Alexander Shnirman

Solomon equations for qubit and two-level systems

  1. Martin Spiecker,
  2. Andrei I. Pavlov,
  3. Alexander Shnirman,
  4. and Ioan M. Pop
We model and measure the combined relaxation of a qubit, a.k.a. central spin, coupled to a discrete two-level system (TLS) environment. We present a derivation of the Solomon equations
starting from a general Lindblad equation for the qubit and an arbitrary number of TLSs. If the TLSs are much longer lived than the qubit, the relaxation becomes non-exponential. In the limit of large numbers of TLSs the populations are likely to follow a power law, which we illustrate by measuring the relaxation of a superconducting fluxonium qubit. Moreover, we show that the Solomon equations predict non-Poissonian quantum jump statistics, which we confirm experimentally.
arxiv pdf

A quantum Szilard engine for two-level systems coupled to a qubit

  1. Martin Spiecker,
  2. Patrick Paluch,
  3. Niv Drucker,
  4. Shlomi Matityahu,
  5. Daria Gusenkova,
  6. Nicolas Gosling,
  7. Simon Günzler,
  8. Dennis Rieger,
  9. Ivan Takmakov,
  10. Francesco Valenti,
  11. Patrick Winkel,
  12. Richard Gebauer,
  13. Oliver Sander,
  14. Gianluigi Catelani,
  15. Alexander Shnirman,
  16. Alexey V. Ustinov,
  17. Wolfgang Wernsdorfer,
  18. Yonatan Cohen,
  19. and Ioan M. Pop
The innate complexity of solid state physics exposes superconducting quantum circuits to interactions with uncontrolled degrees of freedom degrading their coherence. By using a simple
stabilization sequence we show that a superconducting fluxonium qubit is coupled to a two-level system (TLS) environment of unknown origin, with a relatively long energy relaxation time exceeding 50ms. Implementing a quantum Szilard engine with an active feedback control loop allows us to decide whether the qubit heats or cools its TLS environment. The TLSs can be cooled down resulting in a four times lower qubit population, or they can be heated to manifest themselves as a negative temperature environment corresponding to a qubit population of ∼80%. We show that the TLSs and the qubit are each other’s dominant loss mechanism and that the qubit relaxation is independent of the TLS populations. Understanding and mitigating TLS environments is therefore not only crucial to improve qubit lifetimes but also to avoid non-Markovian qubit dynamics.
arxiv pdf

Interacting two-level defects as sources of fluctuating high-frequency noise in superconducting circuits

  1. Clemens Müller,
  2. Jürgen Lisenfeld,
  3. Alexander Shnirman,
  4. and Stefano Poletto
Since the very first experiments, superconducting circuits have suffered from strong coupling to environmental noise, destroying quantum coherence and degrading performance. In state-of-the-art
experiments it is found that the relaxation time of superconducting qubits fluctuates as a function of time. We present measurements of such fluctuations in a 3D-Transmon circuit and develop a qualitative model based on interactions within a bath of background two-level systems (TLS) which emerge from defects in the device material. Assuming both high- and low-frequency TLS are present, their mutual interaction will lead to fluctuations in the noise spectral density acting on the qubit circuit. This model is further supported by direct measurements of energy fluctuations in a single high-frequency TLS.
arxiv pdf