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.

Operating in a deep underground facility improves the locking of gradiometric fluxonium qubits at the sweet spots

  1. Daria Gusenkova,
  2. Francesco Valenti,
  3. Martin Spiecker,
  4. Simon Günzler,
  5. Patrick Paluch,
  6. Dennis Rieger,
  7. Larisa-Milena Pioraş-Ţimbolmaş,
  8. Liviu P. Zârbo,
  9. Nicola Casali,
  10. Ivan Colantoni,
  11. Angelo Cruciani,
  12. Stefano Pirro,
  13. Laura Cardani,
  14. Alexandru Petrescu,
  15. Wolfgang Wernsdorfer,
  16. Patrick Winkel,
  17. and Ioan M. Pop
We demonstrate flux-bias locking and operation of a gradiometric fluxonium artificial atom using two symmetric granular aluminum (grAl) loops to implement the superinductor. The gradiometric
fluxonium shows two orders of magnitude suppression of sensitivity to homogeneous magnetic fields, which can be an asset for hybrid quantum systems requiring strong magnetic field biasing. By cooling down the device in an external magnetic field while crossing the metal-to-superconductor transition, the gradiometric fluxonium can be locked either at 0 or Φ0/2 effective flux bias, corresponding to an even or odd number of trapped fluxons, respectively. At mK temperatures, the fluxon parity prepared during initialization survives to magnetic field bias exceeding 100Φ0. However, even for states biased in the vicinity of 1Φ0, we observe unexpectedly short fluxon lifetimes of a few hours, which cannot be explained by thermal or quantum phase slips. When operating in a deep-underground cryostat of the Gran Sasso laboratory, the fluxon lifetimes increase to days, indicating that ionizing events activate phase slips in the grAl superinductor.

Quantum non-demolition dispersive readout of a superconducting artificial atom using large photon numbers

  1. Daria Gusenkova,
  2. Martin Spiecker,
  3. Richard Gebauer,
  4. Madita Willsch,
  5. Francesco Valenti,
  6. Nick Karcher,
  7. Lukas Grünhaupt,
  8. Ivan Takmakov,
  9. Patrick Winkel,
  10. Dennis Rieger,
  11. Alexey V. Ustinov,
  12. Nicolas Roch,
  13. Wolfgang Wernsdorfer,
  14. Kristel Michielsen,
  15. Oliver Sander,
  16. and Ioan M. Pop
Reading out the state of superconducting artificial atoms typically relies on dispersive coupling to a readout resonator. For a given system noise temperature, increasing the circulating
photon number n¯ in the resonator enables a shorter measurement time and is therefore expected to reduce readout errors caused by spontaneous atom transitions. However, increasing n¯ is generally observed to also increase these transition rates. Here we present a fluxonium artificial atom in which we measure an overall flat dependence of the transition rates between its first two states as a function of n¯, up to n¯≈200. Despite the fact that we observe the expected decrease of the dispersive shift with increasing readout power, the signal-to-noise ratio continuously improves with increasing n¯. Even without the use of a parametric amplifier, at n¯=74, we measure fidelities of 99% and 93% for feedback-assisted ground and excited state preparation, respectively.

Reducing the impact of radioactivity on quantum circuits in a deep-underground facility

  1. Laura Cardani,
  2. Francesco Valenti,
  3. Nicola Casali,
  4. Gianluigi Catelani,
  5. Thibault Charpentier,
  6. Massimiliano Clemenza,
  7. Ivan Colantoni,
  8. Angelo Cruciani,
  9. Luca Gironi,
  10. Lukas Grünhaupt,
  11. Daria Gusenkova,
  12. Fabio Henriques,
  13. Marc Lagoin,
  14. Maria Martinez,
  15. Giorgio Pettinari,
  16. Claudia Rusconi,
  17. Oliver Sander,
  18. Alexey V. Ustinov,
  19. Marc Weber,
  20. Wolfgang Wernsdorfer,
  21. Marco Vignati,
  22. Stefano Pirro,
  23. and Ioan M. Pop
As quantum coherence times of superconducting circuits have increased from nanoseconds to hundreds of microseconds, they are currently one of the leading platforms for quantum information
processing. However, coherence needs to further improve by orders of magnitude to reduce the prohibitive hardware overhead of current error correction schemes. Reaching this goal hinges on reducing the density of broken Cooper pairs, so-called quasiparticles. Here, we show that environmental radioactivity is a significant source of nonequilibrium quasiparticles. Moreover, ionizing radiation introduces time-correlated quasiparticle bursts in resonators on the same chip, further complicating quantum error correction. Operating in a deep-underground lead-shielded cryostat decreases the quasiparticle burst rate by a factor fifty and reduces dissipation up to a factor four, showcasing the importance of radiation abatement in future solid-state quantum hardware.

State preparation of a fluxonium qubit with feedback from a custom FPGA-based platform

  1. Richard Gebauer,
  2. Nick Karcher,
  3. Daria Gusenkova,
  4. Martin Spiecker,
  5. Lukas Grünhaupt,
  6. Ivan Takmakov,
  7. Patrick Winkel,
  8. Luca Planat,
  9. Nicolas Roch,
  10. Wolfgang Wernsdorfer,
  11. Alexey V. Ustinov,
  12. Marc Weber,
  13. Martin Weides,
  14. Ioan M. Pop,
  15. and Oliver Sander
We developed a versatile integrated control and readout instrument for experiments with superconducting quantum bits (qubits), based on a field-programmable gate array (FPGA) platform.
Using this platform, we perform measurement-based, closed-loop feedback operations with 428ns platform latency. The feedback capability is instrumental in realizing active reset initialization of the qubit into the ground state in a time much shorter than its energy relaxation time T1. We show experimental results demonstrating reset of a fluxonium qubit with 99.4% fidelity, using a readout-and-drive pulse sequence approximately 1.5μs long. Compared to passive ground state initialization through thermalization, with the time constant given by T1= 80μs, the use of the FPGA-based platform allows us to improve both the fidelity and the time of the qubit initialization by an order of magnitude.

Phonon traps reduce the quasiparticle density in superconducting circuits

  1. Fabio Henriques,
  2. Francesco Valenti,
  3. Thibault Charpentier,
  4. Marc Lagoin,
  5. Clement Gouriou,
  6. Maria Martínez,
  7. Laura Cardani,
  8. Lukas Grünhaupt,
  9. Daria Gusenkova,
  10. Julian Ferrero,
  11. Sebastian T. Skacel,
  12. Wolfgang Wernsdorfer,
  13. Alexey V. Ustinov,
  14. Gianluigi Catelani,
  15. Oliver Sander,
  16. and Ioan M. Pop
Out of equilibrium quasiparticles (QPs) are one of the main sources of decoherence in superconducting quantum circuits, and are particularly detrimental in devices with high kinetic
inductance, such as high impedance resonators, qubits, and detectors. Despite significant progress in the understanding of QP dynamics, pinpointing their origin and decreasing their density remain outstanding tasks. The cyclic process of recombination and generation of QPs implies the exchange of phonons between the superconducting thin film and the underlying substrate. Reducing the number of substrate phonons with frequencies exceeding the spectral gap of the superconductor should result in a reduction of QPs. Indeed, we demonstrate that surrounding high impedance resonators made of granular aluminum (grAl) with lower gapped thin film aluminum islands increases the internal quality factors of the resonators in the single photon regime, suppresses the noise, and reduces the rate of observed QP bursts. The aluminum islands are positioned far enough from the resonators to be electromagnetically decoupled, thus not changing the resonator frequency, nor the loading. We therefore attribute the improvements observed in grAl resonators to phonon trapping at frequencies close to the spectral gap of aluminum, well below the grAl gap.

Granular aluminum: A superconducting material for high impedance quantum circuits

  1. Lukas Grünhaupt,
  2. Martin Spiecker,
  3. Daria Gusenkova,
  4. Nataliya Maleeva,
  5. Sebastian T. Skacel,
  6. Ivan Takmakov,
  7. Francesco Valenti,
  8. Patrick Winkel,
  9. Hannes Rotzinger,
  10. Alexey V. Ustinov,
  11. and Ioan M. Pop
Superconducting quantum information processing machines are predominantly based on microwave circuits with relatively low characteristic impedance, of about 100 Ohm, and small anharmonicity,
which can limit their coherence and logic gate fidelity. A promising alternative are circuits based on so-called superinductors, with characteristic impedances exceeding the resistance quantum RQ=6.4 kΩ. However, previous implementations of superinductors, consisting of mesoscopic Josephson junction arrays, can introduce unintended nonlinearity or parasitic resonant modes in the qubit vicinity, degrading its coherence. Here we present a fluxonium qubit design using a granular aluminum (grAl) superinductor strip. Granular aluminum is a particularly attractive material, as it self-assembles into an effective junction array with a remarkably high kinetic inductance, and its fabrication can be in-situ integrated with standard aluminum circuit processing. The measured qubit coherence time TR2 up to 30 μs illustrates the potential of grAl for applications ranging from protected qubit designs to quantum limited amplifiers and detectors.

An argon ion beam milling process for native AlOx layers enabling coherent superconducting contacts

  1. Lukas Grünhaupt,
  2. Uwe von Lüpke,
  3. Daria Gusenkova,
  4. Sebastian T. Skacel,
  5. Nataliya Maleeva,
  6. Steffen Schlör,
  7. Alexander Bilmes,
  8. Hannes Rotzinger,
  9. Alexey V. Ustinov,
  10. Martin Weides,
  11. and Ioan M. Pop
We present an argon ion beam milling process to remove the native oxide layer forming on aluminum thin films due to their exposure to atmosphere in between lithographic steps. Our cleaning
process is readily integrable with conventional fabrication of Josephson junction quantum circuits. From measurements of the internal quality factors of superconducting microwave resonators with and without contacts, we place an upper bound on the residual resistance of an ion beam milled contact of 50mΩ⋅μm2 at a frequency of 4.5 GHz. Resonators for which only 6% of the total foot-print was exposed to the ion beam milling, in areas of low electric and high magnetic field, showed quality factors above 106 in the single photon regime, and no degradation compared to single layer samples. We believe these results will enable the development of increasingly complex superconducting circuits for quantum information processing.