit both gate-based and adiabatic quantum computing.
arxiv pdf

Superstrong coupling in circuit quantum electrodynamics

  1. Roman Kuzmin,
  2. Nitish Mehta,
  3. Nicholas Grabon,
  4. Raymond Mencia,
  5. and Vladimir E. Manucharyan
Vacuum fluctuations fundamentally affect an atom by inducing a fnite excited state lifetime along with a Lamb shift of its transition frequency. Here we report the reverse effect: modifcation
of vacuum by a single atom in circuit quantum electrodynamics. Our one-dimensional vacuum is a long section of a high wave impedance (comparable to resistance quantum) superconducting transmission line. It is directly wired to a transmon qubit circuit. Owing to the combination of high impedance and galvanic connection, the transmon’s spontaneous emission linewidth can greatly exceed the discrete transmission line modes spacing. This condition defines a previously unexplored superstrong coupling regime of quantum electrodynamics where many vacuum modes hybridize with each other through interactions with a single atom. We explore this regime by spectroscopically measuring the positions of over 100 consecutive transmission line resonances. The atom reveals itself as a broad peak in the vacuum’s density of states (DOS) together with the Kerr and cross-Kerr interaction of photons at frequencies within the DOS peak. Both dispersive effects are well described by a dissipative Caldeira-Leggett model of our circuit, with the transmon’s quartic anharmonicity treated as a perturbation. Non-perturbative modifications of such a vacuum, including inelastic scattering of single photons, are expected upon replacing the transmon by more anharmonic circuits, with broad implications for simulating critical dynamics of quantum impurity models.
arxiv pdf