Confining the state of light to a quantum manifold by engineered two-photon loss

  1. Zaki Leghtas,
  2. Steven Touzard,
  3. Ioan M. Pop,
  4. Angela Kou,
  5. Brian Vlastakis,
  6. Andrei Petrenko,
  7. Katrina M. Sliwa,
  8. Anirudh Narla,
  9. Shyam Shankar,
  10. Michael J. Hatridge,
  11. Matthew Reagor,
  12. Luigi Frunzio,
  13. Robert J. Schoelkopf,
  14. Mazyar Mirrahimi,
  15. and Michel H. Devoret
Physical systems usually exhibit quantum behavior, such as superpositions and entanglement, only when they are sufficiently decoupled from a lossy environment. Paradoxically, a specially
engineered interaction with the environment can become a resource for the generation and protection of quantum states. This notion can be generalized to the confinement of a system into a manifold of quantum states, consisting of all coherent superpositions of multiple stable steady states. We have experimentally confined the state of a harmonic oscillator to the quantum manifold spanned by two coherent states of opposite phases. In particular, we have observed a Schrodinger cat state spontaneously squeeze out of vacuum, before decaying into a classical mixture. This was accomplished by designing a superconducting microwave resonator whose coupling to a cold bath is dominated by photon pair exchange. This experiment opens new avenues in the fields of nonlinear quantum optics and quantum information, where systems with multi-dimensional steady state manifolds can be used as error corrected logical qubits.

Measurement and Control of Quasiparticle Dynamics in a Superconducting Qubit

  1. Chen Wang,
  2. Yvonne Y. Gao,
  3. Ioan M. Pop,
  4. Uri Vool,
  5. Chris Axline,
  6. Teresa Brecht,
  7. Reinier W. Heeres,
  8. Luigi Frunzio,
  9. Michel H. Devoret,
  10. Gianluigi Catelani,
  11. Leonid I. Glazman,
  12. and Robert J. Schoelkopf
Superconducting circuits have attracted growing interest in recent years as a promising candidate for fault-tolerant quantum information processing. Extensive efforts have always been
taken to completely shield these circuits from external magnetic field to protect the integrity of superconductivity. Surprisingly, here we show vortices can dramatically improve the performance of superconducting qubits by reducing the lifetimes of detrimental single-electron-like excitations known as quasiparticles. Using a contactless injection technique with unprecedented dynamic range, we directly demonstrate the power-law decay characteristics of the canonical quasiparticle recombination process, and show quantization of quasiparticle trapping rate due to individual vortices. Each vortex in our aluminium film shows a quasiparticle „trapping power“ of 0.067±0.005 cm2/s, enough to dominate over the vanishingly weak recombination in a modern transmon qubit. These results highlight the prominent role of quasiparticle trapping in future development of quantum circuits, and provide a powerful characterization tool along the way.

Non-Poissonian Quantum Jumps of a Fluxonium Qubit due to Quasiparticle Excitations

  1. Uri Vool,
  2. Ioan M. Pop,
  3. Katrina Sliwa,
  4. Baleegh Abdo,
  5. Chen Wang,
  6. Teresa Brecht,
  7. Yvonne Y. Gao,
  8. Shyam Shankar,
  9. Michael Hatridge,
  10. Gianluigi Catelani,
  11. Mazyar Mirrahimi,
  12. Luigi Frunzio,
  13. Robert J. Schoelkopf,
  14. Leonid I. Glazman,
  15. and Michel H. Devoret
As the energy relaxation time of superconducting qubits steadily improves, non-equilibrium quasiparticle excitations above the superconducting gap emerge as an increasingly relevant
limit for qubit coherence. We measure fluctuations in the number of quasiparticle excitations by continuously monitoring the spontaneous quantum jumps between the states of a fluxonium qubit, in conditions where relaxation is dominated by quasiparticle loss. Resolution on the scale of a single quasiparticle is obtained by performing quantum non-demolition projective measurements within a time interval much shorter than T1, using a quantum limited amplifier (Josephson Parametric Converter). The quantum jumps statistics switches between the expected Poisson distribution and a non-Poissonian one, indicating large relative fluctuations in the quasiparticle population, on time scales varying from seconds to hours. This dynamics can be modified controllably by injecting quasiparticles or by seeding quasiparticle-trapping vortices by cooling down in magnetic field.

Implementation of low-loss superinductances for quantum circuits

  1. Nicholas A. Masluk,
  2. Ioan M. Pop,
  3. Archana Kamal,
  4. Zlatko K. Minev,
  5. and Michel H. Devoret
The simultaneous suppression of charge fluctuations and offsets is crucial for preserving quantum coherence in devices exploiting large quantum fluctuations of the superconducting phase.
This requires an environment with both extremely low DC and high RF impedance. Such an environment is provided by a superinductance, defined as a zero DC resistance inductance whose impedance exceeds the resistance quantum $R_Q = h/(2e)^2 simeq 6.5 mathrm{kOmega}$ at frequencies of interest (1 – 10 GHz). In addition, the superinductance must have as little dissipation as possible, and possess a self-resonant frequency well above frequencies of interest. The kinetic inductance of an array of Josephson junctions is an ideal candidate to implement the superinductance provided its phase slip rate is sufficiently low. We successfully implemented such an array using large Josephson junctions ($E_J >> E_C$), and measured internal losses less than 20 ppm, self-resonant frequencies greater than 10 GHz, and phase slip rates less than 1 mHz.